# The Breakthrough Listen Search for Intelligent Life:
Technosignature Search of Transiting TESS Targets of Interest

Noah Franz Department of Astronomy, University of California, Berkeley, 501
Campbell Hall 3411, Berkeley, CA, 94720, USA  Department of Physics and
Astronomy, Siena College, 515 Loudon Rd, Loudonville, NY 12211, USA  Steve
Croft Department of Astronomy, University of California, Berkeley, 501
Campbell Hall 3411, Berkeley, CA, 94720, USA  SETI Institute, Mountain View,
California  Andrew P. V. Siemion Department of Astronomy, University of
California, Berkeley, 501 Campbell Hall 3411, Berkeley, CA, 94720, USA  SETI
Institute, Mountain View, California  University of Malta, Institute of Space
Sciences and Astronomy  Raffy Traas  Department of Astronomy, University of
California, Berkeley, 501 Campbell Hall 3411, Berkeley, CA, 94720, USA
Department of Physics, University of Wisconsin - La Crosse, 1725 State Street,
La Crosse, WI 54601, USA  Bryan Brzycki Department of Astronomy, University of
California, Berkeley, 501 Campbell Hall 3411, Berkeley, CA, 94720, USA  Vishal
Gajjar Department of Astronomy, University of California, Berkeley, 501
Campbell Hall 3411, Berkeley, CA, 94720, USA  Howard Isaacson Department of
Astronomy, University of California, Berkeley, 501 Campbell Hall 3411,
Berkeley, CA, 94720, USA  University of Southern Queensland, Toowoomba, QLD
4350, Australia  Matt Lebofsky Department of Astronomy, University of
California, Berkeley, 501 Campbell Hall 3411, Berkeley, CA, 94720, USA  David
H. E. MacMahon  Department of Astronomy, University of California, Berkeley,
501 Campbell Hall 3411, Berkeley, CA, 94720, USA  Danny C. Price Department of
Astronomy, University of California, Berkeley, 501 Campbell Hall 3411,
Berkeley, CA, 94720, USA  International Centre for Radio Astronomy Research,
Curtin University, Bentley, WA 6102, Australia  Sofia Z. Sheikh Department of
Astronomy, University of California, Berkeley, 501 Campbell Hall 3411,
Berkeley, CA, 94720, USA  Julia De Marines  Department of Astronomy,
University of California, Berkeley, 501 Campbell Hall 3411, Berkeley, CA,
94720, USA  Jamie Drew  The Breakthrough Initiatives, NASA Research Park, Bld.
18, Moffett Field, CA, 94035, USA  S. Pete Worden  The Breakthrough
Initiatives, NASA Research Park, Bld. 18, Moffett Field, CA, 94035, USA

###### Abstract

The Breakthrough Listen Initiative, as part of its larger mission, is
performing the most thorough technosignature search of nearby stars.
Additionally, Breakthrough Listen is collaborating with scientists working on
NASA’s Transiting Exoplanet Survey Satellite (TESS), to examine TESS Targets
of Interest (TOIs) for technosignatures. Here, we present a $1-11$ GHz radio
technosignature search of 61 TESS TOIs that were in transit during their
Breakthrough Listen observation at the Robert C. Byrd Green Bank Telescope. We
performed a narrowband Doppler drift search with a minimum S/N threshold of
10, across a drift rate range of $\pm 4$ Hz s-1, with a resolution of 3 Hz. We
removed radio frequency interference by comparing signals across cadences of
target sources. After interference removal, there are no remaining events in
our survey, and therefore no technosignature signals-of-interest detected in
this work. This null result implies that at L, S, C, and X bands, fewer than
52%, 20%, 16%, and 15%, respectively, of TESS TOIs possess a transmitter with
an equivalent isotropic radiated power greater than a few times
$10^{14}\,\textrm{W}$.

technosignatures — search for extraterrestrial intelligence — radio astronomy
— exoplanets

##  1 Introduction

The Search for Extraterrestrial Intelligence (SETI) seeks an answer to the
age-old question: Are we alone in the universe? The modern search for
technosignatures, or signs of intelligent extraterrestrial life, began in the
1960s (Drake, 1961). Due to the limited technology available at the time, this
search was restricted to 1420 MHz, which was hypothesized to be a good
candidate for a universal communication frequency. However, as technology has
developed, technosignature searches have become much more advanced and can
cover much wider bandwidths and larger numbers of targets.

The Breakthrough Listen (BL) Initiative, launched in 2015, will search over 1
million targets for technosignatures over its 10-year lifetime (Worden et al.,
2017). BL operates at optical and radio wavelengths, using a wide variety of
telescopes including the Robert C. Byrd Green Bank Telescope (GBT) in West
Virginia, the Automated Planet Finder (APF) in California, and the CSIRO
Parkes ‘Murriyang’ 64-m radio telescope in Australia. This work presents a
technosignature search of the frequency range $1-11$ GHz using the GBT. The BL
backend on the GBT is capable of simultaneously delivering billions of
frequency channels across several GHz of bandwidth. MacMahon et al. (2018) and
Lebofsky et al. (2019) provide information about the instrument, data formats,
and post-observation data management.

