We present the spectro-timing analysis of the black hole binary system GX 339-4 using AstroSat data extracted at the beginning of outbursts in 2017 and 2019. The joint spectral fitting of LAXPC and SXT data revealed that the source was in a faint low/hard state for both 2017 and 2019 observations with a nearly equal photon index of $\sim$1.57 and $\sim$1.58 and with Eddington ratio, $L/L_{Edd}$, of 0.0011 and 0.0046 respectively. The addition of a reflection component into the spectral modelling improved the fit ($Δχ^2$ $\approx~6$ for 2017 and $Δχ^2$ $\approx~7$ for 2019), pointing towards the presence of weak reflection features arising due to irradiation of the accretion disk. The power density spectrum (PDS) consisted of strong band-limited noise with a break at low frequencies, described with a combination of few zero-centered Lorentzian. The fitting revealed a low-frequency break at $\sim$6~mHz for 2017 and at $\sim$11~mHz for 2019 observation, whose detection is validated by results from independent detectors (LAXPCs and SXT). The break frequency is roughly consistent with results obtained from earlier observations that showed an evolution of the frequency with flux, which is in accordance with the truncated disk model. Associating the break frequency with viscous time scale of the accretion disk, we estimated a truncation radius of $\sim$93 and $\sim$61 gravitational radius for 2017 and 2019 observation, respectively.
We present broadband X-ray spectral-timing analysis of the new Galactic X-ray transient MAXI~J1348--630 using five simultaneous {\it AstroSat} and {\it NICER} observations. Spectral analysis using {\it 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 $\sim 0.9$ and $\sim 6.9$\,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 {\it NICER} (0.5--12 keV) band compar…
SAX J1748.9-2021 is a transient accretion powered millisecond X-ray pulsar located in the Globular cluster NGC 6440. We report on the spectral and timing analysis of SAX J1748.9-2021 performed on AstroSat data taken during its faint and short outburst of 2017. We derived the best-fitting orbital solution for the 2017 outburst and obtained an average local spin frequency of 442.361098(3) Hz. The pulse profile obtained from 3-7 keV and 7-20 keV energy bands suggest constant fractional amplitude ~0.5% for fundamental component, contrary to previously observed energy pulse profile dependence. Our …
We present broadband X-ray spectral analysis of the black hole X-ray binary MAXI J1348--630, performed using five AstroSat observations. The source was in the soft spectral state for the first three and in the hard state for the last two observations. The three soft state spectra were modelled using a relativistic thin accretion disc with reflection features and thermal Comptonization. Joint fitting of the soft state spectra constrained the spin parameter of the black hole $a_*$ $>$ 0.97 and the disc inclination angle $i$ = 32.9$\substack{+4.1 \\ -0.6}$ degrees. The bright and faint hard state…
The Cadmium Zinc Telluride Imager (CZTI) is an imaging instrument onboard AstroSat. This instrument operates as a nearly open all-sky detector above ~60 keV, making possible long integrations irrespective of the spacecraft pointing. We present a technique based on the AstroSat-CZTI data to explore the hard X-ray characteristics of the $γ$-ray pulsar population. We report highly significant ($\sim 30σ$) detection of hard X-ray (60--380 keV) pulse profile of the Crab pulsar using $\sim$5000 ks of CZTI observations within 5 to 70 degrees of Crab position in the sky, using a custom algorithm devel…
We present the results of the X-ray flaring activity of 1ES 1959+650 during October 25-26, 2017 using AstroSat observations. The source was variable in the X-ray. We investigated the evolution of the X-ray spectral properties of the source by dividing the total observation period ($\sim 130$ ksecs) into time segments of 5 ksecs, and fitting the SXT and LAXPC spectra for each segment. Synchrotron emission of a broken power-law particle density model provided a better fit than the log-parabola one. The X-ray flux and the normalised particle density at an energy less than the break one, were foun…
AstroSat is a multi-wavelength satellite launched on 2015 September 28. The CZT Imager of AstroSat on its very first day of operation detected a long duration gamma-ray burst (GRB) namely GRB 151006A. Using the off-axis imaging and spectral response of the instrument, we demonstrate that CZT Imager can localise this GRB correct to about a few degrees and it can provide, in conjunction with Swift, spectral parameters similar to that obtained from Fermi/GBM. Hence CZT Imager would be a useful addition to the currently operating GRB instruments (Swift and Fermi). Specifically, we argue that the C…
