Sudip Chakraborty

We report on the observation of the Compton-thin Seyfert galaxy NGC 5506 performed with the Imaging X-ray Polarimetry Explorer (IXPE). The observation, with a net observing time of 975 ks, was partly simultaneous with NuSTAR (52 ks) and XMM-Newton (37 ks). Using a simple baseline model with IXPE data only, we find an upper limit on the 2–8 keV polarization fraction at the 99% confidence level: Π < 3.1%. We then performed a spectropolarimetric analysis combining data from the three X-ray observatories. While pure spectral modeling confirms the presence of both relativistic and cold distant reflection, in addition to the primary continuum power law, their contributions lead to somewhat different constraints on the coronal Π, depending on the specific assumptions. We also performed Monte Carlo simulations with the radiative transfer code MONK, exploring various coronal geometries. Comparing these results with the IXPE data, and assuming a radially extended corona, as suggested by previous observations of similar sources, the corresponding polarization upper limits require inclination angles below 40°.

The gamma-ray-emitting binary stellar system LS I +61° 303 was observed with the Imaging X-ray Polarimetry Explorer on two successive orbits over orbital phases of 0.74–1.05. Polarization is detected at a significance of 4.2 σ with an average polarization degree of 13.1% ± 3.0% in the 2–8 keV band after background subtraction. This is the second detection of polarization of the X-ray synchrotron emission from a gamma-ray binary and, again, suggests that the magnetic field in the particle acceleration region has a significant ordered component. The orbital motion on the sky of LS I +61° 303 is not well determined, which leads to ambiguity in interpretation of the X-ray electric vector position angle (EVPA) measurement. Use of orbital elements determined via radial velocity measurements combined with radio imaging of variable nebular emission suggests an offset between the X-ray EVPA and the compact object-massive star axis on the order of ∼30°. Such an offset could be produced by Coriolis forces due to binary motion. Use of two different sets orbital elements determined via optical polarimetry suggest either no offset or a perpendicular orientation but require an unexpectedly high inclination, while another set based on the same data but modeled assuming a moderate inclination suggests an offset of 15°. Use of orbital elements derived from modeling of the keV/TeV light curves suggests good alignment between the X-ray EVPA and the compact object-massive star axis. Such alignment was found for the gamma-ray binary PSR B1259-63. If the same physical situation holds for LS I +61° 303, that would favor the orbital elements derived from the keV/TeV light curves.

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6−2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within ∼1 . ′ 5 of the Galactic center, was observed serendipitously during a MAXI J1744−294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6−2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744−294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7% ± 4.0% and polarization angle PA = 122° ± 8°. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2% ± 8.7%, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

The location in which γ -ray are created and emitted within extra-galactic jets is a matter of active debate. One particularly well-suited source for determining the location is the nearby bright radio galaxy 3C 84, harbouring a powerful jet. We investigated the origin of γ -rays that were measured during a recent γ -ray flare by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is almost unpolarised overall, we find that close in time to the γ -ray flare peak regions at parsec-scale distances from the central engine, the linear polarisation increases fractionally. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the γ -ray flare, and combined with insights from concurrent X-ray polarisation measurements, a physically motivated scenario is that the γ -rays are created in this region, in a process consistent with the synchrotron self-Compton mechanism.

Low-luminosity active galactic nuclei (LLAGN) provide a unique view of Comptonization and nonthermal emission from accreting black holes in the low accretion rate regime. However, to decipher the exact nature of the Comptonizing corona in LLAGN, its geometry and emission mechanism must be understood beyond the limits of spectro-timing techniques. Spectropolarimetry offers the potential to break the degeneracies between different coronal emission models. Compton-thin LLAGN provide an opportunity for such spectropolarimetric exploration in the 2–8 keV energy range using the Imaging X-Ray Polarimetry Explorer (IXPE). In this work, we carry out a spectropolarimetric analysis of the first IXPE observation, in synergy with a contemporaneous Nuclear Spectroscopic Telescope Array observation, of an LLAGN: NGC 2110. Using 554.4 ks of IXPE data from 2024 October, we constrain the 99% upper limit on the polarization degree (PD) to be less than 8.3% assuming the corresponding polarization angle (PA) to be aligned with the radio jet, and less than 3.6% if in the perpendicular direction. In the absence of a significant PD detection, the PA remains formally unconstrained, yet the polarization significance contours appear to be aligned with the radio jet, tentatively supporting models in which the corona is radially extended in the plane of the disk. We also carry out detailed Monte Carlo simulations using monk and STOKES codes to test different coronal models against our results and compare the polarization properties between NGC 2110 and brighter Seyferts.

