An Overview of X-Ray Polarimetry of Astronomical Sources

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Article An Overview of X-Ray Polarimetry of Astronomical Sources

Martin C. Weisskopf

NASA/Marshall Space Flight Center (MSFC), ST12, 320 Sparkman Drive, Huntsville, AL 35805, USA; martin.c.weisskopf@nasa.gov; Tel.: +1-256-961-7798

Received: 19 January 2018; Accepted: 1 March 2018; Published: 6 March 2018

Abstract: We review the history of astronomical X-ray polarimetry based on the author’s perspective, beginning with early sounding-rocket experiments by Robert Novick at Columbia University and his team, of which the author was a member. After describing various early techniques for measuring X-ray polarization, we discuss the polarimeter aboard the Orbiting Solar Observatory 8 (OSO-8) and its scientific results. Next, we describe the X-ray polarimeter to have flown aboard the ill-fated original Spectrum-X mission, which provided important lessons on polarimeter design, systematic effects, and the programmatics of a shared focal plane. We conclude with a description of the Imaging X-ray Polarimetry Explorer (IXPE) and its prospective scientific return. IXPE, a partnership between NASA and ASI, has been selected as a NASA Astrophysics Small Explorers Mission and is currently scheduled to launch in April of 2021.

Keywords: X-ray astronomy; X-ray polarimetry; astrophysics

1. Introduction Polarization measurements uniquely probe physical anisotropies—ordered magnetic fields, aspheric matter distributions, and general relativistic coupling to black-hole spin. Polarization measurements in X-ray astronomy add an important, and relatively unexplored, dimension to the parameter space for investigating cosmic X-ray sources and processes and for using extreme astrophysical environments as laboratories for fundamental physics. Specifically, the degree of polarization and the position angle of the electric vector on the sky depend on the conditions under which the X-rays are produced. Therefore, modelling of the physical processes and the geometry must be consistent with the observed polarization. Despite this, only a few experiments have conducted unambiguous measurements of the linear polarization of sources of X-rays. We review the history of X-ray polarimetry, concentrating primarily on the author’s direct participation in such ventures, and describe the results and the experimental techniques. We conclude with a brief description of the Imaging X-ray Polarimetry Explorer (IXPE), a joint venture involving NASA and the Italian Space Agency (ASI), to be launched as a NASA Small Explorer mission in early 2021.

2. Background—Prior to IXPE

2.1. The First Sounding Rocket Experiments The ﬁrst sounding rocket ﬂown in an attempt to measure the linear polarization of a non-solar source of X-rays was launched in July of 1968 by the group at Columbia University under the direction of Professor Robert Novick. The target was Sco X-1. A schematic of the payload is shown in Figure1. The polarimeter exploited the polarization dependence of Compton scattering using lithium as a scattering material. The payload was rotated around the line of sight to mitigate systematic effects, e.g., those imposed by the geometry.

Galaxies 2018, 6, 33; doi:10.3390/galaxies6010033 www.mdpi.com/journal/galaxies Galaxies 2018, 6, x FOR PEER REVIEW 2 of 15 Galaxies 2018, 6, 33 2 of 15 as a scattering material. The payload was rotated around the line of sight to mitigate systematic effects, e.g., those imposed by the geometry.

Figure 1. (a) A single polarimeter element consisting of a block of lithium surrounded by four Figure 1. (a) A single polarimeter element consisting of a block of lithium surrounded by four proportional counters; (b) schematic of the rocket payload, side view; and (c) schematic of the rocket proportionalpayload, counters; front view. (b) schematic of the rocket payload, side view; and (c) schematic of the rocket payload, front view. There are a number of considerations involved in the design of the scattering polarimeter and these considerations, by and large, apply to all X-ray polarimeters. Of course the primary consideration Thereis areto achieve a number high of sensitivity considerations to a polarized involved signal. in theWe designintroduced of thethe scatteringconcept of the polarimeter minimum and these considerations,detectable by polarization and large, (MDP) apply at to the all 99% X-ray conf polarimeters.idence level as the Of parameter course theto consider: primary consideration is to achieve high sensitivity to a polarizedMDP99(%) = signal. [4.29 × 10 We4/M(%)] introduced × [(Rs + RB the)/(RS concept2 × t)]1/2 of the minimum detectable polarization (MDP)The MDP at is the the 99%degree conﬁdence of polarization level detected as the at parameterthe 99% confidence to consider: level independent of the position angle. M is the variation in the signal as a function of the azimuthal angle produced in the 2 1/2 instrument byMDP a 100%99 polarized(%) = [4.29 signal× with104/ RMB =(%)] 0, where× [( RRsB is+ theRB background)/(RS × tcounting)] rate. RS is the source counting rate, and t is the integration time. The MDPThe is lithium the degree scattering of polarization polarimeter is detectedoften referred at the to as 99% a Thomson conﬁdence scattering level polarimeter, independent of the position angle.becauseM theis Thomsom the variation cross-section: in the signal as a function of the azimuthal angle produced in the 222 2 2 2 instrument by a 100% polarizedddσϑ signal//coscossinΩ= with()( emcRB = 0, where RϕϕB +is the background ) counting rate. RS is the source counting rate, and t is the integration time. The lithiumprovides a scattering fairly accurate polarimeter approximation is to often the angular referred dependence to as a of Thomson the scattered scattering photon. Here polarimeter, theta is the polar scattering angle and phi is the azimuthal angle measured from the initial because thepolarization Thomsom direction. cross-section: Of course, for bound electrons, one must account for both coherent and incoherent scattering and, for this experiment, photoelectric absorption: 2 dσ/dΩ = e2/mc2 cos2 ϑ cos2 ϕ + sin2 ϕ provides a fairly accurate approximation to the angular dependence of the scattered photon. Here theta is the polar scattering angle and phi is the azimuthal angle measured from the initial polarization direction. Of course, for bound electrons, one must account for both coherent and incoherent scattering and, for this experiment, photoelectric absorption:

dσcoh 2 2 2 2 2 dΩ = r0hcos ϑ cos ϕ + sin ϕi|F| dσcoh 2 2 2 2 dΩ = r0hcos ϑ cos ϕ + sin ϕiI

