Ankur Kushwaha
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Ankur Kushwaha has a total of 20 co-authors. Here are the top co-authors by number of shared publications:
Anuj Nandi
Space Astronomy Group , ISITE Campus, U. R. Rao Satellite Centre, Outer Ring Road, Marathahalli, Bangalore 560037
5 shared publications- Abhilash R. Sarwade
U.R. Rao Satellite Centre, Indian Space Research Organisation, Bengaluru, India
3 shared publications - Kasiviswanathan Sankarasubramanian
Space Astronomy Group, ISITE-Campus, U. R. Rao Satellite Centre, Bangaluru, Karnataka, India
3 shared publications - Santabrata Das
Indian Institute of Technology Guwahati
3 shared publications - Seshadri Majumder
Department of Physics Indian Institute of Technology Guwahati 781039 Guwahati India
3 shared publications Smrati Verma
U.R. Rao Satellite Centre, Indian Space Research Organisation, Bengaluru, India
3 shared publications
Latest Publications
- Open Access
- 24 June 2026
- Published by Springer Nature in Solar Physics
- Vol. 301 (6) , 96
Iron Fluorescence in X-Class Solar Flares: Aditya-L1/SoLEXS Observations
- 24 June 2026
- Published by Springer Nature in Solar Physics
- Vol. 301 (6) , 96
Open AccessIron Fluorescence in X-Class Solar Flares: Aditya-L1/SoLEXS Observations
- 1 April 2026
- Published by SPIE-Intl Soc Optical Eng in Journal of Astronomical Telescopes, Instruments, and Systems
- Vol. 12 (02) , 1-20
Solar Low-Energy X-ray Spectrometer onboard Aditya-L1 mission: design and qualification
The Solar Low-Energy X-ray Spectrometer (SoLEXS) is one of the scientific payloads onboard the Aditya-L1 mission. The primary science objective of the SoLEXS instrument is to study solar flares spanning from A- to X-class and associated coronal dynamics within the soft X-ray energy range of 2 to 22 keV. The Solar Low-Energy X-ray Spectrometer employs the silicon drift detector (SDD) as the sensing element, chosen for its ability to sustain a high-event rate (∼105 counts/second), while maintaining low electronic noise owing to its intrinsically low detector capacitance. Solar Low-Energy X-ray Spectrometer has a dedicated architecture that integrates an in-house-engineered discrete-component charge-sensitive preamplifiers (CSPA) with SDDs and implements all digital pulse processing operations within a custom Field Programmable Gate Array (FPGA) design developed specifically for the instrument. Solar Low-Energy X-ray Spectrometer achieves an energy resolution of ∼170 eV at 5.9 keV and provides spectral and temporal cadences of 1 s and 0.1 s, respectively. The flight hardware of the instrument is designed, developed, characterized, and qualified for space deployment. The detailed instrument architecture, design approach, component screening and qualification, system-level performance, laboratory characterization, and onboard results of the SoLEXS instrument are presented in this article.
- 1 April 2026
- Published by SPIE-Intl Soc Optical Eng in Journal of Astronomical Telescopes, Instruments, and Systems
- Vol. 12 (02) , 1-20
Solar Low-Energy X-ray Spectrometer onboard Aditya-L1 mission: design and qualification
The Solar Low-Energy X-ray Spectrometer (SoLEXS) is one of the scientific payloads onboard the Aditya-L1 mission. The primary science objective of the SoLEXS instrument is to study solar flares spanning from A- to X-class and associated coronal dynamics within the soft X-ray energy range of 2 to 22 keV. The Solar Low-Energy X-ray Spectrometer employs the silicon drift detector (SDD) as the sensing element, chosen for its ability to sustain a high-event rate (∼105 counts/second), while maintaining low electronic noise owing to its intrinsically low detector capacitance. Solar Low-Energy X-ray Spectrometer has a dedicated architecture that integrates an in-house-engineered discrete-component charge-sensitive preamplifiers (CSPA) with SDDs and implements all digital pulse processing operations within a custom Field Programmable Gate Array (FPGA) design developed specifically for the instrument. Solar Low-Energy X-ray Spectrometer achieves an energy resolution of ∼170 eV at 5.9 keV and provides spectral and temporal cadences of 1 s and 0.1 s, respectively. The flight hardware of the instrument is designed, developed, characterized, and qualified for space deployment. The detailed instrument architecture, design approach, component screening and qualification, system-level performance, laboratory characterization, and onboard results of the SoLEXS instrument are presented in this article.
