Detection of X-Ray Polarized Emission and Accretion-disk Winds with IXPE and NICER in the Black Hole X-Ray Binary 4U 1630−47

Abstract

We detect a high level of polarization in the X-ray emission of the black hole binary 4U 1630–47 in an observation with the Imaging X-ray Polarimetry Explorer. The 2–8 keV polarization degree is 8% at a position angle of 18°, with the polarization degree increasing significantly with energy, from ∼6% at ∼2 keV to ∼11% at ∼8 keV. The continuum emission in the spectrum of simultaneous observations with the Neutron Star Interior Composition Explorer (NICER) is well described with only a thermal disk spectrum, with stringent upper limits to any Comptonized emission from the corona. Together with the lack of significant variability in the Fourier power spectrum, this suggests that the source was in the high soft state at the time of these observations. The NICER spectrum reveals the presence of several absorption lines in the 6–9 keV band that we fit with two ionized absorbers, providing evidence of the presence of a strong disk wind, which supports the idea that the source was in the soft state. Previous measurements of X-ray polarization in other sources in harder states were associated with the corona or the jet in those systems. Given that the corona is significantly absent in this observation of 4U 1630–47, and that the jet in black hole binaries is quenched in the high soft state, we speculate that in this observation of 4U 1630–47, the polarization likely arises from the direct and reflected radiation of the accretion disk in this source.

X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630–47 Challenges the Standard Thin Accretion Disk Scenario

“…Two other works have independently analyzed the same IXPE data sets (Kushwaha et al 2023;Rawat et al 2023), obtaining observational results consistent with those presented in this work. The authors posit that the high PD can be explained by scattering off a wind.…”

Spectropolarimetric study of 4U 1630−47 in steep power-law state with IXPE and NICER

“…In light of this, the launch of the IXPE mission provides an opportunity to investigate the evolution of spectral transitions through X-ray polarimetry and probe the corona geometry . Previously , Rawat et al ( 2023 ), Kushwaha et al ( 2023 ), andRatheesh et al ( 2023 ) reported a significant polarization of 8 per cent for 4U 1630 in the HSS with IXPE . Similarly, Podgorny et al ( 2023 ) and Dovciak et al ( 2023 ) hav e observ ed polarization fraction of ∼1.1 per cent for LMC X-1 and ∼2.27 per cent for Cygnus X-1 in the HSS and the variable soft state, respectively.…”

“…4U 1630 −472 (hereafter 4U 1630) is a transient low-mass X-ray binary source disco v ered almost fiv e decades ago with Uhuru , Ariel V (Jones et al 1976 ), and Vela 5B X-ray monitor (Priedhorsky 1986 ), independently. This source shows particularly soft outbursts with a lack of hard X-ray emission (Capitanio et al 2015 ) and multiple iron absorption lines in the 6-9 keV band (Kubota et al 2007 ;R ó ża ńska et al 2014 ;Miller et al 2015 ;Rawat, Garg & M éndez 2023 ). There is a discrepancy in the measurement of the inclination angle of the source; Kuulkers et al ( 1998 ), and King et al ( 2014 ) reported it as E-mail: rawatdivya838@gmail.com a high-inclination source ( ∼60-70 • ), while R ó ża ńska et al ( 2014 ) proposed that it is a low-inclination system ( i ∼ (11 ± 5) • ).…”

Plane polarisation in Comptonization process: A Monte Carlo study

“…4U 1630-47: IXPE had observed 4U 1630-47 from 23-Aug-2022 to 02-Sept-2022, when the source is in high soft spectral state. The polarisation properties have been analysed by three groups (Rawat et al 2023;Kushwaha et al 2023;Ratheesh et al 2023), and they found that PD is an energy-dependent which increases significantly with energy. In 2-8 keV band PD is ∼0.08 and PA in the sky plane is ∼18 • .…”

“…Sharma et al 2020;Chattopadhyay et al 2019;Chand et al 2019), by INTEGRAL/SPI (McGlynn et al 2007) /IBIS (G.tz et al 2014), by POLAR (Zhang et al 2019), by other instruments, for example, GAP (see in details Chattopadhyay et al 2019). Recently, IXPE has measured polarisation properties of many XRBs, AGNs, pulsar in 2-8 keV energy band (Weisskopf et al 2022;Rawat, Garg, & Méndez 2023;Marshall et al 2022;Jayasurya, Agrawal, & Chatterjee 2023;Pal et al 2023;Marinucci et al 2022;Doroshenko et al 2022), and for few sources the polarisation is an energydependent. Long et al (2022) quantified the polarised emission of Sco X-1 using PolarLight observations in 3-8 keV and noted an energy-dependent polarisation.…”