Figure 1: Sky map of all TESS TOIs (blue dots), with transiting TESS targets
analyzed in this paper overlaid as red x’s. The grey shaded region is the zone
above declination $-20\arcdeg$ where BL targets are usually observed with the
GBT; BL targets below this declination are usually observed at the Parkes
Observatory.

BL employs a variety of strategies for target prioritization. One is to select
targets from catalogs compiled by NASA’s Transiting Exoplanet Survey Satellite
(TESS). As of 2021 June, TESS has found 4,190 new exoplanets, including
confirmed exoplanets and candidates, some of which may have suitable
conditions for life. Traas et al. (2021) recently performed a technosignature
search of 28 TESS targets of interest (TOIs) using the L, S, C, and X-band
receivers at the GBT. Transiting systems are prioritized because Earth is in
the ecliptic for these systems. An ETI may be more likely to send bright
signals out in the direction of their ecliptic, either to intentionally signal
observers who can see their transits, or for purposes such as interplanetary
radar (Traas et al., 2021).

We refine the search of Traas et al. (2021) by selecting systems that were
observed with the GBT during transits of candidate exoplanets, which may
further improve the chance of receiving an extraterrestrial signal. An ETI may
choose to broadcast signals towards their anti-stellar point, knowing that
observers may be monitoring their system during a transit, so there is a
higher likelihood of a transmission being received. In addition, by choosing
to broadcast at this special temporal “Schelling Point” (Sheikh et al., 2020;
Wright et al., 2018; Gajjar et al., 2021), an ETI could enhance signal
detectability for a given transmitter power (relative to an omnidirectional
transmitter) by increasing their antenna gain and beaming a signal in the
opposite direction to their star.

##  2 Observations

BL targets at GBT are observed with an “on/off” ABACAD cadence method
(Lebofsky et al., 2019). The primary target A, is observed, then an “off”
target B, is observed. This method is then repeated twice more with the same
“on” target and two new “off” targets, C and D. Each target in the cadence is
observed for 5 minutes such that the “on” target is observed for a total of 15
minutes and each “off” target is observed for 5 minutes. Comparing the “on”
and “off” scans allows us to differentiate between radio frequency
interference (RFI) signals and a candidate ETI signal, since the latter is
expected to be localized on the sky.

###  2.1 Target Selection

Figure 2: Fraction of the transit observed in the GBT observations. The blue
bins represent all 66 cadences, while the hashed bins indicate a target that
crosses the midpoint of its transit during the observation. Figure 3:
Histogram of the orbital periods of the observed targets.

Observations of TOIs by BL at the GBT are scheduled automatically by selection
from target lists, and not typically deliberately timed to coincide with
transits. By examining ephemerides from ExoFOP-TESS (ExoFOP, 2019) for all
targets observed by BL at GBT as of 2021 June, we determined111Code at:
https://github.com/noahfranz13/BL-TESSsearch that 61 unique targets, across 66
observations, serendipitously transit during their GBT observation. These 61
targets are shown in Figure 1 and Appendix A. TIC 344926234 and TIC 365683032
were observed with two different receivers during two different transits, TIC
376637093 was observed with three different receivers during three different
transits, and TIC 286561122 was observed at C-Band twice during a single
transit. The notch filter regions (Lebofsky et al., 2019) at L ($1200-1340$
MHz) and S ($2300-2360$ MHz) bands are excluded from our analysis.

A histogram of the fraction of each transit observed is shown in Figure 2. The
fraction of transit observed was calculated by dividing the observation time
of the entire cadence by the total transit time of the exoplanet candidate,

$\textrm{FTO}=\frac{t_{\textrm{obs,~{}transit}}}{(t_{\textrm{egress}}-t_{\textrm{ingress}})},$
(1)

where FTO stands for the Fraction of Transit Observed,
$t_{\textrm{obs,~{}transit}}$ is the amount of time in the overlap of the
transit time and observation time, and $t_{\textrm{egress}}$ and
$t_{\textrm{ingress}}$ are the time of egress and ingress, respectively.
Targets that cross the midpoint of their transit, as shown by the hashed bins
in Figure 2, are especially interesting: a narrow-beamed transmitter pointing
away from the host star, perhaps located at the second Lagrange point, would
appear strongest at the midpoint of transit.

Figure 3 shows a histogram of the orbital periods of the TESS TOIs chosen for
this project. These periods are all relatively short, so the TOIs are unlikely
to be terrestrial planets in the habitable zone. Still, ETI may assume it is
easier for us to detect these closer, short period, exoplanets and place a
transmitter there.