Here, F and I are the relevant atomic scattering factors and are, themselves, functions of sin(θ/2)/λ. Figure2 compares the Thomson approximation to actual angular distributions. Galaxies 2018, 6, x FOR PEER REVIEW 3 of 15

dσ 2 coh = rF22cosϑϕ cos 2+ sin 2 ϕ dΩ 0

dσ coh = rI22cosϑϕ cos 2+ sin 2 ϕ dΩ 0 Galaxies 2018, 6, 33 3 of 15 Here, F and I are the relevant atomic scattering factors and are, themselves, functions of sin(θ/2)/λ. Figure 2 compares the Thomson approximation to actual angular distributions.

Cos (polar scattering angle) Azimuthal scattering angle (degrees)

Figure 2. Solid lines are the Thomson approximation and the dots allow for coherent and incoherent Figure 2. Solid lines are the Thomson approximation and the dots allow for coherent and incoherent scattering angles and photoelectric absorption for the lithium blocks. (Left) Cosine of the polar scattering anglesscattering and angle; photoelectric and (right) The absorption azimuthal forscattering the lithium angle. blocks. (Left) Cosine of the polar scattering angle; and (right) The azimuthal scattering angle. This first sounding rocket flight made no detection, providing an upper limit of the order of 20% to the degree of polarization. One needed detectors more sensitive to polarization, especially within This firstthe soundingobserving time rocket limitations flight madeof a sounding no detection, rocket flight. providing This led an the upper group limitto consider of the Bragg order of 20% to the degreereflection of polarization. from crystals One set neededat 45 degrees detectors with re morespect to sensitive the incident to beam polarization, [1]. Bragg especiallycrystals are within the observing timeinherently limitations narrow-band of a sounding devices and, rocket so, the flight. limited This throughput led the might group at tofirst consider be considered Bragg asreflection an from obstacle to the advantages provided by a polarimeter with a 100% modulation factor. There are, as crystals setwe at 45learned, degrees ideally with imperfect respect or to mosaic the incident crystals beamthat optimize [1]. Bragg the throughput. crystals are The inherently next rocket narrow-band devices and,polarimeter, so, the limitedwhich included throughput mosaic graphite might atcrystal first polarimeters be considered sensitive as in an the obstacle first-order to at the 2.6 advantages provided bykeV, a polarimeter is schematically with shown a100% in Figure modulation 3. Figure 4 is factor. a photograph There of are, the aspayload we learned, and the individuals ideally imperfect or involved with its success. mosaic crystals that optimize the throughput. The next rocket polarimeter, which included mosaic graphite The flight was quite successful, despite the fact that one of the doors did not open and the crystal polarimeterspayload sank sensitive into the water in the off first-order the coast of at Wallops 2.6 keV, Island, is schematically and so the film-bearing shown aspect in Figure camera3. Figure4 is a photographcould of not the be payload recovered. and The the target individuals was the Crab involved Nebula and with its itspulsar success. and the experiment provided The flightthe first was detection quite successful, of the X-ray despitepolarization the from fact thatthis system one of [2], the confirming doors did the not synchrotron open and origin the of payload sank the X-rays. Combining the data from both types of polarimeters led to a result of p = 15 ± 5% at a into the waterposition off the angle coast of 156 of ± Wallops 10° measured Island, east and of north. so the film-bearing aspect camera could not be recovered. The target was the Crab Nebula and its pulsar and the experiment provided the first detection of the X-ray polarization from this system [2], confirming the synchrotron origin of the X-rays. Combining the data from both types of polarimeters led to a result of p = 15 ± 5% at a position angle of 156 ± 10◦ measured east of north. Galaxies 2018, 6, x FOR PEER REVIEW 4 of 15

Figure 3. Schematic of sounding rocket 17.09 which featured both lithium scattering polarimeters in Figure 3. Schematicthe forward of section sounding of the rocketpayload 17.09and four which mosaic featured graphite bothcrystallithium Bragg polarimeters, scattering each polarimeters in the forward sectionmounted ofat thean average payload angle and of 45 four degrees mosaic to the inci graphitedent flux crystal and featured Bragg proportional polarimeters, counters each as mounted at detectors. The doors opened once the payload achieved altitude. an average angle of 45 degrees to the incident flux and featured proportional counters as detectors. The doors opened once the payload achieved altitude.

Figure 4. Rocket 17.09 and the science team. From left to right, Prof. Robert Novick, Director of the Columbia Astrophysics laboratory, Gabriel Epstein, graduate student for the crystal polarimeters, Prof. Martin C. Weisskopf, lead for the crystal polarimeters, Prof. Richard Wolff, lead for the lithium scattering polarimeters, and Richard Linke, graduate student for the lithium polarimeters.

2.2. The OSO-8 Polarimeter The next major step forward for X-ray polarimetry was the inclusion of two graphite crystal polarimeters aboard the Orbiting Solar Observatory (OSO-8) [3]. Figure 5 is a photograph of the engineering model of one of the crystal panels. Observation of the Crab, with the contamination of the pulsar removed, yielded a highly significant measurement of p = 19 ± 1% at a position angle of 156.4 ± 1.4° [4] measured east of north, satisfyingly in agreement with the previous measurement and leaving no doubt as to the synchrotron origin of the X-rays.

Galaxies 2018, 6, x FOR PEER REVIEW 4 of 15

Figure 3. Schematic of sounding rocket 17.09 which featured both lithium scattering polarimeters in the forward section of the payload and four mosaic graphite crystal Bragg polarimeters, each Galaxiesmounted2018 at, 6 ,an 33 average angle of 45 degrees to the incident flux and featured proportional counters4 of as 15 detectors. The doors opened once the payload achieved altitude.