Low-Luminosity Active Galactic Nuclei (LLAGN) provides a unique view of Comptonization and non-thermal emission from accreting black holes in the low-accretion rate regime. However, to decipher the exact nature of the Comptonizing corona in LLAGN, its geometry and emission mechanism must be understood beyond the limits of spectro-timing techniques. Spectro-polarimetry offers the potential to break the degeneracies between different coronal emission models. Compton-thin LLAGN provide an opportunity for such spectro-polarimetric exploration in the 2-8 keV energy range using IXPE. In this work, we carry out a spectro-polarimetric analysis of the first IXPE observation, in synergy with a contemporaneous NuSTAR observation, of an LLAGN: NGC 2110. Using 554.4 ks of IXPE data from October 2024, we constrain the 99% upper limit on the Polarization Degree (PD) to be less than 8.3% assuming the corresponding Polarization Angle (PA) to be aligned with the radio jet, and less than 3.6% if in the perpendicular direction. In the absence of a significant PD detection, the PA remains formally unconstrained, yet the polarization significance contours appear to be aligned with the radio jet, tentatively supporting models in which the corona is radially extended in the plane of the disk. We also carry out detailed Monte Carlo simulations using MONK and STOKES codes to test different coronal models against our results and compare the polarization properties between NGC 2110 and brighter Seyferts.

IGR J17591–2342, a transient accretion-powered millisecond X-ray pulsar, was discovered during its 2018 outburst. Here, we present a timing and spectral analysis of the source using AstroSat data of the same outburst. From the timing analysis, we obtain updated values of binary orbital parameters, which reveal an average pulsar spin frequency of 527.4256984(8) Hz. The pulse profiles can be fit well with four harmonically related sinusoidal components with fractional amplitudes of fundamental and second, third, and fourth harmonics as ∼13%, ∼6%, ∼0.9%, ∼0.2%, respectively. The energy-dependent study of pulse profiles in the range of 3 − 20 keV shows that the fractional amplitude of both the fundamental and first overtone is consistent with being constant across the considered energy band. Besides, a decaying trend has been observed for both the fundamental and first overtone in the phase-delay versus energy relation resulting in soft X-ray (2.8-3.3 keV) phase lags of ∼0.05 and ∼0.13 with respect to ≤15 keV photons, for the fundamental and first overtone, respectively. The combined spectra from the Large Area X-ray Proportional Counters and the Soft X-ray Telescope aboard AstroSat in the 1 − 18 keV range can be fit well with an absorbed model consisting of a Comptonization, a blackbody and a Gaussian emission line component yielding as best-fit parameters a blackbody seed photon temperature kT_bb ∼0.95 ± 0.03 keV, and an electron temperature kT_e ∼1.54 ± 0.03 keV. The spectral aspects suggest the scattering of photons from the accretion disc or the neutron star’s surface.

XB 1254−690 is a neutron star low-mass X-ray binary with an orbital period of 3.88 h, and it exhibits energy-dependent intensity dips, thermonuclear bursts, and flares. We present the results of an analysis of a long observation of this source using the AstroSat satellite. The X-ray light curve gradually changed from a high-intensity flaring state to a low-intensity one with a few dips. The hardness–intensity diagram showed that the source is in a high-intensity banana state with a gradually changing flux. Based on this, we divide the observation into four flux levels for a flux-resolved spectral study. The X-ray spectra can be explained by a model consisting of absorption, thermal emission from the disc, and non-thermal emission from the corona. From our studies, we detect a correlation between the temperature of the thermal component and the flux and we examine the implications of our results for the accretion disc geometry of this source.

Aims. Through their radio loudness, lack of thermal UV emission from the accretion disk, and power-law dominated spectra, low-luminosity active galactic nuclei (LLAGNs) display similarity with the hard state of stellar-mass black hole X-ray binaries (BHBs). In this work we perform a systematic hard X-ray spectral study of a carefully selected sample of unobscured LLAGNs using archival NuSTAR data to understand the central engine properties in the lower accretion regime. Methods. We analyzed the NuSTAR spectra of a sample of 16 LLAGNs. We modeled the continuum emission with detailed Comptonization models. Results. We find a strong anti-correlation between the optical depth and the electron temperature of the corona, previously also observed in the brighter Seyferts. This anti-correlation is present irrespective of the shape of the corona, and the slope of this anti-correlation in the log space for LLAGNs (0.68–1.06) closely matches that of the higher accretion rate Seyferts (0.55–1.11) and hard state of BHBs (∼0.87). This anti-correlation may indicate a departure from a fixed disk-corona configuration in radiative balance. Conclusions. Our result, therefore, demonstrates a possible universality in Comptonization processes of black hole X-ray sources across multiple orders of magnitude in mass and accretion rate.

We present the broad-band spectral analysis of all the six hard, intermediate, and soft state NuSTAR observations of the recently discovered transient black hole X-ray binary MAXI J1348−630 during its first outburst in 2019. We first model the data with a combination of a multicolour disc and a relativistic blurred reflection, and, whenever needed, a distant reflection. We find that this simple model scheme is inadequate in explaining the spectra, resulting in a very high iron abundance. We therefore explore the possibility of reflection from a high-density disc. We use two different sets of models to describe the high-density disc reflection: relxill-based reflection models, and reflionx-based ones. The reflionx-based high-density disc reflection models bring down the iron abundance to around the solar value, while the density is found to be |$10^{20.3-21.4}\, \rm cm^{-3}$|⁠. We also find evidence of a high-velocity outflow in the form of ∼7.3 keV absorption lines. The consistency between the best-fitting parameters for different epochs and the statistical significance of the corresponding model indicates the existence of high-density disc reflection in MAXI J1348−630.