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Register a new userPrompt X-ray emission from Fast Radio Bursts -- Upper limits with AstroSat
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Fast Radio Bursts (FRBs) are short lived ($sim$ msec), energetic transients (having a peak flux density of $sim$ Jy) with no known prompt emission in other energy bands. We present results of a search for prompt X-ray emissions from 41 FRBs using the Cadmium Zinc Telluride Imager (CZTI) on AstroSat which continuously monitors $sim70%$ of the sky. Our searches on various timescales in the 20-200 keV range, did not yield any counterparts in this hard X-ray band. We calculate upper limits on hard X-ray flux, in the same energy range and convert them to upper bounds for $eta$: the ratio X-ray to radio fluence of FRBs. We find $eta leq 10^{8-10}$ for hard X-ray emission. Our results will help constrain the theoretical models of FRBs as the models become more quantitative and nearer, brighter FRBs are discovered.
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X-ray and Gamma-ray polarization measurements of the prompt emission of Gamma-ray bursts (GRBs) are believed to be extremely important for testing various models of GRBs. So far, the available measurements of hard X-ray polarization of GRB prompt emission have not significantly constrained the GRB models, particularly because of the difficulty of measuring polarization in these bands. The CZT Imager (CZTI) onboard {em AstroSat} is primarily an X-ray spectroscopic instrument that also works as a wide angle GRB monitor due to the transparency of its support structure above 100 keV. 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 its first year of operation. Among these, 5 GRBs show polarization signatures with $gtrapprox$3$sigma$, and 1 GRB shows $>$2$sigma$ detection significance. We place upper limits for the remaining 5 GRBs. We provide details of the various tests performed to validate our polarization measurements. While it is difficult yet to discriminate between various emission models with the current sample alone, the large number of polarization measurements CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission.
We consider some general implications of bright gamma-ray counterparts to fast radio bursts (FRBs). We show that even if these manifest in only a fraction of FRBs, gamma-ray detections with current satellites (including Swift) can provide stringent constraints on cosmological FRB models. If the energy is drawn from the magnetic energy of a compact object such as a magnetized neutron star, the sources should be nearby and be very rare. If the intergalactic medium is responsible for the observed dispersion measure, the required gamma-ray energy is comparable to that of the early afterglow or extended emission of short gamma-ray bursts. While this can be reconciled with the rotation energy of compact objects, as expected in many merger scenarios, the prompt outflow that yields the gamma-rays is too dense for radio waves to escape. Highly relativistic winds launched in a precursor phase, and forming a wind bubble, may avoid the scattering and absorption limits and could yield FRB emission. Largely independent of source models, we show that detectable radio afterglow emission from gamma-ray bright FRBs can reasonably be anticipated. Gravitational wave searches can also be expected to provide useful tests.
The discovery of periodicity in the arrival times of the fast radio bursts (FRBs) poses a challenge to the oft-studied magnetar scenarios. However, models that postulate that FRBs result from magnetized shocks or magnetic reconnection in a relativistic outflow are not specific to magnetar engines; instead, they require only the impulsive injection of relativistic energy into a dense magnetized medium. Motivated thus, we outline a new scenario in which FRBs are powered by short-lived relativistic outflows (``flares) from accreting black holes or neutron stars, which propagate into the cavity of the pre-existing (``quiescent) jet. In order to reproduce FRB luminosities and rates, we are driven to consider binaries of stellar-mass compact objects undergoing super-Eddington mass-transfer, similar to ultraluminous X-ray (ULX) sources. Indeed, the host galaxies of FRBs, and their spatial offsets within their hosts, show broad similarities with ULXs. Periodicity on timescales of days to years could be attributed to precession (e.g., Lens-Thirring) of the polar accretion funnel, along which the FRB emission is geometrically and relativistically beamed, which sweeps across the observer line of sight. Accounting for the most luminous FRBs via accretion power may require a population of binaries undergoing brief-lived phases of unstable (dynamical-timescale) mass-transfer. This will lead to secular evolution in the properties of some repeating FRBs on timescales of months to years, followed by a transient optical/IR counterpart akin to a luminous red nova, or a more luminous accretion-powered optical/X-ray transient. We encourage targeted FRB searches of known ULX sources.
We present constraints derived from a search of four years of IceCube data for a prompt neutrino flux from gamma-ray bursts (GRBs). A single low-significance neutrino, compatible with the atmospheric neutrino background, was found in coincidence with one of the 506 observed bursts. Although GRBs have been proposed as candidate sources for ultra-high energy cosmic rays, our limits on the neutrino flux disfavor much of the parameter space for the latest models. We also find that no more than $sim1%$ of the recently observed astrophysical neutrino flux consists of prompt emission from GRBs that are potentially observable by existing satellites.
We report results obtained from the study of 12 thermonuclear X-ray bursts in 6 AstroSat observations of a neutron star X-ray binary and well-known X-ray burster, 4U 1636$-$536. Burst oscillations at $sim$581 Hz are observed with 4$-$5$sigma$ confidence in three of these X-ray bursts. The rising phase burst oscillations show a decreasing trend of the fractional rms amplitude at 3$sigma$ confidence,by far the strongest evidence of thermonuclear flame spreading observed with AstroSat. During the initial 0.25 second of the rise a very high value (34.0$pm$6.7%) is observed. The concave shape of the fractional amplitude profile provides a strong evidence of latitude-dependent flame speeds, possibly due to the effects of the Coriolis force. We observe decay phase oscillations with amplitudes comparable to that observed during the rising phase, plausibly due to the combined effect of both surface modes as well as the cooling wake. The Doppler shifts due to the rapid rotation of the neutron star might cause hard pulses to precede the soft pulses, resulting in a soft lag. The distance to the source estimated using the PRE bursts is consistent with the known value of $sim$6 kpc.
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