# Novae in M51: a New, Much Higher Rate from Multi-epoch HST Data

Shifra Mandel,1 Michael M. Shara,2 David Zurek2 Charlie Conroy3 and Pieter van
Dokkum4
1Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027,
USA
2Department of Astrophysics, American Museum of Natural History, New York, NY,
USA
3Department of Astronomy, Harvard University, Cambridge, MA, 02138, USA
4Department of Astronomy, Yale University, New Haven, CT, 06511, USA E-mail:
<EMAIL_ADDRESS>
(Accepted 2022 September 27. Received 2022 September 26; in original form 2022
August 24)

###### Abstract

Accurate determination of the rates of nova eruptions in different kinds of
galaxies give us strong constraints on those galaxies’ underlying white dwarf
and binary populations, and those stars’ spatial distributions. Until 2016,
limitations inherent in ground-based surveys of external galaxies – and dust
extinction in the Milky Way – significantly hampered the determination of
those rates and how much they differ between different types of galaxies.
Infrared Galactic surveys and dense cadence Hubble Space Telescope (HST)-based
surveys are overcoming these limitations, leading to sharply increased nova-
in-galaxy rates relative to those previously claimed. Here we present 14 nova
candidates that were serendipitously observed during a year-long HST survey of
the massive spiral galaxy M51 (the “Whirlpool Galaxy”). We use simulations
based on observed nova light curves to model the incompleteness of the HST
survey in unprecedented detail, determining a nova detection efficiency
$\epsilon=20.3$ percent. The survey’s M51 area coverage, combined with
$\epsilon$, indicates a conservative M51 nova rate of $172^{+46}_{-37}$ novae
yr-1, corresponding to a luminosity-specific nova rate (LSNR) of $\sim
10.4^{+2.8}_{-2.2}$ novae yr-1/$10^{10}L_{\odot,K}$. Both these rates are
approximately an order of magnitude higher than those estimated by ground-
based studies, contradicting claims of universal low nova rates in all types
of galaxies determined by low cadence, ground-based surveys. They demonstrate
that, contrary to theoretical models, the HST-determined LSNR in a giant
elliptical galaxy (M87) and a giant spiral galaxy (M51) likely do not differ
by an order of magnitude or more, and may in fact be quite similar.

###### keywords:

nova, cataclysmic variables – galaxies: stellar content – supernovae: general

††pubyear: 2022††pagerange: Novae in M51: a New, Much Higher Rate from Multi-
epoch HST Data–B

##  1 Introduction

All cataclysmic variables (CVs) are binaries containing a white dwarf (WD)
which accretes matter from a close companion. A nova eruption is a bright (up
to $10^{6}$ $L_{\odot}$) outburst that occurs when the envelope accreted onto
the WD surface ignites in a thermonuclear runaway. Nova characteristics (such
as the recurrence rate, peak luminosity, and decay time) encode information
about the WD and donor star, as well as the binary mass transfer rate during
the millenia between nova eruptions (Hillman et al., 2016, 2020). Novae are
our only means of studying CV populations (and indeed most binary populations)
in galaxies beyond the Local Group. In addition, the most rapidly accreting
WDs in nova binaries can be progenitors of “standard candle” type Ia
supernovae (SNIa) (Hillman et al., 2016), so these stars’ importance extends
beyond the domain of stellar evolution to cosmology.

Given that CVs with high accretion rates from sub-giant companions and/or very
massive WDs are likely SNIa progenitors (Hillman et al., 2016), the dependence
of CV populations on the underlying stellar populations and the environments
of their host galaxies is of great importance for determining whether SNIa are
reliable standard candles. Varying CV populations in different galaxy types
could be an indicator of differing SNIa progenitor channels, with important
implications for the determination of the Hubble constant $H_{0}$ using SNIa
as distance indicators. Differences in CV populations could also hint at
different binary fractions and/or stellar evolution pathways in different
types of galaxies.

Despite their importance in understanding and testing models of binary stellar
evolution, and implications for cosmological standard candles, a lack of
consensus on the actual nova rates in galaxies has persisted for two decades.
On the basis of multiple ground-based surveys, Shafter et al. (2000) and
Shafter et al. (2014) claimed that the luminosity specific nova rates (LSNR,
i.e. annual rate of novae per unit K-band luminosity) in different galaxy
types are all similar, in the range of 1–3 novae yr-1/$10^{10}L_{\odot,K}$. In
contrast, the population synthesis studies of Matteucci et al. (2003), Claeys
et al. (2014) and Chen et al. (2016) suggested that order-of-magnitude
differences in nova rates and LSNR should exist between elliptical, spiral,
and especially starburst galaxies. This is because rapid and massive star
formation should produce a plethora of mass-transferring binaries containing
high-mass WDs in spiral and starburst galaxies. Novae which erupt on high-mass
WDs do so after accreting relatively low-mass envelopes (Yaron et al., 2005).
Such novae can thus erupt more frequently than those associated with low mass
WDs, so that the LSNR in spiral and especially in starburst galaxies are
predicted to greatly exceed the corresponding rates in elliptical galaxies.

Using a Hubble Space Telescope (HST) survey of the massive elliptical galaxy
M87, Shara et al. (2016) showed that ground-based surveys of external galaxies
fail to detect fainter novae and/or those with short decline times and/or
those near the bright centers of galaxies. These effects cause ground-based
surveys to systematically underestimate the true nova rates in galaxies. In
the case of M87, these effects led to the ground-based underestimate of the
M87 nova rate by a factor of 2-4; the HST \- determined LSNR in the K-band was
shown to be $7.88_{-2.6}^{+2.3}$ novae yr-1/$10^{10}L_{\odot},_{K}$ (Shara et
al., 2016). Confirmation of ground-based nova rate underestimates was provided
by Mróz et al. (2016), who demonstrated that the LSNR in the Large Magellanic
Cloud (LMC) is much higher than previous ground-based estimates, and is
comparable to the M87 LSNR. This finding was further supported by De et al.
(2021), who discovered a sizable population of Galactic novae (in the
infrared) that have gone undetected in over a century of optical searches, and
Kawash et al. (2021), who found that approximately half of all Galactic novae
are hidden from current surveys by extinction.

These discoveries (of much higher than previously claimed LSNR) in a giant
elliptical (M87), a barred spiral (the Galaxy), and a dwarf irregular galaxy
(LMC), were carried out via surveys with much longer baselines, denser time
coverage and/or deeper magnitude limits than all previous surveys. They argue
strongly against the claim that the LSNR is relatively low in all galaxies, as
the earlier, shallower and sparser cadence coverage of Shafter et al. (2000)
and Shafter et al. (2014) suggested. They highlight the need for deep,
unbiased surveys of other types of galaxies to confirm that the LSNRs are much
higher than previously thought, and to test whether they vary with galaxy
type, as predicted by binary population synthesis models. In particular, a
giant Sc-type spiral galaxy has not yet been so studied. HST is especially
suited to such investigations because of its unparalleled angular resolution
and consequent sensitivity, its very small and virtually constant point-spread
function, its insensitivity to lunar phase and its immunity to atmospheric
seeing.