We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of 2.5$\times$10$^{35}$ erg s$^{-1}$ in 0.5--30 keV energy range, $\approx$41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detectio…
A NICER and AstroSat view of the neutron star low-mass X-ray binary 1A 1246-588
University of Alabama（阿拉巴马大学）；University of Mumbai（孟买大学）；Tata Institute of Fundamental Research（塔塔基础研究所）；INAF–Osservatorio Astronomico di Cagliari（意大利国家天体物理研究所卡利亚里天文台）
We present spectro-polarimetric analysis of \thisgrb\ using data from \asat, \fermi, and \swift, to provide insights into the physical mechanisms of the prompt radiation and the jet geometry. Prompt emission from \thisgrb\ was very bright (fluence $>10^{-4}$~ergs~cm$^{-2}$) and had a complex structure composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low energy peak $\sim 15$~keV --- which we interpret as a power-law with two breaks, with a synchrotron origin. Alternately, the prompt spectra can also be interpreted as Compton…
We present near-simultaneous Chandra/HST observations of the very faint ($L_{x} < 10^{36}$ erg s$^{-1}$) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3. The Chandra observations constrain the luminosity of M15 X-3 to be $< 10^{34}$ erg s$^{-1}$ in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power-law with break energy $E_{\rm break} = 2.7^{+0.4}_{-0.6}$ keV, and power law indices of $Γ_{1} = 1.3^{+0.1}_{-0.2}$ and $Γ_{2} = 1.9^{+0.2}_{-0.2}$ over a single power law. We fit our new F438W ($B$…
$GRB~160802A$ is one of the brightest gamma-ray bursts (GRBs) observed with $Fermi$ Gamma-ray Burst Monitor (GBM) in the energy range of $10~-~1000$ keV, while at the same time it is surprisingly faint at energies $\gtrsim2$ MeV. An observation with $AstroSat$/CZT Imager (CZTI) also provides the polarisation which helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the $Fermi$/GBM data, and find two main bursting episodes that are clearly separated in time, one particularly faint in higher energies and having certain differences in…
Transitional millisecond pulsars (tMSPs) have emerged in the last decade as a unique class of neutron stars at the crossroads between accretion- and rotation-powered phenomena. In their (sub-luminous) accretion disk state, with X-ray luminosities of order $10^{33}-10^{34}$ erg s$^{-1}$, they switch rapidly between two distinct X-ray modes: the disk-high (DH) and disk-low (DL) states. We present a systematic XMM-Newton and Chandra analysis of the aperiodic X-ray variability of all three currently known tMSPs, with a main focus on their disk state and separating DH and DL modes. We report the di…
We analyze X-ray light curves of the blazar Mrk 421 obtained from the Soft X-ray Imaging Telescope and the Large Area X-Ray Proportional Counter instrument onboard the Indian space telescope $AstroSat$ and archival observations from $Swift$. We show that the X-ray power spectral density (PSD) is a piece-wise power-law with a break, i.e., the index becomes more negative below a characteristic "break-timescale". Galactic black hole X-ray binaries and Seyfert galaxies exhibit a similar characteristic timescale in their X-ray variability that is proportional to their respective black hole mass. X-…
Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB afterglows to systematically probe the late time behaviors of afterglows including jet breaks, flares, and supernova bumps. In particular, the LBT observations have typical flux limits of 25-26 mag in the Sloan r' band, which allows us to extend the temporal baseline for measuring jet breaks by another decade in time scale. We detected four jet breaks (including a "textbook" jet break in GRB070125) and a fifth candidate, all of which are not detectable without deep, late time optical observations. In the other four cas…
We present multiwavelength observations of supernova (SN) 2017hcc with the Chandra X-ray telescope and the X-ray telescope onboard Swift (Swift-XRT) in X-ray bands, with the Spitzer and the TripleSpec spectrometer in near-infrared (IR) and mid-IR bands and with the Karl G. Jansky Very Large Array (VLA) for radio bands. The X-ray observations cover a period of 29 to 1310 days, with the first X-ray detection on day 727 with the Chandra. The SN was subsequently detected in the VLA radio bands from day 1000 onwards. While the radio data are sparse, synchrotron-self absorption is clearly ruled out …
Breaks in the radio spectra of supernova remnants (SNRs) reflect the maximum energy of either shock-accelerated electrons or - in the case of pulsar wind nebulae - of electrons injected by the central pulsar. Otherwise, the break may result from energy losses due to synchrotron aging or it is caused by energy-dependent diffusion. A spectral steepening of the plerionic SNR CTB87 at around 11 GHz was observed in the eighties, but a recent analysis of CTB87's energetic properties based on new radio data raised doubt on it. CTB87 consists of a central compact component surrounded by a diffuse cent…