Figure 4. Rocket 17.09 and the science team. From left to right, Prof. Robert Novick, Director of the Figure 4. Rocket 17.09 and the science team. From left to right, Prof. Robert Novick, Director of the Columbia Astrophysics laboratory, Gabriel Epstein, graduate student for the crystal polarimeters, Columbia Astrophysics laboratory, Gabriel Epstein, graduate student for the crystal polarimeters, Prof.Prof. Martin Martin C. Weisskopf, C. Weisskopf, lead lead for for the the crystal crystal pola polarimeters,rimeters, Prof. Prof. Richard Richard Wolff, Wol leadff, lead for thefor lithiumthe lithium scatteringscattering polarimeters, polarimeters, and and Richard Richard Linke, Linke, gr graduateaduate studentstudent for for the the lithium lithium polarimeters. polarimeters.

2.2. The2.2. OSO-8 The OSO-8 Polarimeter Polarimeter The Thenext next major major step step forward forward for for X-ray X-ray polarime polarimetrytrywas was the the inclusion inclusion of twoof two graphite graphite crystal crystal polarimeterspolarimeters aboard aboard the the Orbiting Orbiting Solar Solar Observator Observatoryy (OSO-8)(OSO-8) [ 3[3].]. Figure Figure5 is 5 ais photograph a photograph of the of the engineeringengineering model model of ofone one of ofthe the crystal crystal panels. panels. ObservationObservation of of the the Crab, Crab, with with the the contamination contamination of of the pulsarthe pulsar removed, removed, yielded yielded a ahighly highly significant signiﬁcant measurementmeasurement of ofp =p 19= 19± 1%± 1% at aat position a position angle angle of of ± ◦ 156.4156.4 ± 1.4° [4]1.4 measured[4] measured east east of ofnorth, north, satisfyingly satisfyingly in in agreement with with the the previous previous measurement measurement and and leaving no doubt as to the synchrotron origin of the X-rays. leaving no doubt as to the synchrotron origin of the X-rays. More recently we were able to check the likelihood that the X-ray measurements of the position angle were on track by calculating the average optical position angle from recent measurements with the Hubble Space Telescope (HST) [5] as a function of the radius of the region surrounding the pulsar. The results are shown in Figure6 and the mean optical position angle is consistent with the OSO-8 X-ray measurements when the nebular size is in the range of 32–38 arcseconds, in excellent agreement with the size observed with the Chandra X-ray Observatory [6]. The OSO-8 satellite featured a number of different instruments, many with different viewing directions. This naturally resulted in an observing program where no one received the amount of observing time that they fully desired. This is generally a problem for instruments that are on an observatory that require long integration times to achieve their objectives, such as spectroscopy or polarimetry. In these cases the signal that one wishes to measure is typically a small fraction of the total ﬂux, even ignoring the background. The limited number of experiments performed with the OSO-8 polarimeter, in general, yielded upper limits (e.g., [7,8]) with the exception of very marginal detections of polarization from Cygnus X-2 [9] and Cygnus X-1 [10]. Galaxies 2018, 6, x FOR PEER REVIEW 5 of 15

More recently we were able to check the likelihood that the X-ray measurements of the position angle were on track by calculating the average optical position angle from recent measurements with the Hubble Space Telescope (HST) [5] as a function of the radius of the region surrounding the pulsar. The results are shown in Figure 6 and the mean optical position angle is consistent with the OSO-8 X-ray measurements when the nebular size is in the range of 32–38 arcseconds, in excellent agreement with the size observed with the Chandra X-ray Observatory [6]. The OSO-8 satellite featured a number of different instruments, many with different viewing directions. This naturally resulted in an observing program where no one received the amount of observing timeGalaxies that 2018 they, 6, x FOR fully PEER REVIEWdesired. This is generally a problem for instruments5 of 15that are on an observatory that Morerequire recently long we wereintegration able to check times the likelih to achieveood that the their X-ray objectives,measurements suchof the position as spectroscopy or polarimetry. Inangle these were caseson track the by calculating signal that the average one wish opticales position to measure angle from is recent typicall measurementsy a small with fraction of the the Hubble Space Telescope (HST) [5] as a function of the radius of the region surrounding the pulsar. total flux, evenThe ignoring results are shownthe background. in Figure 6 and the The mean lim optiitedcal positionnumber angle of is experimentsconsistent with the performed OSO-8 with the OSO-8Galaxies polarimeter,2018X-ray, 6, 33 measurements in general, when yiel theded nebular upper size is limitsin the range (e.g., of 32–38 [7,8]) arcseconds, with the in excellent exception agreement of very 5marginal of 15 detections of polarizationwith the size observed from with Cygn the Chandraus X-2 X-ray[9] and Observatory Cygnus [6]. X-1 [10]. The OSO-8 satellite featured a number of different instruments, many with different viewing directions. This naturally resulted in an observing program where no one received the amount of observing time that they fully desired. This is generally a problem for instruments that are on an observatory that require long integration times to achieve their objectives, such as spectroscopy or polarimetry. In these cases the signal that one wishes to measure is typically a small fraction of the total flux, even ignoring the background. The limited number of experiments performed with the OSO-8 polarimeter, in general, yielded upper limits (e.g., [7,8]) with the exception of very marginal detections of polarization from Cygnus X-2 [9] and Cygnus X-1 [10].

Figure 5. Photograph of the engineering model of the OSO-8 graphite crystal panel. Figure 5. Photograph of the engineering model of the OSO-8 graphite crystal panel. Figure 5. Photograph of the engineering model of the OSO-8 graphite crystal panel.

