###### Abstract

The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide
polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray
band. Measurements are conducted from a stabilized balloon-borne platform,
launched on a 1 million cubic meter balloon from the Esrange Space Center in
Sweden to an altitude of approximately 40 km. Several flights have been
conducted, resulting in two independent measurements of the Crab polarization
and one of Cygnus X-1. Here, a review of the PoGO mission is presented,
including a description of the payload and the flight campaigns, and a
discussion of some of the scientific results obtained to date.

###### keywords:

Compton polarimeter; hard X-rays; Crab, Cygnus X-1; scientific ballooning;
payload design, attitude control

x 10.3390/—— xx Received: date; Accepted: date; Published: date The PoGO+
Balloon-Borne Hard X-ray Polarimetry Mission Mette Friis 1,2, Mózsi Kiss 1,2*,
Victor Mikhalev 1,2, Mark Pearce 1,2 and Hiromitsu Takahashi 3, on behalf of
the PoGO+ Collaboration Correspondence<EMAIL_ADDRESS>Tel.: +46-73-765-24-50

##  1 Introduction

PoGO+ is a balloon-borne hard X-ray Compton polarimetry telescope operating in
the energy range $\sim$20–180 keV. “PoGO” stands for “Polarized Gamma-ray
Observer,” and the plus signifies an upgrade Chauvin1 (2016), specifically
from the PoGOLite “Pathfinder” Chauvin2 (2016), which was launched in 2013
Chauvin3 (2016). The instrument is optimized for point-sources and has a field
of view collimated to about 2∘. Observational targets are the X-ray-bright
sources on the northern hemisphere: the Crab and Cygnus X-1.

By providing two new observational parameters, namely the polarization
fraction and polarization angle, polarimetry is expected to provide new
insight into the emission mechanisms, magnetic fields, and geometries of
sources observed. The relevance of polarimetric studies has been extensively
discussed TheseProceedings1 (2018), and results have been reported in
particular for the Crab system. Many of the results suffer, however, from
systematic effects, leaving open questions about the emission mechanisms at
work in the observed astrophysical sources. The goal of the PoGO mission is to
provide statistically constrained polarimetric observations (a minimum
detectable polarization of $\sim$10% before background subtraction), in a
previously unexplored energy range, with well-understood systematic effects,
using an instrument that has been extensively calibrated with both polarized
and unpolarized beams Chauvin4 (2017).

Here, developments of the PoGO mission are collected, leading up to the 2016
flight and a summary of observational results for the Crab and Cygnus X-1.
Section 2 gives a description of the PoGO+ payload, including the instrument,
attitude control system, and gondola structure. A brief review of the flight
campaigns to date, and the lessons learned from these, is provided in Section
3. In Section 4, scientific results are presented. Finally, Section 5 provides
an outlook on some of the studies that are currently on-going using data
collected with PoGO+.

##  2 The PoGO+ Payload

PoGO+ uses a 61-pixel array of plastic scintillators for reconstructing
azimuthal scattering angles from two-unit interactions (polarization candidate
events) in temporal coincidence. As per the Klein–Nishina formula, such
scattering angles will be modulated by the polarization of the incident photon
beam, following a sinusoidal distribution with a 180∘ period Lei1 (1997). The
scintillators, type EJ-204 from Eljen Technology, are 12 cm tall and about 3
cm across, with a hexagonal cross section, allowing them to be tightly packed
in a honeycomb structure. Copper tubes, approximately 70 cm in length, are
used to collimate the field of view. These are made from 0.5 mm thin copper
sheets, folded into hexagonal cross-section tubes, which are wrapped in 200
$\upmu$m of tin foils followed by the same thickness of lead foils. This
“graded shield” construction of the collimators efficiently suppresses the
out-of-aperture background of the measurements. The plastic scintillators are
surrounded on the bottom and on the sides by a bismuth germanium oxide (BGO)
anticoincidence shield, segmented into 61 pieces on the bottom of the
instrument (one corresponding to each plastic scintillator) and 30 pieces on
the side. The anticoincidence shield is 60 cm tall (compared to the 12 cm
height of the plastic scintillators) and the thickness exceeds 3 cm on each
side. Plastic scintillators and side anticoincidence shield elements are read
out by the same type of photo-multiplier tube, a modified version of Hamamatsu
R7899. In order to cancel out variations in response between individual
detector elements, the entire detector array rotates during observations at a
rate of 1∘/s. The dominant background is from atmospheric neutron interactions
Kole1 (2015), wherefore the entire detector array is surrounded by a passive
polyethylene shield, with a thickness between 5 and 15 cm. Figure 1
illustrates the detector design. A more comprehensive description of the
payload is presented in Chauvin2 (2016).

