73 research outputs found
Eighteen Years of Kilonova Discoveries with Swift
Swift has now completed 18 years of mission, during which it discovered
thousands of gamma-ray bursts (GRBs) as well as new classes of high-energy
transient phenomena. Its first breakthrough result was the localization of
short duration GRBs, which enabled for redshift measurements and kilonova
searches. Swift, in synergy with the Hubble Space Telescope and a wide array of
ground-based telescopes, provided the first tantalizing evidence of a kilonova
in the aftermath of a short GRB. In 2017, Swift observations of the
gravitational wave event GW170817 captured the early UV photons from the
kilonova AT2017gfo, opening a new window into the physics of kilonovae. Since
then, Swift has continued to expand the sample of known kilonovae, leading to
the surprising discovery of a kilonova in a long duration GRB. This article
will discuss recent advances in the study of kilonovae driven by the
fundamental contribution of Swift.Comment: 26 pages, 7 figures, 2 tables. Published as part of the Special Issue
"18 Years of Science with the Neil Gehrels Swift Observatory's
Ultra-Violet/Optical Telescope
Compact Binary Progenitors of Short Gamma-Ray Bursts
In recent years, detailed observations and accurate numerical simulations have provided support to the idea that mergers of compact binaries containing either two neutron stars (NSs) or an NS and a black hole (BH) may constitute the central engine of short gamma-ray bursts (SGRBs). The merger of such compact binaries is expected to lead to the production of a spinning BH surrounded by an accreting torus. Several mechanisms can extract energy from this system and power the SGRBs. Here we connect observations and numerical simulations of compact binary mergers, and use the current sample of SGRBs with measured energies to constrain the mass of their powering tori. By comparing the masses of the tori with the results of fully general-relativistic simulations, we are able to infer the properties of the binary progenitors that yield SGRBs. By assuming a constant efficiency in converting torus mass into jet energy epsilon(sub jet) = 10%, we find that most of the tori have masses smaller than 0.01 Solar M, favoring "high-mass" binary NSs mergers, i.e., binaries with total masses approx >1.5 the maximum mass of an isolated NS. This has important consequences for the gravitational wave signals that may be detected in association with SGRBs, since "high-mass" systems do not form a long-lived hypermassive NS after the merger. While NS-BH systems cannot be excluded to be the engine of at least some of the SGRBs, the BH would need to have an initial spin of approx. 0.9 or higher
A hot cocoon in the ultralong GRB 130925A: hints of a PopIII-like progenitor in a low density wind environment
GRB 130925A is a peculiar event characterized by an extremely long gamma-ray
duration (7 ks), as well as dramatic flaring in the X-rays for
20 ks. After this period, its X-ray afterglow shows an atypical soft
spectrum with photon index 4, as observed by Swift and Chandra,
until s, when XMM-Newton observations uncover a harder spectral
shape with 2.5, commonly observed in GRB afterglows. We find that
two distinct emission components are needed to explain the X-ray observations:
a thermal component, which dominates the X-ray emission for several weeks, and
a non-thermal component, consistent with a typical afterglow. A forward shock
model well describes the broadband (from radio to X-rays) afterglow spectrum at
various epochs. It requires an ambient medium with a very low density wind
profile, consistent with that expected from a low-metallicity blue supergiant
(BSG). The thermal component has a remarkably constant size and a total energy
consistent with those expected by a hot cocoon surrounding the relativistic
jet. We argue that the features observed in this GRB (its ultralong duration,
the thermal cocoon, and the low density wind environment) are associated with a
low metallicity BSG progenitor and, thus, should characterize the class of
ultralong GRBs.Comment: 6 pgs, 3 figs, fig1 revised, ApJL in pres
Joint analysis of gravitational-wave and electromagnetic data of mergers:breaking an afterglow model degeneracy in GW170817 and in future events
On August 17, 2017, Advanced LIGO and Virgo observed GW170817, the first
gravitational-wave (GW) signal from a binary neutron star merger. It was
followed by a short-duration gamma-ray burst, GRB 170817A, and by a non-thermal
afterglow emission. In this work, a combined simultaneous fit of the
electromagnetic (EM, specifically, afterglow) and GW domains is implemented,
both using the posterior distribution of a GW standalone analysis as prior
distribution to separately process the EM data, and fitting the EM and GW
domains simultaneously. These approaches coincide mathematically, as long as
the actual posterior of the GW analysis, and not an approximation, is used as
prior for the EM analysis. We treat the viewing angle, , as shared
parameter across the two domains. In the afterglow modelling with a Gaussian
structured jet this parameter and the jet core angle, , are
correlated, leading to high uncertainties on their values. The joint EM+GW
analysis relaxes this degeneracy, reducing the uncertainty compared to an
EM-only fit. We also apply our methodology to hypothetical GW170817-like events
occurring in the next GW observing run at 140 and 70 Mpc. At 70 Mpc the
existing EM degeneracy is broken, thanks to the inclusion of the GW domain in
the analysis. At 140 Mpc, the EM-only fit cannot constrain nor
because of the lack of detections in the afterglow rising phase.