Figure 6. Comparison of the position angle as measured in the optical with the OSO-8 result. The red Figure 6. Comparisonline is the optical of polarization. the position The angle black line as measuredis the OSO-8in result the with optical errors with shown the in OSO-8blue. result. The red line is the optical polarization. The black line is the OSO-8 result with errors shown in blue. 2.3. The Stellar X-Ray Polarimeter 2.3.Figure The 6. Stellar ComparisonThe X-Ray next Polarimetermajor of the step position for X-ray polarimetryangle as measured was the advent in the of the optical Stellar X-raywith Polarimeterthe OSO-8 (SXRP). result. The red Funded by NASA and Italy, this instrument was designed to fly on the Russian mission Spectrum line isThe the next Roentgen-Gamma.optical major polarization. step for The X-ray polarimeter The polarimetry black was line to isbe was the mount theOSO-8ed advent on resulta slide of thewithand,Stellar occasionally, errors X-ray shown placed Polarimeter in atblue. the (SXRP). Funded by NASA and Italy, this instrument was designed to ﬂy on the Russian mission Spectrum 2.3. TheRoentgen-Gamma. Stellar X-Ray Polarimeter The polarimeter was to be mounted on a slide and, occasionally, placed at the focusThe next of a Danishmajor step foil X-rayfor X-ray telescope. polarimetry The left-hand was the side advent of Figure of the7 is Stellar a schematic X-ray ofPolarimeter SXRP and the(SXRP). Fundedright-hand by NASA side isand a photograph Italy, this ofinstrument the completed was device. designed The to device fly on has the been Russian described mission in a number Spectrum of publications (e.g., [11,12]). Roentgen-Gamma. The polarimeter was to be mounted on a slide and, occasionally, placed at the Unfortunately the collapse of the Soviet Union in 1989 lead to the cancellation of the mission.

SXRP also would have potentially suffered from being placed on an observatory—only 11 days of observing were assigned to SXRP for the ﬁrst year of operation. We should also mention, for potential future renditions of this design, a systematic effect arises when placing a scattering polarimeter at the focus of an X-ray telescope [13]. Galaxies 2018, 6, x FOR PEER REVIEW 6 of 15 focus of a Danish foil X-ray telescope. The left-hand side of Figure 7 is a schematic of SXRP and the right-hand side is a photograph of the completed device. The device has been described in a number of publications (e.g., [11,12]). Unfortunately the collapse of the Soviet Union in 1989 lead to the cancellation of the mission. SXRP also would have potentially suffered from being placed on an observatory—only 11 days of observing were assigned to SXRP for the first year of operation. We should also mention, for potential futureGalaxies renditions2018, 6, 33 of this design, a systematic effect arises when placing a scattering polarimeter at6 the of 15 focus of an X-ray telescope [13].

Figure 7. (Left) Concept of the SXRP. Incident X-rays focused from the telescope converge onto a thin Figure 7. (Left) Concept of the SXRP. Incident X-rays focused from the telescope converge onto a thin graphite crystal. Higher-energy X-rays pass through the crystal and are scattered by a block of graphite crystal. Higher-energy X-rays pass through the crystal and are scattered by a block of beryllium- beryllium-covered lithium. The diffracted and scattered X-rays are detected by imaging proportional covered lithium. The diffracted and scattered X-rays are detected by imaging proportional counters; counters; (right) photograph of the completed device. (right) photograph of the completed device. 2.4. Electron Tracking 2.4. Electron Tracking The next step in the development of detectors for X-ray polarimetry was based on the realization The next step in the development of detectors for X-ray polarimetry was based on the realization that that the polarimeter must be capable of imaging because a large subset of astronomical X-ray sources, the polarimeter must be capable of imaging because a large subset of astronomical X-ray sources, such as such as supernova remnants, are extended. Moreover, as noted in the case of SXRP, many of supernova remnants, are extended. Moreover, as noted in the case of SXRP, many of polarimeters are prone polarimeters are prone to systematic effects which would necessarily complicate the data analysis to systematic effects which would necessarily complicate the data analysis and, possibly, the interpretation and, possibly, the interpretation of the results. In the early 1990s, Austin and Ramsey [14] of the results. In the early 1990s, Austin and Ramsey [14] demonstrated an optical imaging chamber for demonstrated an optical imaging chamber for measuring the details of the tracks produced by the measuring the details of the tracks produced by the photoelectron in the gas of a proportional counter. Since photoelectron in the gas of a proportional counter. Since the direction of the primary photoelectron the direction of the primary photoelectron released in the photoelectric absorption of the incident photon released in the photoelectric absorption of the incident photon depends on the polarization of the depends on the polarization of the incident beam, imaging the track provides information as to the degree of incident beam, imaging the track provides information as to the degree of polarization and the polarization and the position angle of the incident flux. To the extent that the detector pixels were extremely position angle of the incident flux. To the extent that the detector pixels were extremely small small compared to other characteristic lengths, this detector was devoid of systematic effects. A schematic of compared to other characteristic lengths, this detector was devoid of systematic effects. A schematic the device is shown in Figure8 and an image of an electron track is shown in Figure9. The track in Figure9 of the device is shown in Figure 8 and an image of an electron track is shown in Figure 9. The track emphasizes the importance of the track-recognition algorithms as the majority of charge appears at the end in Figure 9 emphasizes the importance of the track-recognition algorithms as the majority of charge of the track where it has lost all sense of the initial direction of the primary photoelectron. Another obvious appears at the end of the track where it has lost all sense of the initial direction of the primary advantage of this device for imaging, at least over an imaging proportional counter, is the higher precision photoelectron. Another obvious advantage of this device for imaging, at least over an imaging associated with the detection of the position of the initial interaction. proportionalGalaxies counter, 2018, 6, x FORis the PEER higher REVIEW precision associated with the detection of the position of7 of the 15 initial interaction.

