On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

GRS 1915+105 was observed by the CGRO/OSSE 9 times in 1995-2000, and 8 of those observations were simultaneous with those by RXTE. We present an analysis of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law-like spectrum extending up to 600 keV without any break. We interpret this emission as strong evidence for the presence of non-thermal electrons in the source. The broad-band spectra cannot be described by either thermal or bulk-motion Comptonization, whereas they are well described by Comptonization in hybrid thermal/non-thermal plasmas.

X-ray and Gamma-ray polarization measurements of the prompt emission of Gamma-ray bursts (GRBs) are believed to be an important tool to test the various models of GRBs. Although there are some reports of hard X-ray polarization measurements of the prompt emission of GRBs, the number of measurements are small to provide statistically significant inputs to the GRB models due to the extreme difficulty of measuring them and quantifying their significance. CZTI onboard AstroSat is primarily an X-ray spectroscopic instrument but works as a wide angle GRB monitor due to the increasing transparency of the CZTI support structure. It also has experimentally verified polarization measurement capability in the 100 − 300 keV energy range and thus provides a unique opportunity to attempt spectro-polarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during the first year of operation. Most of the GRBs show clear polarization signatures with ≥3σ detection significance for 4 GRBs and ∼2.5σ significance for another 3 GRBs. We could place meaningful upper limits for the remaining 4 GRBs. We provide the details of the various tests performed to validate the polarization measurements. While it is difficult to differentiate the various emission models with the current sample of polarization measurements, CZTI in its minimum lifetime of five years is expected to provide a large sample of polarization measurements which would lead to a better understanding of the prompt emission.

We investigate the association between the radio "plateau" states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. To investigate the properties of the source during a superluminal flare, we present GMRT observations during a radio flare which turned out to be a pre-plateau flare as shown by the contemporaneous Ryle telescope observations. A major superluminal ejection was observed at the end of this "plateau" state (Dhawan et al. 2003), associated with highly variable X-ray emission showing X-ray soft dips. This episode, thus has all the three types of radio emission: a pre-plateau flare, a "plateau" state and superluminal jets. We analyze all the available RXTE-PCA data during this episode and show that: (1) the pre-flare "plateau" state consists of a three-component X-ray spectra which includes a multicolor disk-blackbody, a Comptonized component and a power-law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source and identify several candidate superluminal flare events and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the "plateau" state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are observed due to the interaction of the matter blob ejected during the X-ray soft dips, with the steady jet already established during the "plateau" state. This picture can explain all types of radio emission observed from this source in terms of its X-ray emission characteristics.

We present an exhaustive analysis of five broad‐band observations of GRS 1915+105 in two variability states, χ and ω, observed simultaneously by the Proportional Counter Array (PCA) and High‐Energy X‐ray Timing Experiment (HEXTE) detectors aboard the Rossi X‐ray Timing Explorer, and the Oriented Scintillation Spectrometer Experiment (OSSE) detector aboard the Compton Gamma‐ray Observatory. We find all the spectra well fitted by Comptonization of disc blackbody photons, with very strong evidence for the presence of a non‐thermal electron component in the Comptonizing plasma. Both the energy and the power spectra in the χ state are typical of the very high/intermediate state of black hole binaries. The spectrum of the ω state is characterized by a strong blackbody component Comptonized by thermal electrons and a weak non‐thermal tail. We then calculate rms spectra (fractional variability as functions of energy) for the PCA data. We accurately model the rms spectra by coherent superposition of variability in the components implied by the spectral fits, namely a less variable blackbody and more variable Comptonization. The latter dominates at high energies, resulting in a flattening of the rms at high energies in most of the data. This is also the case for the spectra of the quasi‐periodic oscillations present in the χ state. Then, some of our data require a radial dependence of the rms of the disc blackbody. We also study the distance to the source, and find d≃ 11 kpc as the most likely value, contrary to a recent claim of a much lower value.

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Abstract. We examine theoretically the behaviour of the inner accretion disk in GRS 1915+105 when soft X-ray dips are present in the X-ray light curve. We assume the presence of a radial shock in the accretion disk, as in some of the Two Component Advective Flow (TCAF) solutions. We discuss the behaviour of the flux tubes inside a TCAF (which we name Magnetized TCAF or MTCAF model for brevity) and compare various competing forces on the flux tubes. In this MTCAF model, we find that the magnetic tension is the strongest force in a hot plasma of temperature > ∼ 10 10 K and as a result, magnetic flux tubes entering in this region collapse catastrophically, thereby occasionally evacuating the inner disk. We postulate that this magnetic "rubber-band" effect induced evacuated disk matter produces the blobby components of outflows and IR/radio jets. We derive the size of the post-shock region by equating the time scale of the Quasi-Periodic Oscillations to the infall time of accreting matter in the post-shock region and found the shock location to be ∼45−66 rg. We calculate the transition radius rtr, where the Keplerian disk deviates into a sub-Keplerian flow, to be ∼320 rg. Based on the derived X-ray spectral parameters, we calculate the mass of this region to be ∼10 18 g. We conclude that during the X-ray dips the matter in the post-shock region, which manifests itself as the thermal-Compton component in the X-ray spectrum, is ejected, along with some sub-Keplerian matter in the pre-shock region.

GRB 160821A is the third most energetic gamma ray burst observed by the Fermi gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board AstroSat, here we report the most conclusive evidence to date of (i) high linear polarization (66 +26 −27 %; 5.3 σ detection), and (ii) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at 3.5 σ and 3.1 σ detections respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of 1/Γ, where Γ is the Lorentz factor of the outflow.