Table 1: Survey Parameters

Receiver | Frequency | Cadences | Hits | Events | CWTFMa |  $10\sigma$ EIRP${}_{\textrm{min}}$ | Transmitter
---|---|---|---|---|---|---|---
| [GHz] |  |  |  |  | [TW]b | Limit [%]c
L | 1.10 - 1.90 | 5 | 213097 | 172 | 3793 | 167 | 52
S | 1.80 - 2.80 | 17 | 160057 | 33 | 2828 | 393 | 20
C | 4.00 - 7.80 | 21 | 578264 | 57 | 3060 | 788 | 16
X | 7.80 - 11.20 | 23 | 1503241 | 372 | 3686 | 719 | 15
Total | 1.10 - 11.20 | 66 | 2442347 | 634 | - | - | -

  * a

Continuous Waveform Transmitter Figure of Merit (CWTFM) is a figure of merit
that describes the likelihood to find a signal above the
EIRP${}_{\textrm{min}}$ for that receiver.

  * b

Minimum Equivalent Isotropic Radiated Power (EIRP${}_{\textrm{min}}$) is a
measure of the minimum necessary omnidirectional power of a transmitter at
each receiver to be detected.

  * c

The transmitter limit is the maximum percentage of exoplanet candidates in
each frequency range that possess a transmitter.

##  3 Doppler Search

We perform our analysis on fine-frequency resolution spectrograms from the BL
backend at the GBT. As described by Lebofsky et al. (2019) and MacMahon et al.
(2018), the BL backend records spectral data in 187.5 MHz frequency chunks,
with each chunk sent to a separate compute node. Data recorded before early
2021 were spliced together in frequency, one file per receiver, before
archiving. Starting in early 2021, files were instead left in their unspliced
form on the compute nodes, which enables easier parallel processing. The 66
cadences analyzed here represent 21 TB of data in total, most of which were
analyzed in situ on the GBT BL compute nodes. In one observation in our
sample, TIC 365781372 at X-band, the blc40 compute node failed to record data
during a scan, leading to a gap of 187.5 MHz in the spectrum.

Each cadence was analyzed using the BL turboSETI pipeline (Enriquez & Price,
2019). First, FindDoppler identifies narrow-band Doppler-drifting signals in
the filterbank files. Following from Price et al. (2020) and Traas et al.
(2021), we adopt a minimum222turboSETI’s dechirping efficiency is lower for
high drift rate signals, resulting in a higher effective S/N limit. For more
details see Gajjar et al. (2021).  S/N threshold of 10, across a drift rate
range of $\pm 4$ Hz s-1. To maximize efficiency, we parallelized the
processing across all 64 compute nodes available to BL at GBT, greatly
reducing runtime for large amounts of data.

We use the measured orbital periods for our TOIs, applying the methods
presented by Sheikh et al. (2019), to calculate theoretical maximum drift
rates for transmitters in the systems in our sample. We neglect any
contribution from the rotation rates of the planets (which are unknown, but in
many cases may be negligible, since many of our targets have small periods and
are most likely tidally locked). We find that only 2.4% of our targets have
maximum drift rates that lie within $\pm 4$ Hz s-1, suggesting that a search
over a larger drift rate range would be optimal, albeit more computationally
expensive. However, it would be simple (and maybe even common) for ETI to
correct for their drift rate when transmitting a signal, so received signals
would only have small drift rates due to Earth’s orbit and rotation (Sheikh et
al., 2019; Horowitz & Sagan, 1993). Additionally, turboSETI will pick out
bright signals even if the drift rate is not matched correctly.

The second part of the Doppler search is to run the find_event pipeline which
removes signals with no drift rate and compares the hits across each cadence,
eliminating any signals present in both the “on” and “off” observations.
find_event returns events, which are any signals that are present in the “on”
and not “off” observations. Selecting signals that are only present in the
“on” observations removes RFI and isolates signals that are localized on the
sky.

Finally, the plot_event pipeline produces cadence plots for visual inspection,
which allows us to manually eliminate any RFI remaining after the find_event
pipeline. For more information see Enriquez et al. (2017) and Enriquez & Price
(2019).

##  4 Results

Figure 4: Number of hits (grey) and events (black) versus frequency. The
frequency range (band) for each GBT receiver is represented by the colored
regions; the (unshaded) notch filter regions (Lebofsky et al., 2019) at L and
S bands are excluded from our analysis. Values for each band, with the number
of cadences, are shown in Table 1. Note that there are a different number of
cadences at each band so the number of hits and events plotted here should not
be directly compared across bands.