Figure 1: Sketch of the PoGO+ polarimeter. The height is $\sim$1.5 m, and the
total weight is $\sim$600 kg.

Celestial targets are acquired and tracked using a custom attitude control
system and gimbal unit assembly, designed and developed by DST Control,
Sweden. In order to prevent degradation of the polarimetric response due to
shadowing effects from the collimators, the pointing is required to be better
than 5% of the instrument field of view, i.e., within $\sim$0.1∘. Coarse
azimuthal pointing is achieved using a flight train motor coupled directly to
the interface to the balloon rigging. A flywheel provides fine-tuning in
azimuth, and excess momentum can be transferred to the balloon through a
momentum dump system that controls the feedback to the flight train. Brushless
direct drive torque motors are used throughout, with a configuration providing
excellent heat dissipation and electromagnetic compatibility. A detailed
description of the attitude control system hardware and design philosophy is
presented in Stromberg1 (2011). The gimbal unit assembly is shown in Figure 2.

Figure 2: Sketch of the gimbal unit assembly housing the polarimeter. The
framework stands $\sim$2 m tall and has a mass of $\sim$300 kg, excluding the
polarimeter.

Pointing solutions and motor control feedbacks are calculated based on several
attitude inputs. An encoder mounted on the polarimeter axle provides the
telescope elevation. For the azimuth, the backbone of the system is a
differential GPS which, utilizing a 10 m baseline, can provide the desired
pointing accuracy in itself. A three-axis magnetometer serves as a backup in
case of a GPS failure. In the upgrade from PoGOLite to PoGO+, a sun tracker
was additionally included. Using a two-dimensional position-sensitive device
inside a free-moving gimbal, which automatically locates and follows the sun,
this tracker can provide the instrument’s azimuth based on its location and
the position of the sun. These sensors provide absolute information for
instrument pointing. Relative feedback is provided by an optical star tracker
camera, mounted co-axially with the polarimeter. For each X-ray target, an
optical guide star is defined. The camera identifies the pixel coordinates of
the guide star and matches these with the expected coordinates, calculated in
real time based on the instrument position, altitude, and time of day,
correcting for the field rotation of the star field as time goes by. In this
tracking mode, motor feedbacks are generated based on the difference between
the expected and the observed pixel coordinates of the relevant star. An
inclinometer mounted on the gimbal unit assembly provides information on the
gondola pitch and roll. In addition, a micro-mechanical inertial measurement
unit is used, providing an additional layer of redundancy. The unit is
continuously augmented by inputs from the other available sensors and can, in
case other attitude sensors fail, retain attitude information for some time,
until the remaining systems become operational again.

The polarimeter and gimbal unit assembly are housed inside a gondola structure
designed and manufactured by SSC Esrange. The structure provides thermal
shielding, mechanical stiffness for pointing stability, and protection during
launch and landing. It consists of a six-sided light-weight framework with
aluminum ribs and honeycomb composite plates. Solar panels and booms for the
GPS and communication antennas are also affixed onto the structure, as is a
radiator for expelling heat from the polarimeter electronics, using a fluid-
based cooling system. During assembly and disassembly, the gondola can be
separated into an upper part, housing the polarimeter, and a lower part, which
contains batteries, communications equipment, and ancillary hardware. The
assembled PoGO+ payload is shown in Figure 3.