Figure 8. Schematic of the optical imaging chamber. Figure 8. Schematic of the optical imaging chamber. A major advance in electron tracking was made by Costa, Bellazzini, and colleagues utilizing a gas electron multiplier and an ASIC for the readout [15–17]. The subsequent refinement and development of this detector forms the basis for IXPE.

Figure 9. The electron track produced by a 54 keV X-ray as imaged with the optical imaging chamber. The detector gas was a mixture of 2 atmospheres of argon (90%), methane (5%), and trimethylamine (5%). The track is about 14 mm in length. The blob at the top-right is the Auger electron cloud produced by the initial ionization.

3. IXPE IXPE is a collaboration between NASA and ASI, the Italian Space Agency. The institutions and a brief summary of their roles is shown in Figure 10. The current list of co-investigators is given in Table 1. The list of collaborators at the time of selection is given in Table 2. The IXPE science working group, co-chaired by the project scientist, S. L. O’Dell, and Prof. G. Matt is currently establishing the procedures by which participation in IXPE will be extended beyond that listed in these tables.

Galaxies 2018, 6, x FOR PEER REVIEW 7 of 15

Galaxies 2018, 6, 33 Figure 8. Schematic of the optical imaging chamber. 7 of 15

A major advance in electron tracking was made by Costa, Bellazzini, and colleagues utilizing a gas electronA major multiplier advance inand electron an trackingASIC for was the made readout by Costa, [15–17]. Bellazzini, The and subsequent colleagues utilizingrefinement a gas and developmentelectron multiplier of this detector and an ASIC forms for the the basis readout for [15 IXPE.–17]. The subsequent reﬁnement and development of this detector forms the basis for IXPE.

FigureFigure 9. The 9. The electron electron track track produced produced by by a a 54 54 keV keV X-rayX-ray asas imaged imaged with with the the optical optical imaging imaging chamber. chamber. The Thedetector detector gas gas was was a mixture a mixture of of 2 2 atmospheres atmospheres ofofargon argon (90%), (90%), methane methane (5%), (5%), and and trimethylamine trimethylamine (5%).(5%). The The track track is is about about 1414 mm mm in length.in length. The blobThe atblob the top-rightat the top-right is the Auger is electronthe Auger cloud electron produced cloud by the initial ionization. produced by the initial ionization.

3. IXPE 3. IXPE IXPE is a collaboration between NASA and ASI, the Italian Space Agency. The institutions and aIXPE brief is summary a collaboration of their roles between is shown NASA in Figure and ASI, 10. Thethe currentItalian listSpace of co-investigatorsAgency. The institutions is given in and a briefTable summary1. The list of of their collaborators roles is atshown the time in ofFigure selection 10. isThe given current in Table list2. of The co-investigators IXPE science working is given in Tablegroup, 1. The co-chaired list of collaborators by the project at scientist, the time S. of L. selection O’Dell, and is given Prof. G. in MattTable is 2. currently The IXPE establishing science working the group,procedures co-chaired by which by the participation project scientist, in IXPE S. will L. O’Dell, be extended and beyondProf. G. that Matt listed is currently in these tables. establishing the proceduresThe by current which plans participation for IXPE involve in IXPE a Pegasus will be XL extended launch from beyond Kwajalein that Islandlisted inin early these 2021. tables. IXPE will be placed into a nominal orbit of at least 540 km at 0◦ inclination, thus minimizing interaction with the radiation belts. The baseline mission is for two years with at least a one-year extension. ASI is providing the ground station at Malindi and IXPE will use the station at Singapore as a backup. Mission operations will be conducted by Colorado University’s Laboratory for Atmospheric and Space Physics (LASP). Science operations will take place at the Marshall Space Flight Center. Figure 11 shows IXPE, once the boom that establishes the in-flight focal length of nominally 4-m, is deployed. Major elements of the payload and their location are identified in the figure. In what follows, we will concentrate on the optics and the polarization-sensitive detectors. The metrology camera and light emitting diodes (LEDs) are present to measure any alignment changes that might take place during launch and flight. The tip/tilt/rotate mechanism is present to readjust the alignment if necessary. Galaxies 2018, 6, x FOR PEER REVIEW 8 of 15

The current plans for IXPE involve a Pegasus XL launch from Kwajalein Island in early 2021. Galaxies 2018, 6, x FOR PEER REVIEW 8 of 15 IXPE will be placed into a nominal orbit of at least 540 km at 0° inclination, thus minimizing interactionThe with current the plansradiation for IXPE belts. involve The baselinea Pegasus mission XL launch is forfrom two Kwajalein years withIsland at in least early a 2021. one-year extension.IXPE willASI beis providingplaced into the a nominalground stationorbit of atat Malindileast 540 and km IXPEat 0° willinclination, use the thusstation minimizing at Singapore as a interactionbackup. Missionwith the radiationoperations belts. will The be baseline conduc missionted by is forColorado two years University’s with at least Laboratory a one-year for Atmosphericextension. and ASI isSpace providing Physics the ground(LASP). station Science at Malindi operations and IXPEwill willtake use place the stationat the atMarshall Singapore Space Flightas Center. a backup. Mission operations will be conducted by Colorado University’s Laboratory for FigureAtmospheric 11 shows and SpaceIXPE, Physicsonce the (LASP). boom Sciencethat establishes operations the will in-flight take place focal at thelength Marshall of nominally Space 4- Flight Center. m, is deployed. Major elements of the payload and their location are identified in the figure. In what Figure 11 shows IXPE, once the boom that establishes the in-flight focal length of nominally 4- follows,m, is we deployed. will concentrate Major elements on the of the optics payload and and the th polarization-sensitiveeir location are identified detectors. in the figure. The In metrology what camerafollows, and lightwe will emitting concentrate diodes on (LEDs)the optics are and present the polarization-sensitive to measure any alignment detectors. changes The metrology that might take placecamera during and light launch emitting and diodesflight. The(LEDs) tip/tilt/rotate are present mechanismto measure any is presentalignment to changesreadjust that the mightalignment Galaxiesif necessary.take2018 place, 6 , 33 during launch and flight. The tip/tilt/rotate mechanism is present to readjust the alignment8 of 15 if necessary.