The massive Sc-type spiral galaxy M51 (the Whirlpool Galaxy, NGC 5194) has
been surveyed for novae only once, over 20 years ago (Shafter et al., 2000).
Narrowband $H\alpha$ and broadband $R$ images centered on M51, and covering
$16^{\prime}x16^{\prime}$ (the entire galaxy), were taken with the Kitt Peak
National Observatory 4-meter telescope at four well-separated epochs in 1994
and 1995. These data led to the discovery of nine novae. Allowing for gaps in
coverage and other sources of incompleteness (such as a limiting absolute
magnitude detection limit of -7.7 $\pm$ 0.22), Shafter et al. (2000) derived a
rate of 18 $\pm$ 7 novae yr-1 in M51. This corresponds to an M51 LSNR of 1.09
$\pm$ 0.47 novae yr-1/$10^{10}L_{\odot,K}$.

M51 was the subject of an HST observing campaign that began in 2016 and
continued for nearly a year (Conroy et al., 2018). The stated goal of that
survey was to catalog and categorize all luminous stellar variables within a
significant fraction of that galaxy. Given its optimal orientation on the sky
– M51 is face-on, which minimizes internal reddening, and benefits from a
fairly low Galactic extinction that is approximately constant across the field
– this survey also offers an excellent opportunity for the most complete and
unbiased study of the novae in a massive Sc-type spiral galaxy to date.
Although the irregular cadence and sometimes weeks-long gaps between
observations of M51 are not ideal for a nova survey, the proximity of M51
(relative to M87) allows us to detect novae in HST images even if they are
intrinsically faint (hence much too faint to detect from the ground) and/or
after they have dimmed several magnitudes from maximum light. In particular,
the archival HST imaging dataset of M51 enables just the second, head-to-head
comparison (after M87) of ground-based versus HST \- derived nova rates in the
same galaxy.

Section 2 describes the data collected during the M51 HST observing campaign.
Section 3 describes our search for and identification of nova candidates and
their properties. In Section 4 and in section 5 we describe the details of the
simulations we conducted to investigate, respectively, the detection
efficiency and the incompleteness of our HST survey. In Section 6 we discuss
our findings and their implications, and we summarize our results in Section
7.

##  2 HST Imaging Data

Figure 1: HST ACS $I_{814}$-band mosaic of M51. The footprints of the 34
observations used for this study are shown in cyan; the large dashed magenta
circle marks the region consistently observed and included in the photometric
data.

The HST observing campaign of M51 was conducted over the course of 345 days
using the Advanced Camera for Surveys (ACS) Wide-Field Channel (WFC) $F606W$
and $F814W$ filters (hereafter $V_{606}$ and $I_{814}$, respectively). Each of
the 34 imaging epochs, with a total 2.2 ksec exposure in each of the $V_{606}$
and $I_{814}$ bands, covered $\sim 40$ percent of the galaxy’s $I_{814}$ flux
(see Section 6.1). While the average gap between observations was $\sim 10$
days, the survey cadence ranged from 4 to 24 days. Figure 1 shows the HST
fields of view (FOV) of those 34 epochs. Note that the FOVs rotate to maintain
optimal pointing of HST ’s solar panels throughout the course of the year. We
considered only the portion of the FOVs that was covered in every epoch (see
Figure 3 for more details).

To create the M51 star catalog, point-spread-function (PSF) photometry was
performed using the DOLPHOT software package (Dolphin, 2000; Conroy et al.,
2018). The regions around six bright foreground stars were masked to avoid
contamination. The central 10 arcsec were also excluded, because the region is
extremely crowded, making PSF photometry impracticable. A detailed description
of the observations and the methods used to extract the photometric data are
given in Conroy et al. (2018). In total, the HST data yielded photometric
measurements at 34 epochs in two passbands for $\sim 1.39$ million stars.

The survey’s magnitudes are on the Vega zero point (VEGAMAG) system. All
absolute magnitudes were computed assuming an M51 distance modulus of 29.67
with Galactic extinctions in the direction of M51 of $A_{606}=0.086$ and
$A_{814}=0.053$ mag (McQuinn et al., 2016).

##  3 M51 Nova Search and identification

Table 1: Nova Candidates in M51

Nova |  M51 Nucleus Offset |  RA (J2000) |  DEC (J2000) |  Mpeak |  Mpeak |  tpeak,V
---|---|---|---|---|---|---
|  [arcsec] |  [deg] |  [deg] |  ($V_{606}$) |  $(I_{814})$ |  [MJD]
1 |  74.31 |  202.49935 |  47.19936 |  -8.51 |  -8.97 |  57675
2 |  10.77 |  202.46526 |  47.19466 |  -7.99 |  -8.28 |  57936
3 |  23.36 |  202.46145 |  47.19867 |  -7.79 |  -8.27 |  57971
4 |  114.87 |  202.48422 |  47.16494 |  -7.78 |  -8.04 |  57858
5 |  15.55 |  202.47586 |  47.19589 |  -7.73 |  -8.26 |  57946
6 |  45.85 |  202.48117 |  47.18525 |  -7.26 |  -7.68 |  57971
7 |  30.26 |  202.45930 |  47.19994 |  -7.07 |  -7.33 |  57666
8 |  33.21 |  202.47781 |  47.18792 |  -6.97 |  -7.42 |  57666
9 |  119.21 |  202.51632 |  47.20461 |  -6.89 |  -7.22 |  57760
10 |  54.92 |  202.48981 |  47.20186 |  -6.72 |  -7.34 |  57993
11 |  47.62 |  202.48200 |  47.20544 |  -6.41 |  -7.41 |  57760
12 |  67.81 |  202.49248 |  47.18465 |  -6.17 |  -6.48 |  57699
13 |  94.0 |  202.47425 |  47.16934 |  -6.03 |  -6.91 |  57925
14 |  22.89 |  202.47877 |  47.19641 |  -5.53 |  -6.50 |  57666

Fourteen nova candidates discovered in the HST observations of M51, listed in
order of observed peak luminosity.

The four defining characteristics we used to identify potential M51 nova
candidates (cf. Shara et al. (2016)) among the $\sim 1.39$ million stars in
the HST dataset are:

  * •

a peak absolute magnitude brighter than -5 in the $V_{606}$ and $I_{814}$
bands,

  * •

a decrease from peak brightness of at least 2 mag over the duration of the
observing campaign,

  * •

a “blue” color (average near maximum light) of $V_{606}-I_{814}<0.50$ mag, and

  * •

no apparent periodic variability.

These criteria are satisfied by virtually all known novae, and are
deliberately over-conservative so as not to miss any reasonably identifiable
candidates on a first pass. Approximately 1,000 preliminary nova candidates
were selected using the above parameter and light curve constraints. Every
candidate’s light curve was visually inspected, allowing us to weed out
eclipsing, periodic, and spurious sources. [A number of discarded sources
could have been either novae or other highly variable (and similarly
“blue”-colored) stars, like cepheids or luminous blue variables (LBVs). We
retained only sources that showed the canonical single-peaked fast rise and
exponential decline that is most often observed in novae, but not in other
highly variable stars, although nova light curves can take a variety of shapes
(Strope et al., 2010).] All surviving candidates were visually inspected in
the HST images. Fourteen final nova candidates remained.

Table 1 lists the 14 nova candidates we identified in the HST dataset,
including their angular offsets from M51’s nucleus, J2000 coordinates,
absolute magnitudes ($V_{606}$\- and $I_{814}$-band) at maximum light, and the
date of observed maximum brightness. (Note that the actual peak luminosity for
these nova outbursts could have been higher by up to several magnitudes;
because of the gaps between HST observations, most were likely detected during
their decline.) We deliberately omit $t_{2}$ because, due to the large gaps
between the HST observations of M51, the uncertainties in $t_{peak}$ and
$M_{peak}$ – both of which are required to evaluate $t_{2}$ – are too great to
allow for meaningful estimates of the decline time.