Figure 3: The PoGO+ payload and its various constituents. Crash pads, ballast
hoppers, and line-of-sight antennas mounted underneath the gondola are not
shown. One panel of the gondola, the front where the polarimeter is looking
out, has been removed, showing components inside. The structure is $\sim$4 m
tall and the widest part, at the GPS booms, is $\sim$10 m. For flight, the
total suspended weight is almost two tonnes, which includes 450 kg of ballast.

##  3 PoGO Flight Campaigns

PoGO campaigns are conducted from the Swedish Space Corporation (SSC) Esrange
Space Center in Northern Sweden (67.89∘ N, 21.08∘ E). During the summer,
circumpolar winds at a $\sim$40 km altitude move in the westerly direction,
allowing flights from Esrange to Canada. The launch window opens around 1
July, when the angular separation between the Crab and the sun exceeds 15∘,
and closes during August, when the circumpolar vortex starts disintegrating.
There have been four campaigns to date, resulting in three launches.

The year 2011 saw the first test launch of the PoGOLite “Pathfinder.” A leak
of helium from the balloon forced the flight to be terminated prematurely,
resulting in a landing near Nikkaluokta, approximately 100 km to the west of
the Esrange Space Center. Float altitude was never achieved, so scientific
measurements could not be conducted. At landing, only minor damage was
incurred, and the polarimeter array could easily be repaired from spare
detector units.

A second launch attempt was foreseen for the summer of 2012. Due to
unfavorable weather conditions, the payload remained grounded for the duration
of the launch window.

A successful launch of the PoGOLite “Pathfinder” took place on 12 July 2013.
For this particular launch, an overflight agreement had also been secured from
Russian authorities, opening up the possibility for a full polar
circumnavigation, concluding with a termination somewhere over Scandinavia. In
the end, due to a predicted drift in the trajectory towards higher latitudes,
whereby landing in Scandinavia would no longer be certain, the flight was
terminated over Russia on 25 July, resulting in a landing near Norilsk,
$\sim$3000 km east of Moscow. Details of this flight are presented in Chauvin2
(2016). Due to issues related to overheating, scientific operations could only
be concluded during the first three days of the flight. A statistics-limited
measurement of the Crab polarization could, however, be reported Chauvin3
(2016).

The next launch took place on 12 July 2016, now with the upgraded payload,
called “PoGO+”. PoGO+ benefited tremendously from data collected during the
2013 flight of PoGOLite. The polarimeter was extensively redesigned and
optimized Chauvin1 (2016), including (i) new detector units and improved
reflective wrapping yielding higher sensitivity; (ii) improved front-end
electronics with a wider dynamic range and a higher waveform sampling rate
providing improved efficiency for the pulse shape discrimination and event
selection; (iii) additional polyethylene neutron shielding for suppressing the
flight background; and (iv) higher sensitivity for the side anticoincidence
shield, yielding a lower energy threshold for the background rejection. On the
gondola side, an Iridium Pilot antenna had been added, allowing for better
monitoring, operations, and data download rates during over-the-horizon
communications. For the attitude information, the aforementioned sun tracker
had been added, which simplified observations of the Crab system, where close
angular proximity to the sun had previously complicated automatic pointing
operations based on feedback from the optical star tracker camera. Of
particular importance for the scientific observations was the decision to
intersperse background measurements with the on-source observations of the
Crab and Cygnus X-1. Throughout the flight, background fields separated by 5∘
to the east and west of the target were observed, with transitions to/from the
X-ray source occurring automatically approximately every 15 min, and with
almost equal time spent on source and on background fields. This strategy
allowed the temporal evolution of the background rate and its residual
polarization to be studied, and was a prerequisite for correctly determining
the in-flight signal-to-background rate, needed for the background-subtraction
of the flight data. The omission of such interspersed background would have
resulted in a significant degradation of the systematic precision of the
polarimetric results. Figure 4 shows an example of signal (on-source) and
background (off-source) count rates during a Crab observation, underlining the
importance of interspersing the two kinds of observations in this fashion.