Figure 10. Major institutions involved in IXPE. Abbreviations: PI = principal investigator, SE = systems Figureengineering, 10. Major institutionsS&MA = safety involved and mission in IXPE. assurance, Abbreviations:Abbreviati I&T =ons: integration PI = principal and test. investigator, SESE == systems engineering, S&MA == safety and mission assurance, I&T = integration and test.

Figure 11. Schematic rendering of IXPE deployed with the major elements identified.

Figure 11. Schematic rendering of IXPEIXPE deployed with the major elements identiﬁed.identified.

Table 1. IXPE co-Investigators and roles.

L. Baldini Co-I and PI for INFN/Pisa R. Bellazzini Senior Co-I E. Costa Senior Co-I R. Elsner Co-I V. Kaspi Co-I and co-chair of the SWG J. Kolodziejczak Co-I L. Latronico Co-I and PI for INFN/Pisa H. Marshall Co-I G. Matt Co-I and extragalactic theory lead F. Mulieri Co-I S.L. O’Dell Co-I and Project scientist B.D. Ramsey Deputy PI R.W. Romani Co-I and galactic theory lead P. Sofﬁtta Co-I and Italian PI A.Tennant Co-I Galaxies 2018, 6, 33 9 of 15

Table 2. IXPE collaborators.

W. Baumgartner L. Pacciani A. Brez G. Pavlov N. Bucciantini M. Pesce-Rollins E. Churazov P.-O. Petrucci S. Citrano M. Pinchera E. Del Monter J. Poutanen N. Di Lalla M. Razzano I. Donnarumma A. Rubini M. Dovˇciak M. Salvati Y. Evangelista C. Sgrò S. Fabiani F. Spada R. Goosmann G. Spandre S. Gunji L. Stella V. Karas R. Sunyaev M. Kuss R. Taverna A. Manfreda R. Turolla F. Marin K. Wu M. Minuti S. Zane N. Omodei D. Zanetti

3.1. The IXPE Optics The IXPE optics are NiCo mirror shells fabricated at MSFC using electroforming replication (see e.g., [18,19]). The design is illustrated in Figure 12. A single rigid spider is used to support 24 nested mirror shells that comprise one of three identical mirror module assemblies (MMAs). Mounting combs, as shown in the ﬁgure, provide shell attachment points. The light-weight housing is primarily for thermal control. A rear spider is also present, but is only used to limit shell vibrations during launch. The overall properties of the IXPE MMAs are summarized in Table3. Galaxies 2018, 6, x FOR PEER REVIEW 10 of 15

Figure 12. Schematic of a single IXPE mirror module assembly (MMA). (a) Entire MMA; (b) close-up Figure 12. ofSchematic the front spider; of a singleand (c) close-up IXPE mirror of an alignment module comb. assembly (MMA). (a) Entire MMA; (b) close-up of the front spider; and (c) close-up of an alignment comb. Table 3. MMA properties.

Parameter Value Number of mirror modules 3 Number of shells per mirror module 24 Focal length 4001 mm Total shell length 600 mm Range of shell diameters 162–272 mm Range of shell thicknesses 0.18–0.25 mm Shell material Electroformed nickel–cobalt alloy Effective area per mirror module with thermal shields 209 cm2 (@ 2.3 keV); >230 cm2 (3–6 keV) Angular resolution (HPD) ≤25 arcsec Field of view (detector limited) 12.9 arcmin square

3.2. The IXPE Detectors A schematic of the IXPE detector is shown in Figure 13 and characteristics of each of the three detectors are given in Table 4. Figure 14 illustrates two extremely important characteristics of these detectors, namely the measurements of the modulation factor, µ, and verification of the absence of systematic effects to high precision.

Galaxies 2018, 6, 33 10 of 15

Table 3. MMA properties.

3.2. The IXPE Detectors A schematic of the IXPE detector is shown in Figure 13 and characteristics of each of the three detectors are given in Table4. Figure 14 illustrates two extremely important characteristics of these detectors, namely the measurements of the modulation factor, µ, and veriﬁcation of the absence of systematicGalaxies 2018, effects6, x FORto PEER high REVIEW precision. 11 of 15

Figure 13.13. Schematic of the IXPEIXPE detector.detector. Abbreviations:Abbreviations: GEM = gas electron multiplier;multiplier; ADC == analog to digital convertor; DME = dimethyl ether.

Table 4.4. Detector parameters. Parameter Value SensitiveParameter area 15 Value mm × 15 mm Fill gas andSensitive composition area He/DME 15 mm × (20/80)15 mm @ 1 atm DetectorFill gas andwindow composition He/DME 50-µm (20/80) thick @beryllium 1 atm Detector window 50-µm thick beryllium AbsorptionAbsorption and anddrift drift region region depth depth 10 mm 10 mm GEM GEM(gas electron (gas electron multiplier) multiplier) copper-plated copper-plated 50- µ50-µmm liquid-crystal liquid-crystal polymer polymer GEMGEM hole hole pitch pitch 50 50µm µm triangular triangular lattice lattice NumberNumber ASIC ASIC readout readout pixels pixels 300 300× 352 × 352 ASIC pixelated anode Hexagonal @ 50-µm pitch ASIC pixelated anode Hexagonal @ 50-µm pitch Spatial resolution (FWHM) ≤123 µm (6.4 arcsec) @√ 2 keV SpatialEnergy resolution resolution (FWHM) (FWHM) 0.54≤123 keV µm @ (6.4 2 keV arcsec) (∝ E @) 2 keV Energy resolution (FWHM) 0.54 keV @ 2 keV (∝ √E)

Figure 14. (Left) The modulation factor, µ, as a function of energy. The solid line is based on Monte- Carlo simulations. The colored symbols are based on measurements at different times. The dotted lines are simple linear connections; (upper right) Measurements of a 99%-polarized source at 3.7 keV; (lower right) Measurements of an unpolarized source at 5.9 keV.