The light curves for the 14 novae are shown in Figure 2. “Postage stamp”
difference images of all novae in Table 1 for each observing epoch are shown
in Appendix B. The spatial distribution of the novae can be seen in Figure 3,
in which the positions of the 14 novae are overlaid on an image of M51.

Figure 2: $V_{606}$\- and $I_{814}$\- band light curves (shown in blue and
red, respectively) for the 14 nova candidates in M51. Arrows mark upper
limits. Where no error bars are visible, magnitude errors are smaller than the
data point markers.  Figure 3: 2MASS K-band image showing the approximate
“footprint” (cf. Figure 1) of the HST survey (magenta), an area of radius
$\sim 102$ arcsec. The central $10$ arcsec of M51 were excluded from our
photometric analysis. The M51 light fraction covered in this survey - $40$
percent - was computed with the assumption that the light inside the white
circle (radius 220 arcsec) constitutes all the light from M51 (see 6.1 for
more). The locations and IDs of the nova candidates we identified are shown in
turquoise.

##  4 M51 Survey Nova Detection Efficiency

In addition to observing cadence, the observational properties that have the
largest effect on whether a nova outburst will be detected or missed are that
nova’s peak luminosity ($M_{peak}$) and the decline time. The latter is often
defined as $t_{2}$, which is the time required for the luminosity to decrease
from $M_{peak}$ by two magnitudes. The non-uniform cadence of – and large gaps
between – the HST observations of M51 must necessarily hamper the detection of
rapidly fading novae and the peak luminosities $L_{peak}$ of most outbursts.
To quantify the _detection efficiency_ ($\epsilon$) of our survey as a
function of $M_{peak}$ and $t_{2}$ (as opposed to its _incompleteness_ , which
we address in Section 5), we generated an idealized set of artificial novae
covering essentially the entire ranges of observed nova $M_{peak}$ and
$t_{2}$. (Not all combinations of nova $M_{peak}$ and $t_{2}$ are observed in
nature, particularly the combination of most luminous $M_{peak}$ and longest
$t_{2}$, but their inclusion is nonetheless instructive).

We adopted $M_{peak}$ values of $[-10,-9.75,-9.5,\cdots,-5]$ mag and $t_{2}$
values of $[2,5,8,\cdots,152]$ days. All combinations of these 21 values of
$M_{peak}$ and 51 values of $t_{2}$ produced a total of 1,071 artificial nova
light curves. Each artificial nova was assumed to decline exponentially,
beginning at $M_{peak}$ and fading through the HST detection threshold for the
M51 survey, $V_{606}\approx 27.5$ mag. Each synthetic nova outburst was then
begun at day ($t_{0}$) $[0,1,2,\cdots,345]$ (corresponding to the length of
the HST observing campaign of M51) and sampled with the HST observing cadence.
This range of outburst start times was necessary to account for biases in
detectability based on the HST observing cadence near the eruption date, as
the gap between HST epochs varied between 4 and 24 days throughout the
campaign.

This process yielded $346\times 1071=370,566$ light curves, sampled with the
HST observing cadence. Each light curve was evaluated to determine whether the
corresponding nova would have been detected as such via our $V_{606}$-band
selection parameters, listed in Section 3. This yields the survey’s detection
efficiency as a function of $M_{peak}$ and $t_{2}$. The results are plotted in
Figure 4. As expected, the figure demonstrates that novae with brighter
$M_{peak}$ and longer $t_{2}$ were more likely to be detected than their
fainter/faster counterparts, except when their decline times ($t_{2}$)
stretched to months. The latter effect is due to the decrease in observed
variability when the decline time is very long.

The existence of intrinsically faint novae with small $t_{2}$ (the so-called
“faint/fast novae”) was predicted in the 2005 compendium of nova models with a
wide range of WD masses and accretion rates (Yaron et al., 2005). They were
first detected observationally by Kasliwal et al. (2011) in M31, then by Shara
et al. (2016) in M87, and are now understood to be common. Their ubiquity and
importance in determining our survey’s incompleteness are apparent in Figure
4, where we overlaid the best optical samples of Galactic, M31, and M87 novae
that are currently available on the detection efficiency plot just described.

Figure 4: Detection efficiency of synthetic novae for the full range of nova
$M_{peak}$ and $t_{2}$ discussed in Section 4, sampled at the M51 HST
observing cadence. Contours denote a 20 percent change in efficiency.
Galactic, M31, and M87 novae are overlaid in pink, violet, and blue,
respectively.

If they erupted in M51, a significant fraction of all novae observed in M87,
M31 and the Galaxy would nearly always be missed in the current survey because
they display $t_{2}<10$ days. The poor detectability of novae with
$t_{2}\lower 2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}10$ days in our survey
is a consequence of the large gaps between the HST observations of M51. Many
novae are observed to erupt with $t_{2}\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}10$ days, which, as noted above,
are very difficult to detect in this survey. Conversely, almost all novae with
$t_{2}>20$ days and $M_{peak}\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}-8$ would be detectable (assuming a
simplified exponential decline shape111The full range of nova light curve
morphologies, including some that are considerably more complex than the
simplistic models we adopt here, is detailed in Strope et al. (2010). We use
those and other realistic light curves in section 5 below when we determine
our survey’s incompleteness.). No such novae have ever been reported, or
predicted by large suites of nova models (Yaron et al., 2005). The lack of
detection of any such novae in our M51 HST survey (despite the excellent
detection efficiency associated with them) is further evidence that such
objects are very rare, or do not exist.

##  5 Incompleteness Simulations using Observed Novae

To determine the nova rate in M51, we must first measure our survey’s
incompleteness viz. the fraction of novae that erupted in M51 during our
survey but which were not detected. While the idealized, highly simplified
simulations described in Section 4 help us understand how luminosity and
decline time affect the detectability of novae subject to the HST M51 survey
cadence, the reality is more complex. This is because the _shape_ of a nova
light curve, which is generally _not_ exponential throughout, also plays a
crucial role in its detectability, as demonstrated in Figure 5. The decline of
a nova’s brightness is often fastest immediately following the outburst peak,
though the opposite sometimes occurs. Some novae reach a steady brightness
plateau for days or weeks during their declines, while others undergo deep
dips as dust forms in their ejecta. The rates of change in luminosity, and
when those changes occur, vary greatly among well-sampled Galactic novae
(Strope et al., 2010). Thus the irregularly spaced epochs of this survey must
be convolved with a set of realistic light curves, representative of M51
novae, to determine our survey’s incompleteness.

Figure 5: Light curves of two Galactic novae, NQ Vul (orange) and U Sco
(purple). These novae have identical $t_{2}$ (dotted line) but different
decline shapes, resulting in a lower detection efficiency in our M51 survey
for the brighter nova (U Sco). This demonstrates the importance of modelling
detection efficiency using real nova light curves, as $M_{peak}$ and $t_{2}$
alone are not reliable predictors for detectability.

There is no published set of well-sampled light curves of novae in M51 or in
any other Sc-type galaxy. Thus we have produced a series of simulations using
the three most realistic and complete sets of light curves available from
well-observed novae in our Galaxy, M31 and M87. A list of all novae that were
used in the following simulations of 5.1 \- 5.2 are found in Table
LABEL:simnovae in Appendix A. A summary of the results is shown in table 2.