- Open Access
- 6 November 2025
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 545 (2)
Probing the accretion geometry of black hole X-ray binaries: a multimission spectro-polarimetric and timing study
We present a comprehensive spectro-polarimetric and timing analysis of 12 black hole X-ray binaries, namely Cyg X$-$1, 4U 1630$-$47, Cyg X$-$3, LMC X$-$1, 4U 1957$+$115, LMC X$-$3, Swift J1727.8$-$1613, GX 339$-$4, Swift J151857.0$-$572147, IGR J17091$-$3624, MAXI J1744$-$294, and GRS 1915$+$105, using quasi-simultaneous observations from IXPE (Imaging X-ray Polarimetry Explorer), NICER (Neutron star Interior Composition Explorer), NuSTAR (Nuclear Spectroscopic Telescope Array), and AstroSat. Timing analyses reveal type-B and type-C quasi-periodic oscillations across different spectral states, often associated with episodic radio ejections. Broad-band (0.7–60 keV) spectral modelling, employing disc, Comptonization, and reflection components, reveals degeneracies in constraining disc–corona geometries. Polarimetric measurements in 2–8 keV band detect significant polarization degrees (PDs) ranging from 3 per cent–20.6 per cent (1.2 per cent–21.4 per cent) in harder (softer) states, with moderate to strong energy dependence, except for LMC X$-1$, Swift J151857.0–572147, and MAXI J1744$-$294, where no significant polarization is detected. We report polarization detections of Cyg X$-3$ (PD $\sim 21.4~{{\rm per\ cent}}$, soft intermediate state), LMC X$-3$ (PD $\sim 2.4~{{\rm per\ cent}}$, high-soft state), and IGR J17091–3624 (PD $\sim 9~{{\rm per\ cent}}$, low-hard state) using recent IXPE observations. A positive correlation is found between PD and Comptonized photon fraction ($cov_{\rm frac}$), while an anticorrelation is observed with disc-to-Comptonized flux ratio ($F_{\rm ratio}$) across spectral states. Combined timing, spectral, and polarimetric results, together with constraints from radio jet observations, suggest a radially extended corona within a truncated disc for Cyg X$-1$, Swift J1727.8–1613, IGR J17091–3624, and GX 339–4, whereas the disc–corona geometry remains poorly constrained for 4U 1957+115, LMC X$-3$, and 4U 1630–47. We discuss the implications of these findings for understanding accretion geometries and highlight prospects for future X-ray polarimetric studies.- 6 November 2025
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 545 (2)
Open AccessProbing the accretion geometry of black hole X-ray binaries: a multimission spectro-polarimetric and timing study
We present a comprehensive spectro-polarimetric and timing analysis of 12 black hole X-ray binaries, namely Cyg X$-$1, 4U 1630$-$47, Cyg X$-$3, LMC X$-$1, 4U 1957$+$115, LMC X$-$3, Swift J1727.8$-$1613, GX 339$-$4, Swift J151857.0$-$572147, IGR J17091$-$3624, MAXI J1744$-$294, and GRS 1915$+$105, using quasi-simultaneous observations from IXPE (Imaging X-ray Polarimetry Explorer), NICER (Neutron star Interior Composition Explorer), NuSTAR (Nuclear Spectroscopic Telescope Array), and AstroSat. Timing analyses reveal type-B and type-C quasi-periodic oscillations across different spectral states, often associated with episodic radio ejections. Broad-band (0.7–60 keV) spectral modelling, employing disc, Comptonization, and