Folding the GW data into the analysis leads to tighter constraints on
, still leaving unconstrained, requiring instruments with
higher sensitivities, such as Athena.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
DO the FERMI GAMMA-RAY BURST MONITOR and SWIFT BURST ALERT TELESCOPE SEE the SAME SHORT GAMMA-RAY BURSTS?
Compact binary system mergers are expected to generate gravitational radiation detectable by ground-based interferometers. A subset of these, the merger of a neutron star with another neutron star or a black hole, are also the most popular model for the production of short gamma-ray bursts (GRBs). The Swift Burst Alert Telescope (BAT) and the Fermi Gamma-ray Burst Monitor (GBM) trigger on short GRBs (SGRBs) at rates that reflect their relative sky exposures, with the BAT detecting 10 per year compared to about 45 for GBM. We examine the SGRB populations detected by Swift BAT and Fermi GBM. We find that the Swift BAT triggers on weaker SGRBs than Fermi GBM, providing they occur close to the center of the BAT field of view, and that the Fermi GBM SGRB detection threshold remains flatter across its field of view. Overall, these effects combine to give the instruments the same average sensitivity, and account for the SGRBs that trigger one instrument but not the other. We do not find any evidence that the BAT and GBM are detecting significantly different populations of SGRBs. Both instruments can detect untriggered SGRBs using ground searches seeded with time and position. The detection of SGRBs below the on-board triggering sensitivities of Swift BAT and Fermi GBM increases the possibility of detecting and localizing the electromagnetic counterparts of gravitational wave (GW) events seen by the new generation of GW detectors
Searching for the radio remnants of short duration gamma-ray bursts
Neutron star mergers produce a substantial amount of fast-moving ejecta,
expanding outwardly for years after the merger. The interaction of these ejecta
with the surrounding medium may produce a weak isotropic radio remnant,
detectable in relatively nearby events. We use late-time radio observations of
short duration gamma-ray bursts (sGRBs) to constrain this model. Two samples of
events were studied: four sGRBs that are possibly in the local (<200 Mpc)
universe were selected to constrain the remnant non-thermal emission from the
sub-relativistic ejecta, whereas 17 sGRBs at cosmological distances were used
to constrain the presence of a proto-magnetar central engine, possibly
re-energezing the merger ejecta. We consider the case of GRB~170817A/GW170817,
and find that in this case the early radio emission may be quenched by the jet
blast-wave. In all cases, for ejecta mass range of M_ej \lesssim 10^{-2} (5 *
10^{-2}) M_sun, we can rule out very energetic merger ejecta E_ej \gtrsim 5 *
10^{52}(10^{53}) erg, thus excluding the presence of a powerful magnetar as a
merger remnant.Comment: 13 pages, 8 figures, 3 tables. Submitted to MNRA
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