Figure 4: Count rate of two-hit events during the Crab observation on 14 July
2016. Apart from the smooth change as a function of time resulting from the
sidereal motion of the source across the sky, large fluctuations can be seen
both in the on-source (red) and off-source (black) data points. In the absence
of interspersed background measurements, this would be indistinguishable from
a change in the source rate only, resulting in an incorrectly determined
signal-to-background ratio.

Polarimetric results are derived from the azimuthal scattering angle
distributions of these two-hit events. Recorded scattering angles are
corrected for the rotation of the detector array throughout the measurement,
as well as for the instantaneous gondola roll, sampled at 4 Hz. While
distributions for both on-source and off-source measurements are first
inspected in terms of $\chi^{2}$ per degrees of freedom goodness-of-fit with
respect to 180∘ (polarization) and 360∘ (anisotropic background) components,
an unbinned Stokes-based analysis is used to determine final results.
Background is subtracted using the signal-to-background ratio, following
corrections for the slight change in acquisition live-time when observing on-
source versus off-source. Flight results are summarized in the next section.

##  4 Scientific Results from PoGO

Polarimetric results reported for the high-energy emission of the Crab are
limited to a few data points only. At low energies, instruments on the OSO-8
satellite measured polarization at 2.6 and 5.2 keV already in the 1970s
Weisskopf1 (1978). For the higher energies of several hundred keV, results are
reported from the INTEGRAL SPI Chauvin5 (2013); Dean1 (2008), and INTEGRAL
IBIS Moran1 (2016); Forot1 (2008) instruments. The reliability of the INTEGRAL
results may, however, be limited by the fact that these instruments were not
designed as polarimeters and were not calibrated for polarimetry in ground
testing campaigns. PoGOLite, through the “Pathfinder” mission, provided the
first polarimetric results in the intermediate energy range, 20–120 keV
Chauvin3 (2016). The upgraded PoGO+ instrument later provided an independent
study in the range 19–160 keV, confirming the findings from PoGOLite,
significantly improving the statistical precision and producing a result for
the off-pulse (nebula-dominated) and second pulse (“P2”, off-pulse-subtracted)
phase regions, respectively Chauvin6 (2017). Since the PoGO+ results were
published, results from the AstroSat mission have also been reported Vadawale1
(2018), in an energy range complementary to that of PoGO+, 100–380 keV.
Results from these high-energy studies were collected and are shown in Figure
5, where low-energy optical values from the NOT Slowikowska1 (2009) are also
included.

Figure 5: Results from PoGOLite and PoGO+ with other polarimetric studies in
different high-energy emission regimes, for the phase-integrated Crab
measurement (left), and the off-pulse (nebula-dominated) and P2 (off-pulse-
subtracted) phase intervals (right), for the polarization fraction (upper
panels) and polarization angle (lower panels). Low-energy optical results are
also shown.