###  4.1 Technosignature Search

For the rest of this discussion, we refer to a “hit” as any signal present in
a single observation and an “event” as a collection of related hits that
successfully passed through the find_event pipeline. We find 2,442,347 hits
and 634 events which were distributed across the receiver bands as shown in
Figure 4 and Table 1. We show examples of events in Figure 5. After visually
inspecting all 634 events, we find that all of them are consistent with human-
generated RFI. Most commonly, these signals appear to be present — but not
detectable by turboSETI above the S/N threshold — throughout the entire
cadence, indicating a source of interference that is likely local to the
telescope.

(a)

(b)

(c)

(d)

Figure 5: Dynamic spectra (waterfall plots) of 4 representative events from
the 634 event sample. Each plot is a vertical stack of the 6 scans making up
an ABACAD cadence. The vertical axis shows the time since the start of each
scan in the cadence, and the horizontal axis shows the frequency offset from
the event’s starting frequency.

Figures 5a and 5b illustrate signals that seem to appear mostly in the “on”
observations in a given cadence. However, both cadences also have some similar
signals in the “off” observations, and can therefore be ruled out as signals-
of-interest. These signals are broader in frequency than the narrowband
drifting tones turboSETI is designed to search for. Nevertheless, they were
bright enough to rise above turboSETI’s S/N threshold and register as hits.
Although an ETI could transmit signals with a range of bandwidths, the broader
signals in this study were clearly due to RFI. Furthermore, the signal in
Figure 5a is in a frequency range commonly used for aeronautical radar (as are
many of the top-ranked events presented by Enriquez et al. 2017). Likewise,
Figure 5b, given its frequency, is RFI that is likely related to the Iridium
satellite constellation.

Figure 5c shows a waterfall plot of TIC 241076290, a candidate exoplanet with
a tight orbit around its host star, with a period of 0.258 days. This is the
only target in our analysis whose transit is shorter than the 30 minute
observation. In this case we observe only the end of the transit. In the
future, by scheduling specifically timed observations for systems with short
transits, we could look for signals that appear only during the transit. As of
2021 June, in the ExoFOP-TESS catalog there are 31 TOIs with transits shorter
than 30 minutes, which corresponds to 0.74% of TOIs. These TOIs would be
interesting targets for follow up observations.

Figure 5d appears to have a non-linear Doppler shift, suggesting it is
accelerating with respect to the telescope, as might be expected for a
satellite in Earth orbit, and its frequency corresponds to a known satellite
downlink frequency. However, due to the relative motion of satellites (even
geosynchronous satellites) with respect to sidereal targets, they usually
appear in only one or two scans. Instead, Figure 5d has a signal present
throughout the entire cadence. Its presence in the “off” scans rules it out as
an ETI candidate; it may be a pernicious example of a slow-moving satellite
(possibly visible through a telescope sidelobe) that was moving in the same
general direction as the telescope over the course of the 30-minute
observation.

###  4.2 Hit and Event Distribution

The hit and event frequency distributions are shown in Figure 4. Histograms of
the S/N and drift rate distributions are shown in Figure 6. There are
significantly more hits and events at low drift rates, likely produced by RFI
local to the telescope.

Figure 6: Left Column: Histograms of hits (blue) and events (red) as a
function of drift rate split up by receiver. The reported drift rate is
normalized by the center frequency of the hit or event, to produce a value in
units of nHz (e.g., 1 nHz = 1 Hz s-1 at 1 GHz). Right Column: Histograms of
hits (blue) and events (red) as a function of S/N split up by Green Bank
Telescope receiver. Note that not all receivers observed the same number of
targets.

###  4.3 Figures of Merit

To further evaluate our ability to detect ETI signals in this work, we can
compare our figures-of-merit to those from past SETI studies. One such figure-
of-merit is the Drake Figure of Merit (DFM; Drake et al., 1984),

$\textrm{DFM}=\frac{n~{}\Delta f~{}\Omega}{F^{3/2}_{\textrm{min}}},$ (2)

where $n$ is the number of observations at a receiver, $\Delta f$ is the total
frequency range observed, $\Omega$ is the full width half maximum of the
receiver, and $F_{\textrm{min}}$ is the minimum detectable flux. While DFM has
some limitations, as discussed by Enriquez et al. (2017) and Margot et al.
(2021), it is still a useful statistic, especially for surveys across multiple
receivers, such as this one, because it incorporates both the bandwidth
surveyed and the minimum detectable power. Table 2 shows the DFM for this
study in comparison to other recent searches; larger DFMs indicate more
comprehensive searches.

