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We present an improved background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on-board AstroSat. Because of the large collecting area and high pressure, the LAXPC instrument has a large background count rate, which varies during the orbit. Apart from the variation with latitude and longitude during the orbit there is a prominent quasi-diurnal variation which has not been modelled earlier. Using over 5 years of background observations, we determined the period of the quasi-diurnal variation to be 84495 s and using this period, it is possible to account for the variation and also identify time intervals where the fit is not good. These lead to a significant improvement in the background model. The quasi-diurnal variation can be ascribed to the changes in charged particle flux in the near Earth orbit., Comment: Accepted for publication in ApJS

We present timing and spectral results of 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosity of $2.04 \times 10^{37}$ erg s$^{-1}$ and $1.02 \times 10^{37}$ erg s$^{-1}$ respectively. The light curves showed strong pulsation and co-related X-ray intensity variation in SXT (0.5--8.0 keV) and LAXPC (3--60 keV) energy bands. Spin-period and spin-down rate of the pulsar were determined from two observations as $65.35080\pm0.00014$ s , $(-2.10\pm0.8)\times10^{-12}$ Hz s$^{-1}$ at an epoch MJD 58301.61850 and $65.35290\pm0.00017$ s, $(-1.6\pm0.8)\times10^{-12}$ Hz s$^{-1}$ for an epoch MJD 58307.40211. Pulse-shape studies with AstroSat showed energy and intensity dependent variations. The pulsar showed an overall continuous spin-down, over 30 years at an average-rate of $(-2.455\pm0.004)\times10^{-14}$ Hz s$^{-1}$, attributed to propeller-effect in the subsonic-regime of the pulsar, in addition to variations during its outburst activities. Spectra between 0.7--55 keV energy band were well fitted by two continuum models, an absorbed compTT-model and an absorbed power-law with a Fermi-Dirac cutoff (FD-cutoff) model with a black-body. These were combined with an iron-emission line and a cyclotron absorption line. The prominent cyclotron resonance scattering features with a peak absorption energy of $30.48^{+0.33}_{-0.34}$ keV and $30.68^{+0.45}_{-0.44}$ keV for FD-cutoff-model and $30.46^{+0.32}_{-0.28}$ keV and $30.30^{+0.36}_{-0.34}$ keV for compTT-model were detected during two AstroSat observations. These when compared with earlier results, showed long term stability of its average value of $30.23 \pm 0.22$ keV. The pulsar showed pulse-phase as well as luminosity dependent variations in cyclotron-line energy and width and in plasma optical-depth of its spectral continuum., Comment: 25 pages, 18 figures, 3 tables, Accepted for Publication in The Astrophysical Journal

The Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat has three nominally identical detectors for timing and spectral studies in the energy range of 3--80 keV. The performance of these detectors during the five years after the launch of AstroSat is described. Currently, only one of the detector is working nominally. The variation in pressure, energy resolution, gain and background with time are discussed. The capabilities and limitations of the instrument are described. A brief account of available analysis software is also provided., Comment: Accepted for publication in JAA

IGR J17091--3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as `heartbeat', similar to the one observed in GRS 1915+105. In this paper, we report the results of $\sim 125$ ks \textit{AstroSat} observations of this source during the 2016 outburst. For the first time a double peaked QPO (DPQ) is detected in a few time segments of this source with a difference of $\delta f ~\sim12$ mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low frequency (25--48 mHz) `heartbeat' single peak QPO (SPQ) was observed at different intervals of time along with harmonics ($50-95$ mHz). Broadband spectra in the range $0.7-23$ keV, obtained with \textit{SXT} and \textit{LAXPC}, could be fitted well with combination of a thermal Comptonisation and a multicolour disc component model. During \textit{AstroSat} observation, the source was in the Soft-Intermediate State (SIMS) as observed with \textit{Swift/XRT}. We present a comparative study of the `heartbeat' state variability in IGR J17091--3624 with GRS 1915+105. Significant difference in the timing properties is observed although spectral parameters ($\Gamma\sim2.1-2.4$ and $T_\mathrm{max}\sim0.6-0.8$ keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915+105 to that in IGR J17091--3624 as $\sim25:1$. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities., Comment: 17 pages, 13 Figures, 6 Tables; Accepted for publication in MNRAS

We present results from AstroSat observation of the recent outburst of GRO J2058+42, an X-ray pulsar in a Be-binary system. The source was observed on April 10, 2019 by LAXPC and SXT instruments on AstroSat during its declining phase of the latest giant outburst. Light curves showed a strong pulsation of the pulsar with a period of 194.2201 \pm 0.0016 s, and a spin-up rate of (1.65\pm0.06)\times10^{-11} Hz s^{-1}. Intermittent flaring was detected in light curves between 3--80 keV energy band with increase in intensity by up to 1.8 times its average intensity. Pulse profiles obtained between 3--80 keV energy band of the pulsar showed strong dependence on energy. A broad peak was observed in the power density spectrum of the source consistently during AstroSat observations with its peak oscillation frequency of 0.090 Hz along with its higher harmonics, which may be due to quasi-periodic oscillations, a commonly observed phenomenon in transient X-ray pulsars, during their outburst. AstroSat observation also detected cyclotron absorption features in its spectrum corresponding to (9.7--14.4) keV, (19.3--23.8) keV and (37.8--43.1) keV. The pulse phase resolved spectroscopy of the source showed phase dependent variation in its energy and relative strength of these features. The spectrum was well fitted with an absorbed black-body, a Fermi Dirac cut-off model and alternatively with an absorbed CompTT model. Both these models were combined with a Fe-line and three Gaussian absorption lines to account for observed cyclotron resonance scattering features in the spectrum., Comment: 22 pages, 22 figures, 3 tables, Accepted for publication in Astrophysical Journal