###  5.1 Simulation I. Galactic Novae

The largest collection of well-sampled novae starting near maximum brightness
and extending well into the decline phase comprises, not surprisingly,
Galactic novae. Strope et al. (2010) published the light curves of 93 Galactic
novae, most of which were well monitored from outburst peak through several
magnitudes of decline. Of these, 32 have well-determined distances that allow
us to calculate their absolute magnitudes, either from Gaia parallaxes
(Schaefer, 2018) or through the blackbody flux of their giant companions
(Özdönmez et al., 2018).

This largest Galactic nova sample with well-calibrated luminosities is
unfortunately far from unbiased. In particular, it is conspicuously lacking in
faint/fast novae (Kasliwal et al., 2011; Shara et al., 2016), which were
missed throughout 20th century nova searches. As Figure 4 demonstrates, our
M51 survey’s detection efficiency $\epsilon$ for novae with $t_{2}\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}10$ days is close to zero. While we
investigated whether each of these 32 Galactic novae would be identifiable as
such _had it occurred within M51 during our HST observing campaign_ , the lack
of faint/fast novae in the Galactic sample means that its incompleteness
estimate will be an upper limit only. See Section 6.3.1 for more details.

Similarly to the process for the synthetic novae, each of the above 32
Galactic nova light curves was begun at $t_{0}=[-20,-19,-18,$ $\cdots,345]$
days222Given the overall excellent sampling frequency and extended duration of
the Galactic nova light curves, we were able to model outbursts occurring over
a full year, starting 20 days before the HST observing campaign of M51
commenced. and sampled with the HST observing cadence. Linear interpolation
was utilized where small gaps in the Galactic nova light curves coincided with
one of the M51 HST epochs. This provided us with a realistic set of $11,680$
simulated nova light curves. Then, as we had done for the synthetic novae, we
evaluated each of the simulated light curves to determine whether it would
have yielded a nova detection using our $V_{606}$-band selection parameters,
summarized in Section 2.

The results of our detection efficiency evaluations for the 32 Galactic novae
with the most reliable distances are shown in the left panel of Figure 6. The
$M_{peak}$, $t_{2}$, and mean detection efficiency $\epsilon$ shown for each
data point in Figure 6 correspond to the Galactic nova that was used to
generate the simulated light curves. The mean $\epsilon$ of this sample of 32
Galactic novae, which is devoid of any faint/fast types, is a hard upper limit
to the completeness for our M51 survey: $\epsilon=69$ percent.

###  5.2 Simulation II. M87 Novae

Next to our own Galaxy and M31, the massive elliptical galaxy M87 boasts the
largest sample of densely observed nova light curves, the product of a two-
month HST observing campaign (Shara et al., 2016). Due to the brevity of that
survey, approximately half of those light curves are too incomplete for our
simulation studies, but we were able to utilize $15$ M87 novae whose observed
light curves extend beyond $t_{2}$. This dataset provides another independent
test of the detection efficiency $\epsilon$ of the M51 survey, though
faint/slow M87 novae were almost certainly missed. As in the case of the
Galactic novae, this sample of M87 novae provides an independent upper limit
on our M51 survey’s detection efficiency.

We computed the absolute magnitudes for the M87 novae assuming a distance
$d=(16.4\pm 0.5)$ Mpc, corresponding to a distance modulus of $31.1$ mag (Bird
et al., 2010). Each of the selected light curves was then begun at
$t_{0}=[-20,-19,-18,\cdots,345]$ days and sampled with the HST observing
cadence. The final yield for the M87 novae was a set of $5,475$ simulated nova
light curves, which we then evaluated for detectability within the M51 search
parameters.

Figure 6 (center) shows the results of our M87-based nova detection efficiency
evaluations. With a mean of 34 percent, it falls between that of the Galactic
and M31 novae, though much closer to the latter (see below). We note that
although the M87 survey discovered a number of faint/fast novae (Shara et al.,
2017) which would have significantly reduced the overall mean $\epsilon$ of
the M51 survey, most of those novae were excluded from our simulations because
their light curves either dropped below the HST detection limit too quickly,
or were prematurely cut off during the decline period when the survey ended
(the latter also occurred for a few of the brighter novae). Thus, as noted
above, the $\epsilon=34$ percent result for the M87 nova sample, applied to
the M51 survey, is an upper limit.

###  5.3 Simulation III. M31 Novae

The most unbiased sample of novae available to us is that of M31. Unlike our
own Milky Way, where dust extinction and reddening severely hampers our
ability to detect distant novae in the plane of the Galactic disk, M31 affords
us a clearer view of its stellar population. Given that the distances of these
novae are all well determined and internal reddening is minimal, it is not
surprising that our nearest massive galactic neighbor has yielded a
significant number of “faint and fast” novae. As already noted, faint/fast
novae are challenging to detect in external galaxies because they are often
too faint to detect via ground-based observations, which are limited by poor
weather and seeing, bright moon, and easy-to-manage, “a few nights at a time”
block scheduling of telescopes.

For this study, we utilized 29 novae observed during two M31 surveys that are
among the most complete and unbiased available, because of their near-daily
observing cadences over extended periods of time (Arp, 1956; Kasliwal et al.,
2011). We also added 30 well-sampled M31 nova light curves observed by the
Zwicky Transient Facility (ZTF) between 2019-2021
(https://irsa.ipac.caltech.edu/cgi-bin/Gator/nph-scan?projshort=ZTF). We
excluded from consideration all novae in those surveys whose outburst peaks
may have been missed. This brought our M31 sample size to 59, our largest
extragalactic nova sample by far.

Absolute magnitudes for each of the M31 novae were calculated using a distance
modulus of $24.32$ mag (Wagner-Kaiser et al., 2015) and assuming a uniform
extinction $E(B-V)=0.062$ (Schlegel et al., 1998). Each light curve was
initiated at $t_{0}=[-20,-19,-18,\cdots,345]$ days and sampled with the HST
M51 observing cadence. As with the Galactic novae, interpolation was utilized
where small gaps in the light curves coincided with one of the M51 HST epochs.
We were thus able to simulate a set of $21,535$ realistic light curves using
the M31 novae. Once again, the simulated light curves were evaluated for
detectability within the M51 survey.

Figure 6: Detection efficiency $\epsilon$ for simulated light curves from
Galactic (left), M87 (center), and M31 (right) novae. See Sections 5.1–5.3 in
the text for details.

Figure 6 (right panel) shows the detection efficiency $\epsilon$ for the
simulated M31 novae; as before, the $M_{peak}$, $t_{2}$, and $\epsilon$ shown
in the plot correspond to the “parent” nova that was used to generate the
simulated light curves. Not surprisingly, many of the novae with brighter
$M_{peak}$ and longer $t_{2}$ still had higher overall detection rates than
their fainter/faster counterparts. At 20.5 percent, the mean $\epsilon$ for
the M31-based simulations was much lower than that of the Galactic-nova-based
simulations, as expected given the significant subset of faint/fast novae in
the less-biased nova sample of M31.