Table 2: Drake Figure of Merit Study | DFM [GHz m3 W3/2]
---|---
This Study | $2.1\times 10^{32}$
Margot et al. (2021) | $1.11\times 10^{32}$
Gajjar et al. (2021) | $4\times 10^{28}$

A second useful figure-of-merit is the Continuous Waveform Transmitter Figure
of Merit (CWTFM; Enriquez et al., 2017). This describes the likelihood of
finding an ETI signal above a specific minimum Equivalent Isotropic Radiated
Power (EIRP${}_{\textrm{min}}$),

$\textrm{CWTFM}=\zeta_{\textrm{AO}}~{}\frac{\textrm{EIRP}_{\textrm{min}}}{N_{\textrm{stars}}~{}\nu_{\textrm{rel}}}$
(3)

where $N_{\textrm{stars}}$ is the number of pointings in a survey at a
receiver times the number of stars per pointing (assumed to be 1),
$\nu_{\textrm{rel}}$ is the total bandwidth for a receiver normalized by the
central frequency of the receiver, and $\zeta_{\textrm{AO}}$ is a
normalization constant such that CWTFM is 1 for an EIRP equal to that of
Arecibo. EIRP${}_{\textrm{min}}$ is a measure of the necessary power of a
hypothetical omnidirectional antenna, in the most distant star system in our
sample, to be detected by each GBT receiver. We plot the Transmitter Rate
(CWTFM divided by EIRP${}_{\textrm{min}}$) vs. EIRP${}_{\textrm{min}}$ for our
study in comparison to past searches in Figure 7. Technosignature searches
represent compromises between sensitivity (higher sensitivity towards the
left-hand side of the figure) and sky and bandwidth coverage (more stars,
and/or wider fractional bandwidth coverage, towards the bottom of the figure).
Our study occupies a similar region of parameter space to previous studies,
but is the first to achieve wide frequency coverage for a significant number
of stars observed during transit of candidate exoplanets.

Figure 7: Transmitter Rate versus EIRP${}_{\textrm{min}}$ for this study
(represented by the four Y-shaped points) compared to past studies (none of
which specifically targeted systems during transits). The two vertical lines
represent the EIRP of the Arecibo S-Band radar, and the solar power incident
on Earth.

###  4.4 Transmitter Limit

Given our lack of detection of any signal-of-interest, we can calculate the
transmitter limit, or maximum percentage of TESS TOIs at each band that
possess a detectable transmitter based on our search parameters. Price et al.
(2020), Traas et al. (2021), and other authors, calculate this limit using a
one sided 95% Poisson confidence interval with a 50% probability of actually
observing a signal if the transmitter is present (Gehrels, 1986). Given the
small number of cadences observed at L-band, a binomial confidence interval is
a better estimate for the transmitter limit in our case. We list the relevant
limits (95% one-sided binomial interval, with a 50% probability of detecting a
signal if present) in Table 1. Work is ongoing to determine more accurate
detection thresholds by performing signal injection and recovery in BL data.

The TOIs observed in this work all have short periods, as shown in Figure 3.
These targets are very close to their host stars, receiving many hundreds of
times more stellar insolation than terrestrial planets in the habitable zone.
Additionally, some of our targets are exoplanet candidates rather than
confirmed exoplanets. Some caution is therefore warranted in extrapolating the
transmitter limits from the TESS TOIs observed in this work to the entire
population of exoplanets.

##  5 Conclusions

We performed a technosignature search of 61 TESS TOIs, over 66 observations,
that are in transit during their BL observation at the GBT. This could be a
favored time to search for technosignatures from ETI because Earth is in the
ecliptic of these exoplanet candidates as they transit. ETIs may determine
such transits are good Schelling Points, and time their transmissions
accordingly.

After searching the 66 cadences for technosignatures, we did not find any
potential technosignature signals-of-interest. Using this null result, we
constrain the existence of extraterrestrial transmitters brighter than a few
hundred TW to less than 52%, 20%, 16%, and 15% (for L, S, C, and X bands,
respectively) of TESS TOIs that are observed during transit.

##  6 Future Studies

There are numerous ways to extend the work presented in the previous sections.
First, we could analyze targets that serendipitously enter or exit their
secondary transit during their observation. This way, we could search for
signals that appear or disappear as the exoplanet candidates passes behind its
host star.

Second, we could search for signals that appear or disappear at the same time
as targets enter or exit their transit during a BL observation. This would
require observations that cover the entire transit, including a substantial
portion of data taken outside of the ingress and egress of the transit. Since
ETI may assume that Earth is observing the exoplanet during its transit, they
may transmit a beacon to Earth only during the transit.

Third, we could search for signals that appear close to the midpoint of the
transit as viewed from Earth. In this scenario, ETI may direct a narrow,
beamed signal towards their anti-stellar point, which might appear as a signal
that changes in intensity with a Gaussian shape, as Earth is swept by the
transmitter beam. Such a signal could come from a transmitter present on the
anti-stellar point of a tidally-locked planet or, as previously mentioned, a
transmitter placed at an exoplanet’s second Lagrange point.

Fourth, rather than rely on serendipitous scheduling of BL observations of
TESS TOIs, observations could be scheduled during exoplanet candidate
transits. This could enable larger, more thorough studies of exoplanet
candidate transits as a Schelling Point, and as a geometrically-favourable
region for technosignature searches.

