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Herbig Ae/Be (HAeBe) stars are intermediate-mass pre-main-sequence stars, characterized by infrared (IR) excess and emission lines. They are observed to emit X-rays, whose origin is a matter of discussion and not settled yet. X-ray emission is not expected in HAeBe stars, as they lack the subsurface convective zone. In this study, we retrieved observations from the Chandra archive for 62 HAeBe stars, among which 44 sources (detection fraction ∼71  per cent) were detected in X-rays, with 7 being new detections. We use this sample as a test bed to conduct a comparative analysis of the X-ray properties of HAeBe stars and their low-mass counterparts, T Tauri stars (TTSs). Further, we compare the X-ray properties of HAeBe stars and TTSs with optical and IR properties to constrain the X-ray emission mechanism in HAeBe stars. We found no correlation between X-ray emission and disc properties of HAeBe stars, confirming that X-rays are not related to accretion shocks. About 56  per cent of HAeBe stars without any known subarcsec companions have lower plasma temperatures (kT ≤ 2 keV). We observe flaring/variability in HAeBe stars with confirmed low-mass companions. These stars show plasma temperatures > 2 keV, similar to TTSs. Guided by this information, we discuss the role of a T Tauri companion for X-ray emission seen in our sample of HAeBe stars. From the results obtained in this paper, we suggest that X-ray emission from HAeBe stars may not be related to accretion shocks or hidden TTS, but rather can be due to magnetically driven coronal emission. [ABSTRACT FROM AUTHOR]

In eclipsing X-ray binary systems, the direct X-ray emission is blocked by the companion star during the eclipse. We observe only reprocessed emission that contains clues about the environment of the compact object and its chemical composition, ionization levels, etc. We have found flares in some X-ray binaries during their eclipses. The study of eclipse flares provides additional clues regarding the size of the reprocessing region and helps distinguish between different components of the X-ray spectrum observed during the eclipse. In the archival data, we searched for flares during eclipses of high-mass X-ray binaries and found flares in three sources: Vela X-1, LMC X-4, and 4U 1700−37. Comparing spectral properties of the eclipse flare and non-flare data, we found changes in the power-law photon index in all three sources and multiple emission lines in Vela X-1 and 4U 1700-37. The fluxes of prominent emission lines showed a similar increase as the overall X-ray flux during the eclipse flare, suggesting the lines originate in the binary environment and not in the interstellar medium. We also observed a soft excess in 4U 1700-37 that remains unchanged during both eclipse flare and non-flare states. Our analysis suggests that this emission originates from the extremely thin shell of the stellar wind surrounding the photosphere of its companion star. The detection of short (100–200 s) count-rate doubling time-scale in 4U 1700−37 and LMC X-4 indicates that the eclipse reprocessing occurs in a region larger than, but comparable to the size of the companion star. [ABSTRACT FROM AUTHOR]

We present the X-ray spectral and temporal analysis of the black hole X-ray transient Swift  J1658.2–4242 observed by AstroSat. Three epochs of data have been analysed using the JeTCAF model to estimate the mass accretion rates and to understand the geometry of the flow. The best-fitting disc mass accretion rate (⁠|$\dot{m}_{\rm d}$|⁠) varies between |$0.90^{+0.02}_{-0.01}$| and |$1.09^{+0.04}_{-0.03}$| |$\dot{M}_{\rm Edd}$| in these observations, while the halo mass accretion rate changes from |$0.15^{+0.01}_{-0.01}$| to |$0.25^{+0.02}_{-0.01}$| |$\dot{M}_{\rm Edd}$|⁠. We estimate the size of the dynamic corona that varies substantially from |$64.9^{+3.9}_{-3.1}$| to |$34.5^{+2.0}_{-1.5}$|   r g and a moderately high jet/outflow collimation factor stipulates isotropic outflow. The inferred high disc mass accretion rate and bigger corona size indicate that the source might be in the intermediate to soft spectral state of black hole X-ray binaries. The mass of the black hole estimated from different model combinations is ∼14 M⊙. In addition, we compute the quasi-periodic oscillation (QPO) frequencies from the model-fitted parameters, which match the observed QPOs. We further calculate the binary parameters of the system from the decay profile of the light curve and the spectral parameters. The estimated orbital period of the system is 4.0 ± 0.4 h by assuming the companion as a mid or late K-type star. Our analysis using the JeTCAF model sheds light on the physical origin of the spectrotemporal behaviour of the source, and the observed properties are mainly due to the change in both the mass accretion rates and absorbing column density. [ABSTRACT FROM AUTHOR]

We present broad-band X-ray spectral-timing analysis of the new Galactic X-ray transient MAXI J1348–630 using five simultaneous AstroSat and NICER observations. Spectral analysis using AstroSat data identify the source to be in the soft state for the first three observations and in a faint and bright hard state for the next two. Quasi-periodic oscillations at ∼0.9 and ∼6.9 Hz, belonging to the type-C and type-A class are detected. In the soft state, the power density spectra are substantially lower (by a factor >5) for the NICER (0.5–12 keV) band compared to the AstroSat /LAXPC (3–80 keV) one, confirming that the disc is significantly less variable than the Comptonization component. For the first time, energy-dependent fractional rms and time lag in the 0.5–80 keV energy band was measured at different Fourier frequencies, using the bright hard state observation. Hard time lag is detected for the bright hard state, while the faint one shows evidence for soft lag. A single-zone propagation model fits the LAXPC results in the energy band 3–80 keV with parameters similar to those obtained for Cygnus X–1 and MAXI J1820+070. Extending the model to lower energies, reveals qualitative similarities but having quantitative differences with the NICER results. These discrepancies could be because the NICER and AstroSat data are not strictly simultaneous and because the simple propagation model does not take into account disc emission. The results highlight the need for more joint coordinated observations of such systems by NICER and AstroSat. [ABSTRACT FROM AUTHOR]

Ultraluminous X-ray sources (ULXs) are a class of extragalactic, point-like, off-nuclear X-ray sources with X-ray luminosity from ∼1039 to 1041 erg s−1. We investigated the temporal and broadband X-ray spectral properties of the ULX M81 X–6 using simultaneous Suzaku and NuSTAR observations. To understand the nature of the source, we searched for pulsating signals from the source using the NuSTAR observation. However, we failed to identify any strong pulsating signals from the source. Alternatively, the broadband spectral modelling with accreting magnetic neutron star continuum model provides a statistically acceptable fit, and the inferred spectral parameters, and X-ray colours are consistent with other pulsating ULXs. Thus, our analysis suggests that M81 X–6 is another candidate ULX pulsar. [ABSTRACT FROM AUTHOR]