reflection components, reveals degeneracies in constraining disc–corona geometries. Polarimetric measurements in 2–8 keV band detect significant polarization degrees (PDs) ranging from 3 per cent–20.6 per cent (1.2 per cent–21.4 per cent) in harder (softer) states, with moderate to strong energy dependence, except for LMC X$-1$, Swift J151857.0–572147, and MAXI J1744$-$294, where no significant polarization is detected. We report polarization detections of Cyg X$-3$ (PD $\sim 21.4~{{\rm per\ cent}}$, soft intermediate state), LMC X$-3$ (PD $\sim 2.4~{{\rm per\ cent}}$, high-soft state), and IGR J17091–3624 (PD $\sim 9~{{\rm per\ cent}}$, low-hard state) using recent IXPE observations. A positive correlation is found between PD and Comptonized photon fraction ($cov_{\rm frac}$), while an anticorrelation is observed with disc-to-Comptonized flux ratio ($F_{\rm ratio}$) across spectral states. Combined timing, spectral, and polarimetric results, together with constraints from radio jet observations, suggest a radially extended corona within a truncated disc for Cyg X$-1$, Swift J1727.8–1613, IGR J17091–3624, and GX 339–4, whereas the disc–corona geometry remains poorly constrained for 4U 1957+115, LMC X$-3$, and 4U 1630–47. We discuss the implications of these findings for understanding accretion geometries and highlight prospects for future X-ray polarimetric studies. - 1 October 2025
- Published by SPIE-Intl Soc Optical Eng in Journal of Astronomical Telescopes, Instruments, and Systems
- Vol. 11 (04) , 045005-24
Solar Low Energy X-ray Spectrometer on board Aditya-L1: ground calibration and in-flight performance
The Solar Low Energy X-ray Spectrometer (SoLEXS) on board India’s Aditya-L1 mission was launched on September 2, 2023, and commenced solar observations on December 13, 2023, following successful aperture cover deployment. Operating from the sun–Earth L1 Lagrange point, SoLEXS has been providing continuous sun-as-a-star soft X-ray spectroscopy across 2 to 22 keV with 170 eV resolution at 5.9 keV and 1-s temporal cadence since January 6, 2024. The instrument employs two silicon drift detectors with aperture areas of 7.1 and 0.1 mm2 to accommodate the full dynamic range of solar activity from A-class to X-class flares. We present comprehensive ground and on board calibration procedures that establish SoLEXS’s quantitative spectroscopic capabilities. Ground calibration encompassed energy-channel relationships, spectral resolution characterization, instrument response functions, and collimator angular response measurements, with thermo-vacuum testing validating performance stability across operational temperature ranges. On board calibration utilizing an internal Fe55 source demonstrated preserved post-launch spectral resolution (164.9 to 171.2 eV), whereas cross-calibration with GOES-XRS and Chandrayaan-2/XSM confirmed radiometric accuracy and flux agreement. The instrument’s 100% observational duty cycle at L1 enables unprecedented continuous monitoring of solar flare evolution across all intensity classes, providing calibrated data for advancing coronal heating mechanisms, flare energetics, and flare-coronal mass ejection relationship studies through soft X-ray spectroscopy.