##  7 Acknowledgements

The Breakthrough Prize Foundation funds the Breakthrough Initiatives which
manages Breakthrough Listen. The Green Bank Observatory facility is supported
by the National Science Foundation, and is operated by Associated
Universities, Inc. under a cooperative agreement. We thank the staff at Green
Bank Observatory for their support with operations. NF was funded as a
participant in the Berkeley SETI Research Center Research Experience for
Undergraduates Site, supported by the National Science Foundation under Grant
No. 1950897. This research has made use of the Exoplanet Follow-up Observation
Program website, which is operated by the California Institute of Technology,
under contract with the National Aeronautics and Space Administration under
the Exoplanet Exploration Program.

NF thanks all of the 2021 Berkeley SETI interns for their support and
encouragement. In addition, we thank Richard Elkins for his support with
running turboSETI and Daniel Estévez for his insight in identifying the signal
in Figure 5d. We thank the anonymous referee for their comments on the
manuscript.

## References

  * Backus & Project Phoenix Team (2004) Backus, P. R., & Project Phoenix Team. 2004, in American Astronomical Society Meeting Abstracts, Vol. 204, American Astronomical Society Meeting Abstracts #204, 75.04
  * Drake (1961) Drake, F. 1961, Physics Today, 14, 40, doi: https://doi.org/10.1063/1.3057500
  * Drake et al. (1984) Drake, F., Wolfe, J. H., & Seeger, C. L. 1984, SETI science working group report, Tech. rep.
  * Enriquez & Price (2019) Enriquez, E., & Price, D. 2019, turboSETI: Python-based SETI search algorithm.  http://ascl.net/1906.006
  * Enriquez et al. (2017) Enriquez, J. E., Siemion, A., Foster, G., et al. 2017, ApJ, 849, 104, doi: 10.3847/1538-4357/aa8d1b
  * ExoFOP (2019) ExoFOP. 2019, Exoplanet Follow-up Observing Program - TESS, IPAC, doi: 10.26134/EXOFOP3
  * Gajjar et al. (2021) Gajjar, V., Perez, K. I., Siemion, A. P. V., et al. 2021, AJ, 162, 33, doi: 10.3847/1538-3881/abfd36
  * Gehrels (1986) Gehrels, N. 1986, ApJ, 303, 336, doi: 10.1086/164079
  * Gray & Mooley (2017) Gray, R. H., & Mooley, K. 2017, The Astronomical Journal, 153, 110, doi: 10.3847/1538-3881/153/3/110
  * Harp et al. (2016) Harp, G. R., Richards, J., Tarter, J. C., et al. 2016, AJ, 152, 181, doi: 10.3847/0004-6256/152/6/181
  * Horowitz & Sagan (1993) Horowitz, P., & Sagan, C. 1993, ApJ, 415, 218, doi: 10.1086/173157
  * Lebofsky et al. (2019) Lebofsky, M., Croft, S., Siemion, A. P. V., et al. 2019, PASP, 131, 124505, doi: 10.1088/1538-3873/ab3e82
  * MacMahon et al. (2018) MacMahon, D. H. E., Price, D. C., Lebofsky, M., et al. 2018, PASP, 130, 044502, doi: 10.1088/1538-3873/aa80d2
  * Margot et al. (2021) Margot, J.-L., Pinchuk, P., Geil, R., et al. 2021, The Astronomical Journal, 161, 55, doi: 10.3847/1538-3881/abcc77
  * Price et al. (2020) Price, D. C., Enriquez, J. E., Brzycki, B., et al. 2020, The Astronomical Journal, 159, 86, doi: 10.3847/1538-3881/ab65f1
  * Sheikh et al. (2020) Sheikh, S. Z., Siemion, A., Enriquez, J. E., et al. 2020, AJ, 160, 29, doi: 10.3847/1538-3881/ab9361
  * Sheikh et al. (2019) Sheikh, S. Z., Wright, J. T., Siemion, A., & Enriquez, J. E. 2019, ApJ, 884, 14, doi: 10.3847/1538-4357/ab3fa8
  * Siemion et al. (2013) Siemion, A. P. V., Demorest, P., Korpela, E., et al. 2013, The Astrophysical Journal, 767, 94
  * Traas et al. (2021) Traas, R., Croft, S., Gajjar, V., et al. 2021, AJ, 161, 286, doi: 10.3847/1538-3881/abf649
  * Tremblay & Tingay (2020) Tremblay, C. D., & Tingay, S. J. 2020, PASA, 37, e035, doi: 10.1017/pasa.2020.27
  * Worden et al. (2017) Worden, S. P., Drew, J., Siemion, A., et al. 2017, Acta Astronautica, 139, 98, doi: 10.1016/j.actaastro.2017.06.008
  * Wright et al. (2018) Wright, J. T., Sheikh, S., Almár, I., et al. 2018, NASA Technosignatures Workshop, arXiv e-prints, arXiv:1809.06857.  https://arxiv.org/abs/1809.06857