The PoGO+ results Chauvin6 (2017) reveal a polarization fraction of (20.9
$\pm$ 5.0)% for the phase-integrated Crab measurement. This relatively high
value indicates synchrotron radiation from a magnetically ordered and
therefore compact emission region. The observed polarization angle is (131.3
$\pm$ 6.8)∘, compatible with the pulsar spin axis which is identified as
having angle (124.0 $\pm$ 0.1)∘ Ng1 (2004) projected on the sky. NuSTAR
imaging Madsen1 (2015) shows that high energy emission is predominantly coming
from more central regions of the Crab system. The alignment of the measured
polarization angle with the pulsar spin axis is thus consistent with the
toroidal ring region Nakamura1 (2007) of the Crab system dominating the hard
X-ray emission observed by PoGO+. Spatially resolved measurements from HST
Moran2 (2013) reveal high levels of polarization from the synchrotron knot and
from the wisp structures, exhibiting polarization angles of (124.7 $\pm$ 1.0)∘
and 124∘–130∘, respectively. A vector map of the full nebula instead shows a
peak distribution around polarization angle 165∘. Such fine details are not
resolved in hard X-rays, but the agreement with the polarization angle
determined by PoGO+ suggests that these features have counterparts also in the
energy range of the latter instrument, $\sim$19–160 keV.

Taken at face value, published polarimetric results for the Crab from
different instruments indicate an increase of polarization fraction as a
function of energy. However, recent results from AstroSat Vadawale1 (2018) are
in agreement with PoGO results Chauvin6 (2017), which do not show a drastic
increase as seen by INTEGRAL IBIS Moran1 (2016); Forot1 (2008). This strain
illustrates the difficulty of deriving polarimetric results, polarization
fraction in particular, from instruments not calibrated as polarimeters.

##  5 Summary and Outlook

Unlike satellite-based measurements, scientific ballooning missions allow
modifications and upgrades to the payload design to be implemented based on
flight data and experience, increasing the chance of success in a re-flight.
In this fashion, PoGO+ has successfully conducted polarimetric observations of
the Crab system and Cygnus X-1 in the energy range $\sim$20–180 keV, based on
experience from the flight of the PoGOLite “Pathfinder.” Results presented for
the Crab Chauvin6 (2017) are the first in this energy range. Independent
results from AstroSat Vadawale1 (2018) have been found to be in agreement,
strengthening the confidence in the results and countering a rapid increase of
polarization fraction with energy as suggested by some measurements Moran1
(2016); Forot1 (2008). A follow-up to the AstroSat observations, and the claim
Vadawale1 (2018) of varying polarization properties in the Crab off-pulse
region, has also been conducted Chauvin8 (2018), wherein PoGO+ data has been
revisited, allowing this claim to be addressed. For Cygnus X-1, PoGO+ results
Chauvin7 (2018) allow the emission geometry predicted by different models to
be constrained.

What is currently being investigated is the possibility of an energy-dependent
analysis of the polarization data, for the Crab and/or for Cygnus X-1. As
PoGO+ is optimized for polarimetry and not calorimetry, this would be a Monte-
Carlo-driven study, relying on the previously benchmarked simulation results
Chauvin4 (2017).

There are currently no plans to re-fly PoGO+, but the experience from the PoGO
mission is used for the development of a new instrument, called SPHiNX
(“Satellite Polarimeter for High eNergy X-rays”) Fei1 (2018), which has a wide
field of view and is intended for gamma-ray bursts.

###### Acknowledgements.

This research was supported in Sweden by The Swedish National Space Board, The
Knut and Alice Wallenberg Foundation, and The Swedish Research Council. In
Japan, support was provided by the Japan Society for Promotion of Science and
ISAS/JAXA. SSC are thanked for providing expert mission support and launch
services at Esrange Space Center. DST Control developed the PoGO+ attitude
control system under the leadership of J.-E. Strömberg. Contributions from
past collaboration members and students are acknowledged. M.F., V.M., M.P. and
H.T. attended the Workshop on behalf of the Collaboration. The manuscript was
prepared by M.K. All authors have read the manuscript and contributed
comments. The authors declare no conflict of interest.