- 1 October 2025
- Published by SPIE-Intl Soc Optical Eng in Journal of Astronomical Telescopes, Instruments, and Systems
- Vol. 11 (04) , 045005-24
Solar Low Energy X-ray Spectrometer on board Aditya-L1: ground calibration and in-flight performance
The Solar Low Energy X-ray Spectrometer (SoLEXS) on board India’s Aditya-L1 mission was launched on September 2, 2023, and commenced solar observations on December 13, 2023, following successful aperture cover deployment. Operating from the sun–Earth L1 Lagrange point, SoLEXS has been providing continuous sun-as-a-star soft X-ray spectroscopy across 2 to 22 keV with 170 eV resolution at 5.9 keV and 1-s temporal cadence since January 6, 2024. The instrument employs two silicon drift detectors with aperture areas of 7.1 and 0.1 mm2 to accommodate the full dynamic range of solar activity from A-class to X-class flares. We present comprehensive ground and on board calibration procedures that establish SoLEXS’s quantitative spectroscopic capabilities. Ground calibration encompassed energy-channel relationships, spectral resolution characterization, instrument response functions, and collimator angular response measurements, with thermo-vacuum testing validating performance stability across operational temperature ranges. On board calibration utilizing an internal Fe55 source demonstrated preserved post-launch spectral resolution (164.9 to 171.2 eV), whereas cross-calibration with GOES-XRS and Chandrayaan-2/XSM confirmed radiometric accuracy and flux agreement. The instrument’s 100% observational duty cycle at L1 enables unprecedented continuous monitoring of solar flare evolution across all intensity classes, providing calibrated data for advancing coronal heating mechanisms, flare energetics, and flare-coronal mass ejection relationship studies through soft X-ray spectroscopy.
- Open Access
First detection of X-ray polarization in thermal state of LMC X-3: Spectro-polarimetric study with IXPE
We report a comprehensive spectro-polarimetric study of the black hole binary LMC X$-3$ using simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR} observations in 0.5$-$20 keV energy band. The broad-band energy spectrum (0.5$-$20 keV) with {\it NICER} and {\it NuSTAR} is well described by the disc emission of temperature $\sim$1.1 keV and a weak Comptonizing tail beyond $\sim$10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of $\sim$96\%. The lack of variability ($rms$$\sim$0.5\%) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04$\pm$0.40\% (>7$σ$) at a polarization angle (PA) of $-$44.24$^{\circ}$$\pm$3.77$^{\circ}$ (>7$σ$) is found in 2$-$8 keV energy range of {\it IXPE}. In addition, PD is seen to increase with energy up to $\sim$4.35$\pm$0.98\% (>3$σ$) in 4$-$8 keV band. Further, we attempt to constrain the source spin ($a_{*}$) using broad-band spectral modelling that indicates a weakly rotating `hole' in LMC X$-3$ with $a_{*}=0.273_{-0.012}^{+0.011}-0.295_{-0.021}^{+0.008}$ ($90\%$ confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X$-3$ is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings.Open AccessFirst detection of X-ray polarization in thermal state of LMC X-3: Spectro-polarimetric study with IXPE
We report a comprehensive spectro-polarimetric study of the black hole binary LMC X$-3$ using simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR} observations in 0.5$-$20 keV energy band. The broad-band energy spectrum (0.5$-$20 keV) with {\it NICER} and {\it NuSTAR} is well described by the disc emission of temperature $\sim$1.1 keV and a weak Comptonizing tail beyond $\sim$10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of $\sim$96\%. The lack of variability ($rms$$\sim$0.5\%) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04$\pm$0.40\% (>7$σ$) at a polarization angle (PA) of $-$44.24$^{\circ}$$\pm$3.77$^{\circ}$ (>7$σ$) is found in 2$-$8 keV energy range of {\it IXPE}. In addition, PD is seen to increase with energy up to $\sim$4.35$\pm$0.98\% (>3$σ$) in 4$-$8 keV band. Further, we attempt to constrain the source spin ($a_{*}$) using broad-band spectral modelling that indicates a weakly rotating `hole' in LMC X$-3$ with $a_{*}=0.273_{-0.012}^{+0.011}-0.295_{-0.021}^{+0.008}$ ($90\%$ confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X$-3$ is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings. - Open Access