##  Appendix A Target List

Table 3: Our sample of TESS TOIs that transited during their observation at the GBT. Target Name | TOI | RA (hrs) | Dec (∘) | Distance (pc) | Orbital | Observation | Band
---|---|---|---|---|---|---|---
|  |  |  |  | Period (days) | Start Time (UTC) |
TIC438490744 | 529.01 | 6.683726312 | 16.58970997 | 63.0507 | 1.665878 | 5/31/2021 21:36 | C
TIC147950620 | 1194.01 | 11.18809073 | 69.96478246 | 149.667 | 2.310602 | 6/14/2020 20:37 | C
TIC458478250 | 1165.01 | 15.47643744 | 66.35871037 | 126.261 | 2.255296 | 11/24/2019 23:54 | S
TIC344926234 | 634.01 | 10.10383058 | 3.946493286 | 92.8368 | 0.49359 | 11/24/2019 14:43 | S, C
TIC78154865 | 638.01 | 9.857820581 | -4.123319468 | 96.6355 | 0.493826 | 2/27/2020 1:37 | S
TIC365781372 | 627.01 | 5.461566558 | 7.918525328 | 629.834 | 1.13889 | 11/30/2019 5:03 | X
TIC270468559 | 571.01 | 9.022952699 | 6.097088972 | 405.238 | 4.641843 | 9/11/2020 19:08 | C
TIC121338379 | 498.01 | 8.606046796 | -3.860202132 | 188.364 | 0.275043 | 1/19/2020 8:34 | S
TIC375542276 | 1163.01 | 19.60611228 | 19.63922729 | 148.342 | 3.07765 | 12/15/2019 1:12 | L
TIC468880077 | 438.01 | 3.766972039 | 9.9903089 | 72.4646 | 5.8076 | 1/27/2021 23:58 | C
TIC459942762 | 430.01 | 4.018554669 | 4.540889327 | 66.5727 | 0.58644 | 12/21/2019 6:56 | X
TIC280437559 | 969.01 | 7.675771778 | 2.098612197 | 77.2554 | 1.823737 | 1/11/2020 9:20 | X
TIC425206121 | 508.01 | 7.433963292 | 7.615772707 | 300.276 | 4.611733 | 1/19/2020 3:44 | S
TIC178367144 | 966.01 | 8.226143814 | -1.982782058 | 253.985 | 3.409244 | 1/19/2020 8:02 | S
TIC138168780 | 1651.01 | 6.319529079 | 73.82755828 | 235.479 | 3.764988 | 3/29/2021 18:57 | L
TIC73104318 | 1674.01 | 4.114769716 | 58.46544652 | 201.645 | 7.45494 | 2/10/2020 4:40 | X
TIC422756130 | 1695.01 | 1.461444537 | 72.29660211 | 45.1309 | 3.134319 | 5/25/2020 18:35 | S
TIC285674856 | 1570.01 | 3.546414139 | 51.88450172 | 294.123 | 1.74626 | 2/17/2020 20:06 | X
TIC241076290 | 1560.01 | 1.935519308 | 52.58547107 | 560.222 | 0.25792 | 6/28/2020 16:07 | S
TIC348673213 | 1639.01 | 2.387149453 | 56.57002561 | 153.986 | 0.901465 | 4/19/2020 16:04 | C
TIC292321872 | 1572.01 | 2.126808592 | 45.50016306 | 505.331 | 8.66698 | 11/13/2020 9:44 | L
TIC294471966 | 1446.01 | 20.1334245 | 51.36180671 | 133.863 | 6.31719 | 6/30/2020 0:34 | C
TIC409183335 | 1667.01 | 5.453849363 | 38.59745582 | 225.941 | 3.32125 | 11/13/2020 12:58 | L
TIC286561122333Note that this target was observed twice at C-Band and both cadences overlap transits for this TOI. | 1658.01 | 4.391744889 | 35.49511432 | 506.864 | 0.67994 | 3/23/2020 0:16 | C
TIC311035838 | 1419.01 | 13.73959477 | 48.02856107 | 134.554 | 2.899733 | 6/20/2020 5:37 | S
TIC327579226 | 1532.01 | 0.315744444 | 57.20064898 | 259.563 | 8.90592 | 11/10/2020 4:26 | X
TIC365683032 | 1354.01 | 20.81199235 | 51.91068918 | 245.776 | 1.42904 | 4/18/2020 8:56 | S, X
TIC241040309 | 1559.01 | 1.381155015 | 48.95536157 | 685.251 | 3.46479 | 7/18/2020 13:06 | X