## References

  * Chauvin1 (2016) Chauvin, M.; Jackson, M.; Kawano, T.; Kiss, M.; Kole, M.; Mikhalev, V.; Moretti, E.; Takahashi, H.; Pearce, M. Optimising a balloon-borne polarimeter in the hard X-ray domain: From the PoGOLite Pathfinder to PoGO+. Astropart. Phys. 2016, 82, 99–107, doi:10.1016/j.astropartphys.2016.06.005.
  * Chauvin2 (2016) Chauvin, M.; Florén, H.H.; Jackson, M.; Kamae, T.; Kawano, T.; Kiss, M.; Kole, M.; Mikhalev, V.; Moretti, E.; Olofsson, G.; et al. The design and flight performance of the PoGOLite Pathfinder balloon-borne hard X-ray polarimeter. Exp. Astron. 2016, 41, 17–41, doi:10.1007/s10686-015-9474-x.
  * Chauvin3 (2016) Chauvin, M.; Florén, H.H.; Jackson, M.; Kamae, T.; Kawano, T.; Kiss, M.; Kole, M.; Mikhalev, V.; Moretti, E.; Olofsson, G.; et al. Observation of polarized hard X-ray emission from the Crab by the PoGOLite Pathfinder. MNRAS Lett. 2016, 456, L84–L88, doi:10.1093/mnrasl/slv177.
  * TheseProceedings1 (2018) Various authors. In The Bright Future of Astronomical X-ray Polarimetry, a special issue of Galaxies; Marin, F., Ed. MDPI: Basel, Switzerland, 2018, ISSN 2075-4434.
  * Chauvin4 (2017) Chauvin, M.; Friis, M.; Jackson, M.; Kawano, T.; Kiss, M.; Mikhalev, V.; Ohashi, N.; Stana, T.; Takahashi, H.; Pearce, M. Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO+. Nucl. Instrum. Meth. A 2017, 859, 125–133, doi:10.1016/j.nima.2017.03.027.
  * Lei1 (1997) Lei, F.F.; Dean, A.A.; Hills, G.L. Compton Polarimetry in Gamma-Ray Astronomy. Space Sci. Rev. 1997, 82, 309–388, doi:10.1023/A:1005027107614.
  * Kole1 (2015) Kole, M.; Pearce, M.; Salinas, M.M. A model of the cosmic ray induced atmospheric neutron environment. Astropart. Phys. 2015, 62, 230–240, doi:10.1016/j.astropartphys.2014.10.002.
  * Stromberg1 (2011) Strömberg, J.-E. A Modular Solution for Science Platform Stabilisation. In Proceedings of the 20th Symposium on European Rocket and Balloon Programmes and Related Research; Ouwehand, L., Ed.; ESA Communications ESTEC: Noordwijk, The Netherlands, 2011; pp. 353–358, ISBN 978-92-9092-264-3.
  * Weisskopf1 (1978) Weisskopf, M.M.; Silver, E.E.; Kestenbaum, H.H.; Long, K.K.; Novick, R. A precision measurement of the X-ray polarisation of the Crab nebula without pulsar contamination. Astrophys. J. Lett. 1978, 220, L117–L121, doi:10.1086/182648.
  * Chauvin5 (2013) Chauvin, M.; Roques, J.J.; Clark, D.D.; Jourdain, E. Polarimetry in the hard X-ray domain with INTEGRAL SPI. Astrophys. J. 2013, 769, 137, doi:10.1088/0004-637X/769/2/137.
  * Dean1 (2008) Dean, A.A.; Clark, D.D.; Stephen, J.J.; McBride, V.V.; Bassani, L.; Bazzano, A.; Bird, A.A.; Hill, A.A.; Shaw, S.S.; Ubertini, P. Polarized Gamma-Ray Emission from the Crab. Science 2008, 321, 1183–1185, doi:10.1126/science.1149056.