- 4 October 2023
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society: Letters
- Vol. 527 (1) , L76-L81
First detection of X-ray polarization in thermal state of LMC X-3: spectro-polarimetric study with IXPE
We report a comprehensive spectro-polarimetric study of the black hole binary LMC X-3 using simultaneous Imaging X-ray Polarimetry Explorer (IXPE), NICER, and NuSTAR observations in 0.5–20 keV energy band. The broad-band energy spectrum (0.5–20 keV) with NICER and NuSTAR is well described by the disc emission of temperature ∼1.1 keV and a weak Comptonizing tail beyond ∼10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of ∼96 per cent. The lack of variability (rms ∼ 0.5 per cent) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04 ± 0.40 per cent (>7σ) at a polarization angle of −44.24° ± 3.77° (>7σ) is found in 2–8 keV energy range of IXPE. In addition, PD is seen to increase with energy up to ∼4.35 ± 0.98 per cent (>3σ) in 4–8 keV band. Further, we attempt to constrain the source spin (a*) using broad-band spectral modelling that indicates a weakly rotating ‘hole’ in LMC X-3 with $a_{*}=0.273_{-0.012}^{+0.011}\,\text{to}\,0.295_{-0.021}^{+0.008}$ (90 per cent confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X-3 is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings.- 4 October 2023
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society: Letters
- Vol. 527 (1) , L76-L81
Open AccessFirst detection of X-ray polarization in thermal state of LMC X-3: spectro-polarimetric study with IXPE
We report a comprehensive spectro-polarimetric study of the black hole binary LMC X-3 using simultaneous Imaging X-ray Polarimetry Explorer (IXPE), NICER, and NuSTAR observations in 0.5–20 keV energy band. The broad-band energy spectrum (0.5–20 keV) with NICER and NuSTAR is well described by the disc emission of temperature ∼1.1 keV and a weak Comptonizing tail beyond ∼10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of ∼96 per cent. The lack of variability (rms ∼ 0.5 per cent) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04 ± 0.40 per cent (>7σ) at a polarization angle of −44.24° ± 3.77° (>7σ) is found in 2–8 keV energy range of IXPE. In addition, PD is seen to increase with energy up to ∼4.35 ± 0.98 per cent (>3σ) in 4–8 keV band. Further, we attempt to constrain the source spin (a*) using broad-band spectral modelling that indicates a weakly rotating ‘hole’ in LMC X-3 with $a_{*}=0.273_{-0.012}^{+0.011}\,\text{to}\,0.295_{-0.021}^{+0.008}$ (90 per cent confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X-3 is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings. - Open Access
Spectro-polarimetry of 4U 1957+115 with IXPE: Effects of spin and returning radiation on polarised emission of black hole in thermal state
We present a comprehensive spectro-polarimetric study of persistent Black hole X-ray binary 4U $1957\!+\!115$ with {\it IXPE} and {\it NICER} observations. The source is observed in disk dominated thermal state with disk temperature, kT$_{in}\approx1.4$ keV. The emission during thermal state from the source is found to be moderately polarized and \textit{IXPE} measures a degree of polarization (PD) $= 1.95\pm0.37\%$ ($>4.79σ$) along with a polarization angle (PA) $= -42.78^{\circ}\pm5.41^{\circ}$ in the energy range of $2-8$ keV. PD is found to be an increasing function of energy, whereas PA indicates switching within the energy range which could be due to high inclination and the returning radiation within the system. Simultaneous energy spectra ($0.6-10$ keV) from {\it NICER} are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.988\pm0.001\,(1σ)$, which is corroborated by {\it NICER} observations. Finally, we discuss the implications of our findings.Open AccessSpectro-polarimetry of 4U 1957+115 with IXPE: Effects of spin and returning radiation on polarised emission of black hole in thermal state