To quantify our completeness we carried out the following set of simulations.
In each trial, we chose one of the M31 nova at random, initiated its outburst
on a random day as described above, and analyzed its light curve to determine
whether that nova was detectable within the M51 survey, using our
$V_{606}$-band selection parameters. We continued selecting novae at random
and evaluating them for detectability until the number detected, $N_{obs}$,
reached 14. The“true” number of novae needed to reach $N_{obs}=14$ during each
trial was recorded as $N_{tr}$. These trials were repeated $10^{5}$ times,
yielding the distribution of $10^{5}$ $N_{tr}$ shown in Figure 7. The mean and
1-sigma ($\pm 34.1$ percent) widths of the $N_{tr}$ distribution are
$69^{+19}_{-15}$ novae, corresponding to a nova detection efficiency
$\epsilon=20.3^{+5.6}_{-4.3}$ percent.

The above value is in agreement with the mean detection rate of
$\epsilon=20.5$ percent for the full dataset of M31 simulations. The mean
$\epsilon$ values for the ZTF, Arp (1956), and Kasliwal et al. (2011) subsets
are also consistent, at 20, 22, and 18 percent, respectively.

Figure 7: Distribution of the “true” number of (randomly sampled) novae,
$N_{tr}$, required to recover the number of observed novae, $N_{obs}=14$, in
$10^{5}$ trials. The dashed line marks the mean value of $N_{tr}=69$ novae.
$\frac{N_{obs}}{<N_{tr}>}$ = 20.3 percent, which best approximates the
detection efficiency $\epsilon$ of our M51 nova survey if the nova populations
of M31 and M51 are not very dissimilar.

##  6 Discussion

###  6.1 Detection Efficiency and the True Nova Rate in M51

[b]  Simulation $<M_{peak}>$a $<t_{2}>$a Input Trials % Detected Reference
(Abs. mag) (days) Novae (Mean $\epsilon$) Synthetic -7.5 77 1071 370,566
$<65^{b}$ This work Galactic -7.7 22 32 11,680 $<69^{c}$ 1 M87 -7.9 16 15
5,475 $<34^{d}$ 2 M31 -7.1 22 59 21,535 $20.3^{+5.6}_{-4.3}$ 3, 4, 5

Table 2: Nova detection efficiency from simulations

  * a

Parameters listed are averaged values from the distributions of observed input
novae that the sets of simulated light curves were generated from.

  * b

This value is a strong upper limit since this simulation assigned equal weight
to all simulated novae, including those with simultaneously large $M_{peak}$
and $t_{2}$; no such novae are known to exist.

  * c

This value is a strong upper limit since Galactic surveys are strongly biased
against the detection of faint/fast novae (see Section 6.3.1 for more).

  * d

This value is an upper limit since the M87 survey was biased against the
detection of faint novae, and the fast novae that had been detected were
largely excluded from our sample (Section 5.2).

References (for “input” nova light curves): (1) Strope et al. (2010); (2)
Shara et al. (2016); (3) Arp (1956); (4) Kasliwal et al. (2011); (5) ZTF
(https://irsa.ipac.caltech.edu/cgi-bin/Gator/nph-scan?projshort=ZTF).

The simulations described in section 5 indicate that less than a quarter of
the novae that underwent an outburst in M51 during the HST M51 survey were
detected, assuming that the nova populations of M51 and M31 are similar. While
it might be tempting to simply lump all of the M31, M87 and Galactic novae of
Table 3 into a single simulation, we again emphasize that the most biased
sample is that of the Galactic novae, which miss almost all faint, fast novae.
Furthermore, as Figure 5 demonstrates, “fast” novae cannot be quantified by
$t_{2}$ alone, since the light curve decline rate preceding and following the
two-magnitude-decline point plays a much larger role in a nova’s
detectability. This light curve shape effect is evident in Figure 6, where
many of the M31 novae (right panel) have much lower detection efficiency than
their Galactic counterparts (left panel) within the same $M_{peak}$ – $t_{2}$
parameter space. That’s because the overall shape (especially the rapid
declines) of many M31 nova light curves makes them difficult to detect in our
own Galaxy. Theory and simulations (Yaron et al., 2005) predict that a similar
population of faint/fast Galactic novae should exist. These Galactic novae
have not yet been discovered because, at minimum, months-long, large area,
$\sim$ nightly infrared (to minimize reddening effects) surveys are required
for their detection. Lumping the M87, M31 and Galactic novae underestimates
the number of faint fast novae and overestimates our detection efficiency in
M51. We adopt the detection efficiency $\epsilon$ yielded by our M31
simulations because we consider this estimate to be the most robust, given the
relative size and lack of observational bias in that sample.

Figure 8 shows that the $K$-band light distribution in M51 (which follows the
distribution of red giants with masses $\sim$ 1 M⊙) is in good agreement with
the spatial distribution of novae in M51 (see Figure 3 for the area included
in this analysis). This is consistent with the distribution of novae found in
the massive elliptical galaxy M87, which was also shown to follow the $K$-band
light (Shara et al., 2016).

Figure 8: Luminosity profile of M51 in K-band (black), I-band (red), and
V-band (blue) light. The cumulative distribution of nova candidates we
identified in our survey is shown in grey. Note that this plot does not show
all of M51’s light at these radii; it includes the fraction of the galaxy
light (and detected novae) that overlap with this survey’s HST -observed area.

As Figure 3 shows, our photometric catalog covered a region that encompasses
$\sim 40$ percent of the $K$-band light from M51. To calculate this fraction,
we first determined the amount of light in the area that consistently remained
within the field of view of the HST survey, a circle of radius $\sim 102$
arcsec. We then subtracted the inner $10$ arcsec, which was excluded from our
photometric catalog because its high surface brightness and stellar density
presents a challenge for typical photometry tools. We computed the fraction of
M51 light included in our study under the assumption that the total light from
M51 is encompassed within a circle of radius $\sim 220$ arcsec. We used a
2MASS K-band image to get the photon counts within the specified areas,
yielding a fraction of $\sim 40$ percent of the M51 $K$-band light covered by
this survey.

We now have in place the three key values needed to derive the M51 nova rate.
These are:

  * •

the fraction of M51’s light consistently observed in the HST campaign (Figure
3), 40 percent;

  * •

the determination that the novae “follow the light” in M51 (Figure 8); and

  * •

the nova detection efficiency for this HST campaign, $\sim 20.3^{+5.6}_{-4.3}$
percent.

Then, under the assumption that the underlying CV population in M51 resembles
that of M31, we conclude that the intrinsic nova rate in M51 is
$172^{+46}_{-37}$ novae yr-1. This M51 nova rate is nearly an order of
magnitude higher than previously claimed (Shafter et al., 2000).

###  6.2 Nova Rates and Distributions in Different Galaxy Types

Although binary population synthesis models have predicted that nova rates
($\nu$, normalized by K-band luminosity $L_{K}$) should vary with Hubble type
(Matteucci et al., 2003; Claeys et al., 2014; Chen et al., 2016), earlier
ground-based surveys found little evidence for this (Shafter et al., 2000).
Given that this survey confirms what several others have already suggested –
that ground-based surveys tend to miss a large fraction of nova outbursts
(Shara et al., 2016; Mróz et al., 2016) – it is now time to reevaluate the
evidence.

