PoGO+ polarimetric constraint on the synchrotron jet emission of Cygnus X-1

Abstract

We report a polarimetric constraint on the hard X-ray synchrotron jet emission from the Cygnus X-1 black-hole binary system. The observational data were obtained using the PoGO+ hard X-ray polarimeter in July 2016, when Cygnus X-1 was in the hard state. We have previously reported that emission from an extended corona with a low polarization fraction is dominating, and that the polarization angle is perpendicular to the disk surface. In the soft gamma-ray regime, a highly-polarized synchrotron jet is reported with INTEGRAL observations. To constrain the polarization fraction and flux of such a jet component in the hard X-ray regime, we now extend analyses through vector calculations in the Stokes QU plane, where the dominant corona emission and the jet component are considered simultaneously. The presence of another emission component with different polarization angle could partly cancel out the net polarization. The 90% upper limit of the polarization fraction for the additional synchrotron jet component is estimated as <10%, <5%, and <5% for polarization angle perpendicular to the disk surface, parallel to the surface, and aligned with the emission reported by INTEGRAL data, respectively. From the 20-180 keV total flux of 2.6 × 10 −8 erg s −1 cm −2 , the upper limit of the polarized flux is estimated as < 3 × 10 −9 erg s −1 cm −2 .

Hard X-ray Polarimetry -- An overview of the method, science drivers and recent findings

“…PoGO+ Friis et al, 2018), a dedicated balloon borne hard X-ray polarimeter found the source to be unpolarized in its hard state (12 − 18 July 2016) in 20-180 keV (Chauvin et al, 2018a(Chauvin et al, , 2019. They placed an upper limit of 5.6 % at position angle of 154±31 • .…”

“…In recent times, there are some interesting polarization measurement reports in hard X-rays from Cadmium Zinc Telluride Imager (CZTI) on board AstroSat satellite (Bhalerao et al, 2017;Rao et al, 2016), balloon borne instrument, PoGO+ , POLAR on board Chinese space laboratory (Produit et al, 2018;Sun et al, 2016), GAP on board IKAROS spacecraft (Yonetoku et al, 2006) for bright hard X-ray sources (Vadawale et al, 2018;Chattopadhyay et al, 2019;Chauvin et al, 2018aChauvin et al, , 2019Chauvin et al, , 2018bZhang et al, 2019;Yonetoku et al, 2011Yonetoku et al, , 2012. These reports along with those from last two decades (McConnell et al, 2002;Coburn & Boggs, 2003;Laurent et al, 2011;Jourdain et al, 2012;Forot et al, 2008;Dean et al, 2008;Moran et al, 2013;Götz et al, 2009Götz et al, , 2013McGlynn et al, 2007bMcGlynn et al, , 2009 have significantly enhanced the interest in the X-ray polarimetry (Marin, 2018).…”

“…There has also been a significant effort in extracting polarization information from instruments which are not specifically meant for polarimetry measurements like RHESSI (McConnell et al, 2002;Coburn & Boggs, 2003;Boggs et al, 2006;McConnell et al, 2007), IN-TEGRAL (Laurent et al, 2011;Jourdain et al, 2012;Dean et al, 2008;Forot et al, 2008), AstroSat (Chattopadhyay et al, 2014a;Vadawale et al, 2015) and Hitomi (Aharonian et al, 2018). Measurements from these experiments in the last two decades have provided some important scientific inputs in understanding emission mechanism and geometry in X-ray pulsars (Vadawale et al, 2018;Forot et al, 2008;Jourdain & Roques, 2019;Chauvin et al, 2018b;Aharonian et al, 2018), magnetic field structure in Pulsar Wind Nebula (PWN) (Vadawale et al, 2018;Dean et al, 2008;Chauvin et al, 2018b), disk-jet interplay in black hole X-ray binaries (XRBs) (Laurent et al, 2011;Jourdain et al, 2012;Chauvin et al, 2018aChauvin et al, , 2019, hard X-ray emission mechanism in GRB prompt emission (Zhang et al, 2019;Chattopadhyay et al, 2019;Yonetoku et al, 2011Yonetoku et al, , 2012Götz et al, 2009), and emission process behind solar flares (McConnell et al, 2007;Suarez-Garcia et al, 2006;Boggs et al, 2006;Zhitnik et al, 2006). Though in some cases, results are marred with large uncertainties and a firm conclusion is not possible, most of these findings are extremely interesting as they pose new challenges to the existing theories.…”

Feasibility of Observing Gamma-ray Polarization from Cygnus X-1 Using a CubeSat

“…The emission between 250 -400 keV, which contains Comptonized photons (plus reflection) and some contribution from the jet, has a somewhat smaller polarization degree of (40 ± 10)%, based on SPI measurement (Jourdain et al 2012), but undetected by IBIS with a 20% upper limit. At even lower energies, SPI reported a 20% upper limit for 130 -230 keV (Jourdain et al 2012) and PoGO+ gave an 8.6% upper limit for 20 -180 keV (Chauvin et al 2018), which argues for an extended, rather than compact, Compton cloud region (Chauvin et al 2018(Chauvin et al , 2019.…”

A polarized view of the hot and violent universe

“…One technique, however, is still lagging behind: polarimetry. In fact, after the measurement, at 2.6 and 5.2 keV, of the 19% average polarization of the Crab Nebula and a tight upper limit of about 1% to the accreting neutron star Scorpius X-1 (not surprisingly, the two brightest X-ray sources in the sky), obtained by OSO-8 in the 70s, no more observations have been performed in the classical X-ray band, even if some interesting results have been obtained in hard X-rays by balloon flights like POGO+ [1][2][3][4] and X-Calibur [5,6], and in soft gamma-rays by INTEGRAL [7][8][9], Hitomi [10], and AstroSAT [11]. The lack of polarimetric measurements implies that we are missing vital physical and geometrical information on many sources which can be provided by the two additional quantities polarimetry can provide: the polarization degree, which is related to the level of asymmetry in the systems (not only in the distribution of the emitting matter, but also in the magnetic or gravitational field), and the polarization angle, which indicates the main orientation of the system.…”