We present a comprehensive spectro-polarimetric study of persistent Black hole X-ray binary 4U $1957\!+\!115$ with {\it IXPE} and {\it NICER} observations. The source is observed in disk dominated thermal state with disk temperature, kT$_{in}\approx1.4$ keV. The emission during thermal state from the source is found to be moderately polarized and \textit{IXPE} measures a degree of polarization (PD) $= 1.95\pm0.37\%$ ($>4.79σ$) along with a polarization angle (PA) $= -42.78^{\circ}\pm5.41^{\circ}$ in the energy range of $2-8$ keV. PD is found to be an increasing function of energy, whereas PA indicates switching within the energy range which could be due to high inclination and the returning radiation within the system. Simultaneous energy spectra ($0.6-10$ keV) from {\it NICER} are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.988\pm0.001\,(1σ)$, which is corroborated by {\it NICER} observations. Finally, we discuss the implications of our findings. - 8 June 2023
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society: Letters
- Vol. 524 (1) , L15-L20
IXPE and NICER view of black hole X-ray binary 4U 1630–47: First significant detection of polarized emission in thermal state
We present a detailed spectro-polarimetric study of black hole X-ray binary 4U 1630−47 during its 2022 outburst with IXPE and NICER observations. The source is observed in disc dominated thermal state (kTin ≈ 1.4 keV) with clear detection of absorption features at 6.69 ± 0.01 keV and 6.97 ± 0.01 keV from both NICER as well as IXPE spectra, likely indicating a coupling of disc-wind. A significant degree of polarization (PD) $= 8.33\pm 0.17~{{\ \rm per\ cent}}$ and polarization angle (PA) =17.78° ± 0.60° in the energy range of 2 − 8 keV are measured with IXPE. PD is found to be an increasing function of energy whereas PA remains roughly same within the energy range. Simultaneous energy spectra from NICER in the range of 0.5 − 12 keV are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast }=0.920\pm 0.001\, (1\sigma)$ which is corroborated by NICER observations. Finally, we discuss the implications of our findings.- 8 June 2023
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society: Letters
- Vol. 524 (1) , L15-L20
IXPE and NICER view of black hole X-ray binary 4U 1630–47: First significant detection of polarized emission in thermal state
We present a detailed spectro-polarimetric study of black hole X-ray binary 4U 1630−47 during its 2022 outburst with IXPE and NICER observations. The source is observed in disc dominated thermal state (kTin ≈ 1.4 keV) with clear detection of absorption features at 6.69 ± 0.01 keV and 6.97 ± 0.01 keV from both NICER as well as IXPE spectra, likely indicating a coupling of disc-wind. A significant degree of polarization (PD) $= 8.33\pm 0.17~{{\ \rm per\ cent}}$ and polarization angle (PA) =17.78° ± 0.60° in the energy range of 2 − 8 keV are measured with IXPE. PD is found to be an increasing function of energy whereas PA remains roughly same within the energy range. Simultaneous energy spectra from NICER in the range of 0.5 − 12 keV are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast }=0.920\pm 0.001\, (1\sigma)$ which is corroborated by NICER observations. Finally, we discuss the implications of our findings.- Open Access
IXPE and NICER view of Black hole X-ray binary 4U 1630-47: First significant detection of polarized emission in thermal state
We present a detailed spectro-polarimetric study of Black hole X-ray binary 4U $1630-47$ during its $2022$ outburst with IXPE and NICER observations. The source is observed in disk dominated thermal state (kT$_{in}\approx1.4$ keV) with clear detection of absorption features at $6.69\pm0.01$ keV and $6.97\pm0.01$ keV from both NICER as well as IXPE spectra, likely indicating a coupling of disk-wind. A significant degree of polarization(PD) $= 8.33\pm0.17\%$ and polarization angle(PA) $=17.78^{\circ}\pm0.60^{\circ}$ in the energy range of $2-8$ keV are measured with IXPE. PD is found to be an increasing function of energy whereas PA remains roughly same within the energy range. Simultaneous energy spectra from NICER in the range of $0.5-12$ keV are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.927\pm0.001\,(1σ)$ which is corroborated by NICER observations. Finally, we discuss the implications of our findings.Open AccessIXPE and NICER view of Black hole X-ray binary 4U 1630-47: First significant detection of polarized emission in thermal state