Assuming a value of $L_{K}=16.6\times 10^{10}L_{\odot,K}$ for M51 (Shafter et
al., 2000) and a rate of $172^{+46}_{-37}$ novae yr-1, we find a luminosity-
specific nova rate (LSNR) of $\sim 10.4^{+2.8}_{-2.2}$ novae
yr-1/$10^{10}L_{\odot,K}$ for M51. Both rates are nearly an order of magnitude
higher than the ground-based values from Shafter et al. (2000). Remarkably,
Shara et al. (2016) found a luminosity-specific nova rate of
$7.88^{+2.3}_{-2.6}$ novae yr-1/$10^{10}L_{\odot,K}$ for M87, within the error
range of our HST -derived rate for M51. The similarity of these values is
inconsistent with Chen et al. (2016), who predicted an order-of-magnitude-
higher LSNR value for M51-like galaxies than for M87-like elliptical galaxies.
The best evidence now in hand demonstrates that the difference between the
LSNRs of M87 and M51 may be much smaller than that predicted. Note that the
M87 nova rate was conservatively calculated, assuming no incompleteness (Shara
et al., 2016). Applying our incompleteness analysis to the M87 HST survey may
well yield an even higher nova rate for that galaxy – even closer to the LSNR
we observe for M51.

Figure 9: Luminosity-specific nova rates for an array of galaxies. All values
are from Della Valle & Izzo (2020) (table 7), with the exception of M83 (which
was taken from Shafter et al. (2021)), and the two galaxies with HST -based
nova rates: M51 and M87. For the former, we adopted a K-band luminosity of
$L_{K}=16.6\times 10^{10}L_{\odot,K}$ (Shafter et al., 2000). The larger black
arrow highlights the difference between the previously published LSNR and our
newly determined rate for M51 (diamond marker). The smaller arrow denotes the
increase in LSNR for M87 established via the high-cadence HST survey by Shara
et al. (2016), compared to earlier values calculated by Shafter et al. (2000)
and Curtin et al. (2015) through ground-based studies.

Figure 9 shows the published luminosity-specific nova rates for 13 different
galaxies. While our newly extrapolated rate for M51 appears to be higher than
those of the 12 other galaxies, this may not in fact be the case, as none of
the other 12 nova rates were derived using our detailed detectability
analysis. Comparable studies of other galaxies may well yield equally high
luminosity-specific nova rates by improving incompleteness corrections.
Similarly, high-cadence surveys will also increase nova rates universally,
because more frequent observations lower incompleteness. Such is the case for
M87 and the LMC, whose LSNRs are already higher than the 10 remaining
galaxies’. Unlike most of the rates shown in Figure 9, the LSNRs for these two
galaxies were extrapolated from surveys with near-daily cadence. Hence they
can be considered far more reliable than their sparsely surveyed counterparts
– particularly with respect to faint/fast novae.

###  6.3 Additional Uncertainties

The following is a discussion of the various uncertainties in our analysis,
related to a) the selection of novae, and b) the detection parameters, that
were used for the simulations described in Section 5.

####  6.3.1 Nova population differences between galaxies

Our analysis relies on several assumptions which are not definitive and which
impact our final results. Chief among them is the assumption that the nova
population in M51 resembles that of M31. As noted above, and shown in Table 2,
assuming instead a nova population like that in our Galactic sample would
result in a significantly higher detection efficacy for this survey, and
consequently, a lower derived nova rate estimate. We repeat that the nova rate
determined using the M31 dataset is almost certainly more reliable, given that
the M31 sample a) includes faint-fast novae which are conspicuously absent
from the Galactic sample, b) is compiled from unbiased333We sourced the M31
novae from surveys selected for their high cadence and long duration. Combined
with the relatively (compared to our Galaxy) low variation in reddening and
accurate distance to M31, which provided us with well-constrained absolute
magnitudes, this resulted in the most unbiased sample available, as detailed
in Section 5.3. surveys, and c) boasts a much larger sample size.

The survey efficiency results of the simulation based on M87 novae are more
consistent with those derived from the M31 dataset than its Galactic
counterpart. However, as Table 2 shows, that sample is biased towards the
brighter novae. The larger distance to M87, combined with the relatively short
duration of the M87 HST survey, hampered the detection of well-sampled novae
of longer duration and lower $M_{peak}$. We also note that as a massive
elliptical galaxy with an overall older stellar population, the nova
distribution in M87 may be different from that of its spiral counterparts…
which is why we did not simply lump together the M31 and M87 nova samples.

Figure 10: $M_{peak}$ distributions for Galactic (pink), M31 (violet), M87
(blue), and M51 (grey) novae. For M51, all $M_{peak}$ values are lower limits
as the outburst “peaks” were likely missed for most novae due to the survey
observing cadence. The dotted lines denote median values.  Figure 11:
Cumulative $t_{2}$ distributions for Galactic (pink), M31 (violet), and M87
(blue) novae used in the simulations described in Section 5. A larger sample
of Galactic novae, including 40 whose distances are not well constrained, is
shown in black. Dotted lines denote the median values. The inset shows the
distributions in log scale.

Figure 10 shows the distributions of $M_{peak}$ for the Milky Way (32 novae),
M31 (59 novae), M87 (15 novae), and M51 (14 novae) sample light curves. Due to
the large gaps between the HST observations of M51, we cannot determine the
actual peak luminosities of those novae; we can only determine their lower
limits. The fainter “tail” of the M87 nova distribution is deficient. We
attribute this to observational selection, i.e. the difficulty in detecting
such faint novae, even with HST , at the distance of M87. In addition, the
relatively short duration of that survey prevented the detection of slowly-
declining novae, which have lower $M_{peak}$ . The higher-$M_{peak}$ tail of
M51 is missing as well – again, not surprisingly – as one might expect from a
survey with (on average) 10 day gaps, missing most luminous, fast novae near
maximum luminosity.

Figure 10 also appears to show a dearth of Galactic novae with $M_{peak}$
fainter than $\sim-6.75$. This is at least partly due to a selection bias that
favors the detection of brighter Galactic novae, as described in Section 5.1.
Note that Özdönmez et al. (2018) and Shafter (2017) find a mean $M_{peak}$ of
$-7.2$ for a subset of Galactic novae. In the case of the former, novae with
well-determined distances were selected; for the latter, it was predominantly
novae from the Galactic bulge. Since our own selection criteria were stricter
(requiring well-determined distances and well-sampled light curves), it is not
surprising that the mean $M_{peak}$ for our sample is higher (Table 2). This
further supports our claim that the resulting detection efficiency $\epsilon$
for our limited Galactic sample is an overestimate. In contrast, the mean
$M_{peak}$ for our M31 sample is consistent with not only the abovementioned
estimates for larger subsets of Galactic novae, but also the entire dataset of
observed novae in M31 (Özdönmez et al., 2018; Shafter, 2017) – and not just
the limited subset of well-sampled novae from unbiased surveys included in our
simulation. This consistency further supports our adaptation of the $\epsilon$
value yielded by the M31 simulations.

Perhaps even more telling are the distributions of $t_{2}$ in the different
galaxies. Unlike the absolute magnitudes, $t_{2}$ values are well constrained
for 72 Galactic novae, since the latter can be determined from apparent
magnitudes, even when the distance to the source is unknown. As Figure 11
shows, the overall distribution of $t_{2}$ for the novae in our M31 dataset is
similar to that of the Galactic novae. This is true both for the larger
distribution of 72 novae and for the subset of 32 novae (with well-constrained
distance estimates) that we utilized in the simulations described in Section
5.1. This further indicates that our M31 sample is a good representation of
the typical nova population within a barred spiral galaxy.