TIC312862941 | 1638.01 | 1.021431709 | 55.69799904 | 126.283 | 0.915094 | 4/20/2020 0:22 | C
TIC137881699 | 1781.01 | 10.03817034 | 53.95082988 | 935.456 | 2.972133 | 9/11/2020 23:33 | C
TIC149833117 | 1717.01 | 6.975231953 | 67.67733006 | 188.086 | 4.052173 | 6/5/2020 4:12 | S
TIC368536386 | 1666.01 | 5.961338651 | 36.76580927 | 428.948 | 1.69433 | 9/14/2020 7:13 | C
TIC376682699 | 1511.01 | 22.69014554 | 69.07445015 | 544.233 | 1.10264 | 11/8/2020 15:36 | X
TIC376637093 | 1516.01 | 22.67230188 | 69.50372602 | 247.054 | 2.05603 | 5/19/2020 22:39 | S, C, X
TIC327011842 | 1576.01 | 1.564455335 | 45.01032893 | 493.702 | 0.78424 | 7/18/2020 11:58 | X
TIC44631965 | 1461.01 | 1.482420079 | 35.86484113 | 359.959 | 3.568678 | 5/29/2020 19:06 | C
TIC142090065 | 1715.01 | 5.271560572 | 79.73772521 | 182.907 | 2.826937 | 9/4/2020 7:39 | X
TIC198212955 | 1242.01 | 16.57021523 | 60.19589615 | 110.015 | 0.381481 | 7/29/2020 8:39 | C
TIC138017750 | 1608.01 | 3.386736393 | 33.07814949 | 100.635 | 2.472722 | 10/26/2020 0:59 | S
TIC26433869 | 1607.01 | 3.7876164 | 30.14950686 | 329.591 | 1.03578 | 7/3/2020 18:25 | X
TIC353367071 | 1663.01 | 5.995649857 | 33.50698402 | 402.261 | 2.37532 | 9/14/2020 8:18 | C
TIC272625214 | 1613.01 | 23.75456899 | 62.14267079 | 304.76 | 5.24666 | 7/27/2020 16:11 | C
TIC129979528 | 1599.01 | 2.447511416 | 37.55044553 | 121.944 | 1.219868 | 9/17/2020 7:05 | X
TIC341815767 | 1819.01 | 17.83467483 | 54.63614716 | 160.295 | 3.09374 | 12/21/2020 19:24 | X
TIC457138169 | 1770.01 | 9.424525742 | 50.9088635 | 163.438 | 1.09254 | 8/14/2020 13:08 | C
TIC371673488 | 1497.01 | 22.88221213 | 59.85095835 | 405.174 | 0.8158 | 12/20/2020 1:44 | X
TIC15863518 | 1713.01 | 6.701367042 | 39.84291832 | 138.371 | 0.557201 | 12/13/2020 0:47 | X
TIC389182138 | 1391.01 | 22.90899711 | 54.16180798 | 115.746 | 2.72687 | 10/10/2020 6:51 | C
TIC235905185 | 1829.01 | 19.39182508 | 78.75421665 | 479.529 | 6.289555 | 12/1/2020 4:15 | X
TIC191284318 | 1458.01 | 0.63819763 | 42.46306636 | 226.637 | 2.77598 | 11/10/2020 7:20 | X
TIC358631536 | 1343.01 | 21.17169156 | 48.4642791 | 400.034 | 3.40304 | 12/24/2020 18:22 | S
TIC274942910 | 1325.01 | 21.52843349 | 41.79747049 | 52.4946 | 1.07922 | 12/24/2020 22:09 | S
TIC233720539 | 1815.01 | 18.42528104 | 63.48810973 | 617.233 | 2.55532 | 1/14/2021 1:44 | X
TIC38686737 | 432.01 | 3.857704881 | -10.6140933 | 746.646 | 2.24704 | 1/14/2021 5:19 | X
TIC117979455 | 422.01 | 4.786847839 | -17.25336165 | 124.504 | 0.63322 | 1/17/2021 5:29 | S
TIC328167090 | 1384.01 | 22.11089078 | 55.68625098 | 235.218 | 0.71255 | 4/4/2021 13:04 | L
TIC154741689 | 2170.01 | 10.95424616 | 89.08691789 | 206.368 | 9.27688 | 3/17/2021 6:56 | C
TIC427730490 | 2040.01 | 23.48497742 | 71.50646786 | 144.717 | 3.86085 | 3/22/2021 20:38 | X
TIC321688498 | 2290.01 | 21.43996743 | 68.64052458 | 58.0924 | 0.38623 | 3/22/2021 22:53 | X
TIC393911494 | 2106.01 | 13.81189142 | 44.9117615 | 121.167 | 0.633259 | 3/28/2021 11:38 | S
TIC285542903 | 2060.01 | 0.884664039 | 60.61811644 | 914.062 | 2.26584 | 4/19/2021 17:49 | C