  * Moran1 (2016) Moran, P.; Kyne, G.; Gouiffès, C; Laurent, P.; Hallinan, G.; Redfern, R.M.; Shearer, A. A recent change in the optical and $\gamma$-ray polarization of the Crab nebula and pulsar. MNRAS 2016, 456, 2974–2981, doi.org/10.1093/mnras/stv2780.
  * Forot1 (2008) Forot, M.; Laurent, P.; Grenier, I.I.; Gouiffès, C.; Lebrun, F. Polarization of the Crab pulsar and nebula as observed by the INTEGRAL$/$IBIS Telescope. Astrophys. J. Lett. 2008, 688, L29–L32, doi:10.1086/593974.
  * Chauvin6 (2017) Chauvin, M.; Florén, H.-G.; Friis, M.; Jackson, M.; Kamae, T.; Kataoka, J.; Kawano, T.; Kiss, M.; Mikhalev, V.; Mizuno, T.; et al. Shedding new light on the Crab with polarized X-rays. Sci. Rep. 2017, 7, 7816, doi:10.1038/s41598-017-07390-7.
  * Vadawale1 (2018) Vadawale, S.S.; Chattopadhyay, T.; Mithun, N.N.; Rao, A.A.; Bhattacharya, D.; Vibhute, A.; Bhalerao, V.V.; Dewangan, G.G.; Misra, R.; Paul, B.; et al. Phase-resolved X-ray polarimetry of the Crab pulsar with the AstroSat CZT Imager. Nat. Astron. 2018, 2, 50–55, doi:10.1038/s41550-017-0293-z.
  * Slowikowska1 (2009) Słowikowska, A.; Kanbach, G.; Kramer, M.; Stefanescu, A. Optical polarization of the Crab pulsar: Precision measurements and comparison to the radio emission. MNRAS 2009, 397, 103–123, doi:10.1111/j.1365-2966.2009.14935.x.
  * Ng1 (2004) Ng, C.-Y.; Romani, R.W. Fitting pulsar wind tori. Astrophys. J. 2004, 601, 479–484, doi:10.1086/380486.
  * Madsen1 (2015) Madsen, K.K.; Reynolds, S.; Harrison, F.; An, H.; Boggs, S.; Christensen, F.F.; Craig, W.W.; Fryer, C.C.; Grefenstette, B.B.; Hailey, C.C.; et al. Broadband X-ray imaging and spectroscopy of the Crab nebula and pulsar with NuSTAR. Astrophys. J. 2015, 801, 66, doi:10.1088/0004-637X/801/1/66.
  * Nakamura1 (2007) Nakamura, Y.; Shibata, S. Polarization of the Crab Nebula with disordered magnetic components. MNRAS 2007, 381, 1489–1498, doi:10.1111/j.1365-2966.2007.12172.x.
  * Moran2 (2013) Moran, P.; Shearer, A.; Mignani, R.R.; Słowikowska, A.; De Luca, A.; Gouiffès, C.; Laurent, P. Optical polarimetry of the inner Crab nebula and pulsar. MNRAS 2013, 433, 2564–2575, doi:10.1093/mnras/stt931.
  * Chauvin7 (2018) Chauvin, M.; Florén, H.-G.; Friis, M.; Jackson, M.; Kamae, T.; Kataoka, J.; Kawano, T.; Kiss, M.; Mikhalev, V.; Mizuno, T.; et al. A constraint on the accretion geometry of the black hole in Cygnus X-1 with X-ray polarimetry. Submitted.
  * Chauvin8 (2018) Chauvin, M.; Florén, H.-G.; Friis, M.; Jackson, M.; Kamae, T.; Kataoka, J.; Kawano, T.; Kiss, M.; Mikhalev, V.; Mizuno, T.; et al. The PoGO+ view on Crab off-pulse hard X-ray polarisation. Accepted for publication in MNRAS Lett. _arXiv_ 2018, arXiv:1802.07775 [astro-ph.HE]
  * Fei1 (2018) Xie, F.; Pearce, M. A study of background conditions for SPHiNX – the satellite-borne gamma-ray burst polarimeter. In The Bright Future of Astronomical X-ray Polarimetry, a special issue of Galaxies; Marin, F., Ed. MDPI: Basel, Switzerland, 2018, ISSN 2075-4434.