####  6.3.2 Visual identification (or mis-identification) of nova light curves

Of the approximately 1,000 preliminary nova candidates selected for visual
inspection from the full M51 HST dataset, only 14 light curves were clearly
those of transients. Given the appearance of many of the simulated light
curves (described in Section 5), which were based on observed nova samples
that included a broader variety of shapes, we have good reason to believe that
a significant number of novae embedded in the M51 data went unidentified,
because, upon review, their light curves did not resemble the canonical nova
decline shapes (Section 3). We applied a similar visual test to a subset of
the simulated light curves that were marked as “detected” by our nova
selection parameters, and found that as many as half of those light curves do
not pass the by-eye inspection. This indicates that our survey detection
efficiency could be overestimated by a factor of two – and consequently, that
the intrinsic M51 nova rate could well be double our estimate of
$172^{+46}_{-37}$ novae yr-1. Out of an abundance of caution, we decided
against incorporating this aspect of our analysis into our results. But we
note that the M51 nova rate of $172^{+46}_{-37}$ novae yr-1 is a conservative
determination.

Another factor to consider is that the 14 nova candidates we identified
haven’t been spectroscopically confirmed. In the unlikely event that a small
subset of them are not actually novae, the extrapolated nova rate for M51
would be reduced. For example, if two of the 14 sources are not in fact novae,
the rate would be decreased to $\sim 148$ novae yr-1 – which is still far
higher than the previously published rate of 18 novae yr-1 (Shafter et al.,
2000). However, we consider this scenario highly improbable; of the two
classes of variable massive stars that can reach luminosities equivalent to
novae, Mira variables are far redder than our nova candidates, and luminous
blue variables typically rise in luminosity over months to years, not days, as
is the case for our sources.

####  6.3.3 Color limitations

Among the crucial selection parameters used to winnow nova candidates from the
enormous photometric dataset of M51 was the color near maximum light. Most
novae exhibit a “blue” color near maximum light of $V-I<0.50$ mag, but this
property is by no means universal. Since we lacked sufficient “color” data for
most of the novae in our simulation samples, we did not employ this selection
criterion for the simulated set. Therefore, the detection efficiency vis-à-vis
the actual M51 HST data may be – once again – overestimated, and our derived
nova rate consequently underestimated.

####  6.3.4 Under- (or over-) representation of faint/fast novae in simulated
samples

The number of observed faint/fast nova light curves available to us is rather
small. Given the difficulty in detecting such novae, their intrinsic ubiquity
is nearly impossible to reliably determine from current data. In our analysis,
we assumed that the novae contained in our M31 dataset constitute a
representative sample, although faint/fast novae are likely underrepresented.
We also cannot rule out that such faint/fast novae do not commonly occur in
other galaxies, including M51 – though we consider it far more likely that
their detection in M31 and M87 was a result of superior detection efficiency
in those surveys (compared to Galactic ones).

####  6.3.5 Magnitude uncertainties

Our analysis assumed that extinction internal to M51 is negligible and
approximately uniform. Similarly, for the M31 novae used in our simulations,
we did not account for possible differential extinction. This could introduce
additional errors in magnitude that were not incorporated into our analysis.
However, we found that arbitrarily making all M31 light curves one magnitude
fainter had no significant effect on the mean detection efficiency for the M31
simulations. This indicates that internal extinction effects are likely
negligible.

We also note that the novae in our M31 dataset were observed in similar but
not identical filters as the M51 HST novae. As noted above, varying the
luminosity of our sample novae by up to one magnitude had no discernible
effect on their overall detectability, so a slight shift in filter wavelength
should not significantly influence our results.

##  7 Conclusions

We conducted a study of the nova rate in M51 using a year-long HST survey and
realistic simulations to thoroughly test the nova detection efficiency
$\epsilon$ of that survey. Our incompleteness simulations modelled the
detectability of well-observed M31 novae with unprecedented detail. This
allowed us to extrapolate M51’s intrinsic nova rate with an unparalleled
degree of accuracy – under the assumption that the nova population in M51
resembles that of M31. We find that the nova rate in M51 is $\approx
172^{+46}_{-37}$ novae yr-1, and its luminosity-specific nova rate is $\sim
10.4^{+2.8}_{-2.2}$ novae yr${}^{-1}/10^{10}L_{\odot,K}$. Both these rates are
nearly an order of magnitude higher than the previous published value based on
ground-based observations (Shafter et al., 2000). In contradiction to
theoretical predictions of order-of-magnitude differences in LSNRs between
elliptical and spiral galaxies, M51 and M87 appear to display $\sim$
comparable LSNRs. The novae in M51 closely follow the $K$-band light
distribution in that galaxy, similar to the novae of M87.

## Acknowledgements

This work was supported by the National Science Foundation Graduate Research
Fellowship Program under Grant No. DGE 2036197. Any opinions, findings, and
conclusions or recommendations expressed in this material are those of the
authors and do not necessarily reflect the views of the National Science
Foundation. CC and PvD acknowledge support from NASA Grant HST-GO-14704.001.
We would like to thank Jay Strader for his valuable critiques and suggestions,
which helped to significantly improve an earlier draft of this paper.

facilities: HST(ACS), AAVSO, ZTF

## Data Availability

The M51 photometry and simulation data underlying this article will be shared
on reasonable request to the first author. The other photometry data are
publicly available from the references provided in Table 2.

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##  Appendix A Novae used in simulations

Some of the Galactic novae listed in this table have two or more values of
$M_{peak}$ and $t_{2}$ reported in the literature. Where available, we took
$M_{peak}$ from the AAVSO data compiled by Strope et al. (2010), provided that
the sampling was $\sim$ daily around the clearly observed peak. We define
$t_{2}$ as the time it took for the nova to decline from $M_{peak}$ by two
magnitudes. Given that the nova luminosity can change by as much as $1-2$ mag
over a timescale of hours, it is not surprising that near-simultaneous
observations yielded different values. For consistency, and to reflect the
fact that the HST M51 epochs were $2.2$ ksec each (and therefore could have
easily missed the absolute outburst “peak” even if the observation occurred on
the day of peak brightness), we utilized the AAVSO values even when a higher
$M_{peak}$ was reported elsewhere.