Galaxies 2018, 6, x FOR PEER REVIEW 11 of 15

Figure 13. Schematic of the IXPE detector. Abbreviations: GEM = gas electron multiplier; ADC = analog to digital convertor; DME = dimethyl ether.

Table 4. Detector parameters.

Parameter Value Sensitive area 15 mm × 15 mm Fill gas and composition He/DME (20/80) @ 1 atm Detector window 50-µm thick beryllium Absorption and drift region depth 10 mm GEM (gas electron multiplier) copper-plated 50-µm liquid-crystal polymer GEM hole pitch 50 µm triangular lattice Number ASIC readout pixels 300 × 352 ASIC pixelated anode Hexagonal @ 50-µm pitch Galaxies 2018, 6Spatial, 33 resolution (FWHM) ≤123 µm (6.4 arcsec) @ 2 keV 11 of 15 Energy resolution (FWHM) 0.54 keV @ 2 keV (∝ √E)

Figure 14. (Left) The modulation factor, µ, as a function of energy. The solid line is based on Monte- Figure 14. (Left) The modulation factor, µ, as a function of energy. The solid line is based on Carlo simulations. The colored symbols are based on measurements at different times. The dotted Monte-Carlo simulations. The colored symbols are based on measurements at different times. The dotted lines are simple linear connections; (upper right) Measurements of a 99%-polarized source at 3.7 keV; lines are simple linear connections; (upper right) Measurements of a 99%-polarized source at 3.7 keV; (lower right) Measurements of an unpolarized source at 5.9 keV. (lower right) Measurements of an unpolarized source at 5.9 keV.

3.3. The IXPE Science IXPE opens a new window on the X-ray universe—imaging X-ray polarimetry. This provides powerful and unique capabilities as compared to all other current X-ray astronomy missions. IXPE, when compared to the polarimeters of OSO-8, will be able to reach the same sensitivity, but with an observing time that is a factor of 100 lower. Moreover, IXPE is the only observatory that can provide, simultaneously, spectral, imaging, timing, and polarization data from the sources it observes. This capability allows one to address key questions in astronomy and astrophysics and provide new scientiﬁc results and constraints for theoretical models. Amongst these questions are:

• What is the spin of an accreting stellar mass black hole? • What are the geometry and magnetic ﬁeld strength in magnetars? • Was the black hole at the center of the Milky Way an active galactic nucleus in the recent past? • What is the magnetic ﬁeld structure in synchrotron X-ray sources? • What are the geometries and origins of X-rays from both isolated and accreting pulsars?

For our first example of IXPE science we consider measuring black hole spin in the black hole-induced twisted space-time. The example is GRX1915+105, a micro-quasar in a binary system in an accretion-dominated state. In this case a scattering disk forms about the black hole and is fed by material from the companion star. Scattering polarizes the thermal disk emission and the twisted space-time rotates the polarization depending on the distance of the photon from the black hole. The rotation of the polarization vector is more dominant at high energies as the inner portions of the disk are hotter, producing higher energy X-rays. The impact on the polarization depends heavily on the spin of the black hole as shown in Figure 15 which shows the variation of the degree of polarization and the position angle (relative to an arbitrary zero) as a function of energy for different assumed values of the spin parameter, a. The figure also shows the results of fitting these curves assuming a 200 ksec observation with IXPE. Including priors on the disk orientation, the spin parameter can be measured to the following precision depending on the spin: a = 0.50 ± 0.04, 0.900 ± 0.008, 0.99800 ± 0.0003. Another fascinating example of the potential of IXPE is illustrated in Figure 16 where we show the results of pulse-phase polarimetry for the 11 s pulsating magnetar 1RXS J170849.0-400910, under two conditions: (1) ignoring the effects of quantum electrodynamics (QED) in modeling the source of the pulsed Galaxies 2018, 6, 33 12 of 15 emission in a super-strong magnetic field; or (2) accounting for QED. A simple light curve of intensity versus pulse phase cannot distinguish these two cases; the pulse-phase polarimetry clearly can. Supernova remnants and pulsar wind nebulae are obvious candidates for imaging polarimetry and many such observations will be part of the IXPE observing plan. A fascinating example of exploiting the imaging capability involves observation of the region near the fairly small (only a few million solar masses) black hole, SgR A*, at the center of our galaxy. The experiment is illustrated in Figure 17. The molecular clouds shown will reflect X-rays if emitted from SgR A*. Due to the distances involved the reflected X-rays will have been emitted by the black hole several hundred years ago. Since the scattering angle is approximately 90 degrees, the position angle of the polarized flux will point back towards SgR A* identifying it as the source of X-rays. In this case, fluxes from SgR B2 [20] would imply that SgR A* was a million times more intense just a short few hundred years ago than it is today. We conclude this discussion with the example of using IXPE to image and study the polarization properties of the extragalactic active galaxy, Cen A and its jet. Figure 18 illustrates a 1.5 msec observation and illustrates a number of regions which may be easily resolved. Note that the angular resolution also allows one to simultaneously study two ultra-luminous X-ray sources (one is shown) that are in the full field. Table5 shows the minimum detectable polarization that may be achieved based on this investment of observing time. Note that the table includes the effects of dilution by unpolarized diffuse emission. Galaxies 2018, 6, x FOR PEER REVIEW 13 of 15 This is a very important experiment for understanding the physical processes that form and maintain these X-ray-emitting jets. Radio polarization [21] implies that the field is aligned with the direction of the jet. fieldGalaxies is aligned 2018, 6, xwith FOR PEERthe direction REVIEW of the jet. However, different models for electron acceleration13 of may15 However,predict different modelsdependences for electron in X-rays. acceleration may predict different dependences in X-rays. field is aligned with the direction of the jet. However, different models for electron acceleration may predict different dependences in X-rays.