We present a detailed spectro-polarimetric study of Black hole X-ray binary 4U $1630-47$ during its $2022$ outburst with IXPE and NICER observations. The source is observed in disk dominated thermal state (kT$_{in}\approx1.4$ keV) with clear detection of absorption features at $6.69\pm0.01$ keV and $6.97\pm0.01$ keV from both NICER as well as IXPE spectra, likely indicating a coupling of disk-wind. A significant degree of polarization(PD) $= 8.33\pm0.17\%$ and polarization angle(PA) $=17.78^{\circ}\pm0.60^{\circ}$ in the energy range of $2-8$ keV are measured with IXPE. PD is found to be an increasing function of energy whereas PA remains roughly same within the energy range. Simultaneous energy spectra from NICER in the range of $0.5-12$ keV are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.927\pm0.001\,(1σ)$ which is corroborated by NICER observations. Finally, we discuss the implications of our findings. - 5 August 2021
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 507 (2) , 2602-2613
AstroSat and MAXI view of Cygnus X-1: Signature of an ‘extreme’ soft nature
We present a detailed spectral and timing analysis of Cygnus X-1 with multi-epoch observations, during 2016–2019, by SXT and LAXPC onboard AstroSat. We model the spectra in broad energy range of |$0.5\!-\!70.0\, \rm {keV}$| to study the evolution of spectral properties while Cygnus X-1 transited from hard state to an extreme soft state via intermediate states in 2017. Simultaneous timing features are also examined by modelling the power density spectra in |$3.0\!-\!50.0\, \rm {keV}$|. We find that during high-soft state (HSS) observations, made by AstroSat on 2017 October 24 (MJD 58050), the energy spectrum of the source exhibits an inner disc temperature (|$kT\rm _{in}$|) of |$0.46 \pm 0.01\, \rm {keV}$|, a very steep photon index (Γ) of 3.15 ± 0.03 along with a fractional disc flux contribution of ∼45 per cent. The power density spectrum in the range of |$0.006\!-\!50.0\, \rm {Hz}$| is also very steep with a power-law index of 1.12 ± 0.04 along with a high root mean square value of ∼25 per cent. Comparing the spectral softness of HSS with those of previously reported, we confirm that AstroSat observed Cygnus X-1 in the ‘softest’ state. The lowest MAXI spectral hardness ratio of ∼0.229 corroborates the softest nature of the source. Moreover, we estimate the spin of the black hole by continuum-fitting method, which indicates that Cygnus X-1 is a maximally rotating ‘hole’. Further, Monte Carlo simulations are performed to estimate the uncertainty in spin parameter, which is constrained as a* > 0.9981 with 3σ confidence interval. Finally, we discuss the implications of our findings.
- 5 August 2021
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 507 (2) , 2602-2613
AstroSat and MAXI view of Cygnus X-1: Signature of an ‘extreme’ soft nature
We present a detailed spectral and timing analysis of Cygnus X-1 with multi-epoch observations, during 2016–2019, by SXT and LAXPC onboard AstroSat. We model the spectra in broad energy range of |$0.5\!-\!70.0\, \rm {keV}$| to study the evolution of spectral properties while Cygnus X-1 transited from hard state to an extreme soft state via intermediate states in 2017. Simultaneous timing features are also examined by modelling the power density spectra in |$3.0\!-\!50.0\, \rm {keV}$|. We find that during high-soft state (HSS) observations, made by AstroSat on 2017 October 24 (MJD 58050), the energy spectrum of the source exhibits an inner disc temperature (|$kT\rm _{in}$|) of |$0.46 \pm 0.01\, \rm {keV}$|, a very steep photon index (Γ) of 3.15 ± 0.03 along with a fractional disc flux contribution of ∼45 per cent. The power density spectrum in the range of |$0.006\!-\!50.0\, \rm {Hz}$| is also very steep with a power-law index of 1.12 ± 0.04 along with a high root mean square value of ∼25 per cent. Comparing the spectral softness of HSS with those of previously reported, we confirm that AstroSat observed Cygnus X-1 in the ‘softest’ state. The lowest MAXI spectral hardness ratio of ∼0.229 corroborates the softest nature of the source. Moreover, we estimate the spin of the black hole by continuum-fitting method, which indicates that Cygnus X-1 is a maximally rotating ‘hole’. Further, Monte Carlo simulations are performed to estimate the uncertainty in spin parameter, which is constrained as a* > 0.9981 with 3σ confidence interval. Finally, we discuss the implications of our findings.