Table 3: Novae Used in Simulations. Nova |  $m_{peak}$ |  $t_{2}$ |  Filter
---|---|---|---
|  (mag) |  (days) |
M31 Novae
M31 (Arp ’56) 1 |  16.43 |  3.0 |  $m_{pg}$
M31 (Arp ’56) 2 |  16.43 |  2.5 |  $m_{pg}$
M31 (Arp ’56) 4 |  18.15 |  10.0 |  $m_{pg}$
M31 (Arp ’56) 5 |  15.99 |  12.0 |  $m_{pg}$
M31 (Arp ’56) 6 |  16.37 |  9.9 |  $m_{pg}$
M31 (Arp ’56) 7 |  16.01 |  14.0 |  $m_{pg}$
M31 (Arp ’56) 8 |  17.0 |  9.0 |  $m_{pg}$
M31 (Arp ’56) 10 |  17.04 |  5.3 |  $m_{pg}$
M31 (Arp ’56) 12 |  16.0 |  13.0 |  $m_{pg}$
M31 (Arp ’56) 13 |  17.04 |  29.0 |  $m_{pg}$
M31 (Arp ’56) 15 |  16.9 |  15.0 |  $m_{pg}$
M31 (Arp ’56) 16 |  17.26 |  12.0 |  $m_{pg}$
M31 (Arp ’56) 17 |  17.2 |  24.0 |  $m_{pg}$
M31 (Arp ’56) 19 |  17.72 |  31.0 |  $m_{pg}$
M31 (Arp ’56) 20 |  17.19 |  17.0 |  $m_{pg}$
M31 (Arp ’56) 21 |  17.4 |  20.0 |  $m_{pg}$
M31 (Arp ’56) 23 |  17.38 |  28.0 |  $m_{pg}$
M31 (Arp ’56) 24 |  17.81 |  23.0 |  $m_{pg}$
M31 (Arp ’56) 25 |  17.56 |  3.0 |  $m_{pg}$
M31 (Arp ’56) 26 |  18.0 |  29.0 |  $m_{pg}$
M31 (Arp ’56) 30 |  18.02 |  53.9 |  $m_{pg}$
M31N 2007-10a |  17.55 |  9.0 |  g
M31N 2008-07b |  18.9 |  14.4 |  g
M31N 2008-08c |  17.1 |  29.0 |  g
M31N 2008-09a |  17.8 |  21.0 |  g
M31N 2008-09c |  16.8 |  13.0 |  g
M31N 2008-10b |  18.1 |  5.0 |  g
M31N 2008-11a |  18.2 |  8.0 |  g
M31N 2008-12b |  17.05 |  24.0 |  g
ZTF21aagkzve |  17.26 |  9.0 |  ZTF-g
ZTF21aagkzve |  17.09 |  15.0 |  ZTF-r
ZTF21acbcfmh |  16.53 |  8.9 |  ZTF-g
ZTF21acbcfmh |  16.35 |  15.0 |  ZTF-r
ZTF21abjiotr |  19.25 |  – |  ZTF-g
ZTF21abjiotr |  18.39 |  30.0 |  ZTF-r
ZTF20acstbfh |  17.68 |  3.0 |  ZTF-g
ZTF20acplkub |  18.89 |  – |  ZTF-g
ZTF20acplkub |  18.31 |  7.9 |  ZTF-r
ZTF20acoqrpm |  16.81 |  11.0 |  ZTF-g
ZTF20acgigfo |  17.6 |  9.0 |  ZTF-g
ZTF20acgigfo |  17.0 |  12.9 |  ZTF-r
ZTF20acfucwr |  16.95 |  14.9 |  ZTF-g
ZTF20acfucwr |  17.02 |  18.0 |  ZTF-r
ZTF20abqhsxb |  17.81 |  12.0 |  ZTF-r
ZTF19acxrihd |  18.11 |  35.0 |  ZTF-g
ZTF19acxrihd |  17.97 |  58.0 |  ZTF-r
ZTF19adakuos |  17.53 |  28.0 |  ZTF-g
ZTF19adakuos |  17.39 |  19.0 |  ZTF-r
ZTF19acqprad |  18.51 |  40.0 |  ZTF-g
ZTF19acqprad |  18.36 |  59.0 |  ZTF-r
ZTF19acnfsij |  18.33 |  30.8 |  ZTF-r
ZTF19acgfhfd |  17.11 |  30.0 |  ZTF-g
ZTF19acgfhfd |  16.82 |  19.0 |  ZTF-r
ZTF19acfsteg |  16.72 |  14.0 |  ZTF-g
ZTF19acfsteg |  16.6 |  44.0 |  ZTF-r
ZTF19abirmkt |  18.12 |  – |  ZTF-g
ZTF19abirmkt |  17.91 |  170.0 |  ZTF-r
ZTF19abfvpjh |  17.31 |  10.0 |  ZTF-g
ZTF19abfvpjh |  16.87 |  12.9 |  ZTF-r
M87 Novae
1 |  21.84 |  16.0 |  V
2 |  22.25 |  10.0 |  V
3 |  22.64 |  2.5 |  V
4 |  22.74 |  8.0 |  V
5 |  22.99 |  9.0 |  V
6 |  23.18 |  14.0 |  V
7 |  23.21 |  8.0 |  V
9 |  23.42 |  15.2 |  V
10 |  23.51 |  18.0 |  V
11 |  23.57 |  31.0 |  V
12 |  23.58 |  33.8 |  V
15 |  23.75 |  22.0 |  V
19 |  23.9 |  9.5 |  V
20 |  23.94 |  39.8 |  V
23 |  24.16 |  4.8 |  V
Galactic Novae
V356 Aql |  6.97 |  37.0 |  V
V603 Aql |  -0.5 |  5.8 |  V
V1370 Aql |  7.7 |  9.8 |  V
V1494 Aql |  4.11 |  8.5 |  V
V705 Cas |  5.69 |  33.0 |  V
V723 Cas |  7.08 |  17.0 |  V
V842 Cen |  4.9 |  29.5 |  V
V476 Cyg |  1.94 |  7.2 |  V
V1330 Cyg |  9.9 |  119.0 |  V
V1974 Cyg |  4.27 |  16.2 |  V
HR Del |  3.58 |  68.0 |  V
DN Gem |  3.58 |  13.0 |  V
DQ Her |  1.56 |  40.0 |  V
V446 Her |  4.83 |  17.7 |  V
V533 Her |  3.0 |  20.0 |  V
CP Lac |  2.02 |  6.0 |  V
DK Lac |  5.9 |  18.0 |  V
IM Nor |  7.84 |  26.0 |  V
RS Oph |  4.94 |  6.8 |  V
GK Per |  0.19 |  7.0 |  V
RR Pic |  0.95 |  14.0 |  V
CP Pup |  0.7 |  4.8 |  V
T Pyx |  6.77 |  45.0 |  V
V3890 Sgr |  8.05 |  5.2 |  V
U Sco |  7.7 |  2.5 |  V
V992 Sco |  7.7 |  10.0 |  V
FH Ser |  4.5 |  46.2 |  V
V382 Vel |  2.77 |  6.5 |  V
NQ Vul |  6.19 |  2.5 |  V
PW Vul |  6.41 |  3.8 |  V
QU Vul |  5.33 |  20.8 |  V
QV Vul |  7.13 |  38.0 |  V

##  Appendix B Novae in Difference Images (“Postage Stamps”)

Figure 12 shows the 14 novae that we identified in M51, as they appeared in
each observing epoch. To create these images, we first aligned the four
dithered ACS images from each epoch using the TweakReg WCS alignment task and
drizzled them together with DrizzlePac, following the process described in
Hoffmann et al. (2021). Separately, we created a composite image of M51 by
“drizzling” together the individual images from all 34 epochs, again following
the procedure described in Hoffmann et al. (2021). We then subtracted the
composite image from each of the 34 individual drizzled images, creating the
difference images shown below. This process allows us to clearly observe the
appearance and decline of the novae. Each “postage stamps” cutout is 1 arcsec
x 1 arcsec in size. Images corresponding to the epochs of observed peak
luminosity are marked with cyan ticks.

(a) Nova 1

(b) Nova 2

(c) Nova 3

(d) Nova 4

(e) Nova 5

(f) Nova 6

(g) Nova 7

(h) Nova 8

(i) Nova 9

(j) Nova 10

(k) Nova 11

(l) Nova 12

(m) Nova 13

(n) Nova 14

Figure 12: $1\arcsec\times 1\arcsec$ $V_{606}$ difference image cutouts of the
14 nova candidates in M51 for the 34 epochs of the HST observing campaign.
Images corresponding to the epochs of observed peak luminosity are marked with
cyan ticks.