Figure 15. (Left) Linear polarization versus energy for various black hole spin parameters; (right ) Figure 15. (Left) Linear polarization versus energy for various black hole spin parameters; (right) Position PositionFigure angle 15. (Left with) Linear respect polarization to an arbitrary versus refere energynce. for Dotted various lines black are hole based spin on parameters; model calculations. (right) angle with respect to an arbitrary reference. Dotted lines are based on model calculations. Solid lines are fits SolidPosition lines areangle fits with to the respect data tobase and arbitrary on a 200 refere ksec IXPEnce. Dotted observation. lines are based on model calculations. to the data based on a 200 ksec IXPE observation. Solid lines are fits to the data based on a 200 ksec IXPE observation.

Figure 16. Left to right: flux, degree of polarization, and position angle, all versus the pulse phase for Figure 16. Left to right: flux, degree of polarization, and position angle, all versus the pulse phase for Figure1RXS 16. J170849.0-400910. Left right Blue and red curves represent QED accounted for and not accounted for, 1RXS J170849.0-400910.to : ﬂux, Blue degree and red of curves polarization, represen andt QED position accounted angle, for alland versus not accounted the pulse for, phase respectively. Data points and errors are for a 250-ksec observation. for 1RXSrespectively. J170849.0-400910. Data points Blueand errors and red are curvesfor a 250-ksec represent observation. QED accounted for and not accounted for, respectively. Data points and errors are for a 250-ksec observation.

FigureFigure 17. 17. (Left (Left) Looking) Looking down down on on the the galactic galactic center. TheThe reredd asterisk asterisk is is Sagittarius Sagittarius A* A* and and the the blue blue objectobject to tothe the left left is isa arepresentation representation of of the the molemolecular cloud SgRSgR B2. B2. X-rays X-rays emitted emitted by by SgR SgR A* A* may may be be scatteredscattered to toIXPE IXPE by by SgR SgR B2; B2; ( right(right) )View View ofof thethe galactic centercenter from from IXPE. IXPE. The The square square in inthe the top top right right of ofthe the figure figure represents represents the the size size of of the the IXPEIXPE field of view andand thethe HPD HPD is is the the half-power-diameter half-power-diameter showing that SgR B2 is clearly resolved. The dashed line pointing back to SgR A* represents the showing that SgR B2 is clearly resolved. The dashed line pointing back to SgR A* represents the position angle of the scattered X-rays and the solid lines represent the measured uncertainty. position angle of the scattered X-rays and the solid lines represent the measured uncertainty.

Galaxies 2018, 6, x FOR PEER REVIEW 13 of 15 field is aligned with the direction of the jet. However, different models for electron acceleration may predict different dependences in X-rays.

Figure 15. (Left) Linear polarization versus energy for various black hole spin parameters; (right) Position angle with respect to an arbitrary reference. Dotted lines are based on model calculations. Solid lines are fits to the data based on a 200 ksec IXPE observation.

Figure 16. Left to right: flux, degree of polarization, and position angle, all versus the pulse phase for Galaxies1RXS2018 J170849.0-400910., 6, 33 Blue and red curves represent QED accounted for and not accounted for,13 of 15 respectively. Data points and errors are for a 250-ksec observation.

Figure 17. (Left) Looking down on the galactic center. The red asterisk is Sagittarius A* and the blue Figure 17. (Left) Looking down on the galactic center. The red asterisk is Sagittarius A* and the blue object to the left is a representation of the molecular cloud SgR B2. X-rays emitted by SgR A* may be object to the left is a representation of the molecular cloud SgR B2. X-rays emitted by SgR A* may be scattered to IXPE by SgR B2; (right) View of the galactic center from IXPE. The square in the top right scattered to IXPE by SgR B2; (right) View of the galactic center from IXPE. The square in the top right of of the figure represents the size of the IXPE field of view and the HPD is the half-power-diameter the ﬁgure represents the size of the IXPE ﬁeld of view and the HPD is the half-power-diameter showing showing that SgR B2 is clearly resolved. The dashed line pointing back to SgR A* represents the that SgR B2 is clearly resolved. The dashed line pointing back to SgR A* represents the position angle position angle of the scattered X-rays and the solid lines represent the measured uncertainty. of the scattered X-rays and the solid lines represent the measured uncertainty. Galaxies 2018, 6, x FOR PEER REVIEW 14 of 15

Figure 18. IXPE image of Cen A. The half-power diameter (HPD) is the small circle on the left. An Figure 18. IXPE image of Cen A. The half-power diameter (HPD) is the small circle on the left. An ultra- ultra-luminous source of X-rays (ULX) is identified together with the core of Cen A and a number luminous source of X-rays (ULX) is identiﬁed together with the core of Cen A and a number knots. knots. The minimum detectable polarizations based on a 1.5 msec IXPE observation are given in Table 5. The minimum detectable polarizations based on a 1.5 msec IXPE observation are given in Table5.

Table 5. MDP for different regions of Cen A for 1.5 msec. Table 5. MDP for different regions of Cen A for 1.5 msec. Region MDP99 CoreRegion MDP<7.0%99 Jet Core <7.0%10.9% Knot A +Jet B 10.9%17.6% 17.6% KnotKnot C A + B 16.5% Knot C 16.5% KnotKnot F F 23.5%23.5% KnotKnot G G 30.9%30.9% ULXULX 14.8%14.8%

Conflicts of Interest: The author declares no conflict of interest.

Conﬂicts of Interest: The author declares no conﬂict of interest. References

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