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 oneof 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.

The origins of X-ray and radio emissions during an X-ray binary outburst are comparatively better understood than those of ultraviolet, optical and infrared radiation. This is because multiple competing mechanisms – emission from intrinsic and irradiated disk, secondary star emission, synchrotron emissionfrom jet and/or non-thermal electron cloud, etc – peak in these mid-energy ranges. Ascertaining the true emission mechanism and segregating the contribution of different mechanisms, if present, is important for correct understanding of the energetics of the system and hence its geometry and other properties. We have studied the multi-wavelength spectral energy distribution of the galactic X-ray binary GRS 1716-249 ranging from near infrared ($5 \times 10^{-4}$ keV) to hard X-rays (120 keV) using observations from AstroSat, Swift, and Mount Abu Infrared Observatory. Broadband spectral fitting suggests that the irradiated accretion disk dominates emission in ultraviolet and optical regimes. The near infrared emission exhibits some excess thanthe prediction of the irradiated disk model, which is most likely due to Synchrotron emission from jets as suggested by radio emission. Irradiation of the inner disk by the hard X-ray emission from the Corona also plays a significant role in accounting for the soft X-ray emission.

With its large effective area at hard X-rays, high time resolution and having co-aligned other instruments, AstroSat/LAXPC was designed to usher in a new era in rapid variability studies and wide spectral band measurements of the X-ray binaries. Over the last five years, the instrument has successfully achieved to a significant extent these Science goals. In the coming years, it is poised to make more important discoveries. This paper highlights the primary achievements of AstroSat/LAXPC in unraveling the behavior of black hole and neutron star systems and discusses the exciting possibility of the instrument’s contributionto future science.

An alternative scheme is described to estimate the layer 1 LAXPC 20 background for faint sources where the source contribution to the 50–80 keV count rate is less than 0.25 counts/s (15 milli-crabs or $6\times 10^{-11}$ ergs/s/cm$^2$). We consider 12 blank sky observations and based on their 50–80 keV count rate in 100 second time-bins, generate four template spectra which are then used to estimate the background spectrum and lightcurve for a given faint source observation. The variance of the estimated background subtracted spectra for the 12 blank sky observations is taken as the energy dependent systematic uncertainty which will dominate over the statistical one for exposures longer than 5 ks. The estimated 100 second time bin background lightcurve in the 4–20 keV band with a 3% systematic error matches with the blank sky ones.The 4–20 keV spectrum can be constrained for a source with flux $\gtrapprox$1 milli-crab. Fractional rms variability of 10% can be determined for a $\sim$5 milli-crab source lightcurve binned at 100 seconds. To illustrate the scheme, the lightcurves and spectra of three different blank sky observations, three AGN sources (Mrk 0926, Mrk 110, NGC 4593) and LMC X-1 are shown.

We present the timing results of out-of-eclipse observations of Centaurus X-3 spanning half a binary orbit, performed on 12–13 December, 2016 with the Large Area X-ray Proportional Counter (LAXPC) on-board AstroSat. The pulse profile was confirmed to exhibit a prominent pulse peak with a secondary inter-pulse. The systemic spin period of the pulsar was found to be $4.80188 \pm 0.000085$ s in agreement with its spin up trend. The spin up timescale seems to have increased to $7709\pm 58$ yr that points to negative torque effects in the inner accretion disk. We also report the derived values of projected semi-major axis and orbital velocity of the neutron star.

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418,due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milky Way. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical imagesfrom Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66$+$112227.8, the farthest star($\sim$17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate ($4–7.4 \times 10^{–4} \ M_{\odot}$ yr$^{–1}$) in these fireballs, estimated from UVIT flux densities, to beincreasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain thepuzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy.

We report the results of an observation of low-mass X-ray binary GX $3+1$ with AstroSat’s large area X-ray proportional counter (LAXPC) and soft X-ray telescope (SXT) instruments on-board for the first time. We have detected type-I thermonuclear burst ($\sim$15 s) present in the LAXPC 20 light curve with a double peak feature at higher energies and our study of the hardness–intensity diagram reveals that the source was in a soft banana state. The pre-burst emission could be described well by a thermally Comptonized model component. The burst spectra is modeled adopting a time-resolved spectroscopic method using a single color blackbody model added to the pre-burst model, to monitor the parametric changes as the burst decays. Basedon our time-resolved spectroscopy, we claim that the detected burst is a photospheric radius expansion (PRE) burst. During the PRE phase, the blackbody flux is found to be approximately constant at an averaged value of $\sim$2.56 in $10^{-8}$ ergs s$^{-1}$ cm$^{-2}$ units. On the basis of literature survey, we infer that AstroSat/LAXPC 20 has detected a burst from GX $3+1$ after more than a decade, which is also a PRE one. Utilizing the obtained burst parameters, we provide a new estimation to the source distance, which is ${\sim}9.3 \pm 0.4$ kpc, calculated for anisotropic burst emission. Finally, we discuss and compare our findings with the published literature reports.