A multiwavelength study of the relativistic tidal disruption canidate Sw J2058+05 at late times

Swift J2058.4+0516 (Sw J2058+05, hereafter) has been suggested as the second member (after Sw J1644+57) of the rare class of tidal disruption events accompanied by relativistic ejecta. Here we report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of Sw J2058+05 from 3 months to 3 yr post-discovery in order to study its properties and compare its behavior with that of Sw J1644+57. Our main results are as follows. (1) The long-term X-ray light curve of Sw J2058+05 shows a remarkably similar trend to that of Sw J1644+57. After a prolonged power-law decay, the X-ray flux drops off rapidly by a factor of ≳160 within a span of Δt/t ≤ 0.95. Associating this sudden decline with the transition from super-Eddington to sub-Eddington accretion, we estimate the black hole mass to be in the range of 104−6 M⊙. (2) We detect rapid (≲500 s) X-ray variability before the dropoff, suggesting that, even at late times, the X-rays originate from close to the black hole (ruling out a forward-shock origin). (3) We confirm using HST and VLBA astrometry that the location of the source coincides with the galaxy's center to within ≲400 pc (in projection). (4) We modeled Sw J2058+05's ultraviolet/optical/infrared spectral energy distribution with a single-temperature blackbody and find that while the radius remains more or less constant at a value of 63.4±4.5 AU (∼1015 cm) at all times during the outburst, the blackbody temperature drops significantly from ∼ 30,000 K at early times to a value of ∼ 15,000 K at late times (before the X-ray dropoff). Our results strengthen Sw J2058+05's interpretation as a tidal disruption event similar to Sw J1644+57. For such systems, we suggest the rapid X-ray dropoff as a diagnostic for black hole mass

The early- and late-time spectral and temporal evolution of ORB 050716

We report on a comprehensive set of observations of gamma-ray burst 050716, detected by the Swift satellite and subsequently followed-up rapidly in X-ray, optical and near-infrared (NIR) wavebands. The prompt emission is typical of long-duration bursts, with two peaks in a time interval of T90= 68 s (15–350 keV). The prompt emission continues at lower flux levels in the X-ray band, where several smaller flares can be seen on the top of a decaying light curve that exhibits an apparent break around 220 s post-trigger. This temporal break is roughly coincident with a spectral break. The latter can be related to the extrapolated evolution of the break energy in the prompt γ-ray emission, and is possibly the manifestation of the peak flux break frequency of the internal shock passing through the observing band. A possible 3σ change in the X-ray absorption column is also seen during this time. The late-time afterglow behaviour is relatively standard, with an electron distribution power-law index of p= 2; there is no notable temporal break out to at least 10 d. The broad-band optical/NIR to X-ray spectrum indicates a redshift of z≳ 2 for this burst, with a host-galaxy extinction value of EB−V≈ 0.7 that prefers a small magellanic cloud (SMC)-like extinction curve

The extreme, red afterglow of GRB 060923A: Distance or dust?

Gamma-ray bursts (GRBs) are powerful probes of the early Universe, but locating and identifying very distant GRBs remain challenging. We report here the discovery of the K-band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high-z burst (z≳ 11). However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely z≲ 2.8) and therefore that dust is the likely cause of the very red-afterglow colour. This being the case, it is one of the few instances so far found of a GRB afterglow with high-dust extinction

An Ultraviolet Spectrum of the Tidal Disruption Flare ASASSN-14li

We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{\mathrm{UV}} = 3.5 \times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a High-Velocity Cloud), and narrow absorption and broad (FWHM $\approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $\Delta v = -(250$-400) km s$^{-1}$. Together with the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars

A Precise Distance to the Host Galaxy of the Binary Neutron Star Merger GW170817 Using Surface Brightness Fluctuations

The joint detection of gravitational waves (GWs) and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of physical processes: heavy element synthesis via the r-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the "standard siren" technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC 4993. In this Letter we measure the surface brightness fluctuation (SBF) distance to NGC 4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope (HST). For the preferred F110W passband we derive a distance modulus of $(m-M)\,=33.05\pm 0.08\pm 0.10$ mag, or a linear distance d = 40.7 ± 1.4 ± 1.9 Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC 4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC 4993 implies a Hubble constant H 0 = 71.9 ± 7.1 km s−1 Mpc−1. A comparison of our result to the GW-inferred value of H 0 indicates a binary orbital inclination of i gsim 137°. The SBF technique can be applied to early-type host galaxies of BNS mergers to ~100 Mpc with HST and possibly as far as ~300 Mpc with the James Webb Space Telescope, thereby helping to break the inherent distance-inclination degeneracy of the GW data at distances where many future BNS mergers are likely to be detected

Optical and near-infrared observations of the GRB020405 afterglow

We report on photometric, spectroscopic and polarimetric monitoring of the optical and near-infrared (NIR) afterglow of GRB020405. Ground-based optical observations, performed with 8 different telescopes, started about 1 day after the high-energy prompt event and spanned a period of ~10 days; the addition of archival HST data extended the coverage up to ~150 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of Balmer and oxygen emission lines in the optical spectrum of the host galaxy indicates that the GRB is located at redshift z =0.691. Fe II and Mg II absorption systems are detected at z= 0.691 and at z = 0.472 in the afterglow optical spectrum. The latter system is likely caused by absorbing clouds in the galaxy complex located ~2 '' southwest of the GRB020405 host. Hence, for the first time, the galaxy responsible for an intervening absorption line system in the spectrum of a GRB afterglow is spectroscopically identified. Optical and NIR photometry of the afterglow indicates that, between 1 and 10 days after the GRB, the decay in all bands is consistent with a single power law of index $\alpha = 1.54\pm 0.06$. The late-epoch VLT J-band and HST optical points lie above the extrapolation of this power law, so that a plateau (or "bump") is apparent in the VRIJ light curves at 10-20 days after the GRB. The light curves at epochs later than day ~20 after the GRB are consistent with a power-law decay with index $\alpha' = 1.85\pm 0.15$. While other authors have proposed to reproduce the bump with the template of the supernova (SN) 1998bw, considered the prototypical "hypernova", we suggest that it can also be modeled with a SN having the same temporal profile as the other proposed hypernova SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A single polarimetric R-band measurement shows that the afterglow is polarized, with $P = 1.5\pm 0.4$% and polarization angle $\theta = 172^{\circ}\pm 8^{\circ}$. Broad-band optical-NIR spectral flux distributions show, in the first days after the GRB, a change of slope across the J band which we interpret as due to the presence of the electron cooling frequency $\nu_{\rm c}$. The analysis of the multiwavelength spectrum within the standard fireball model suggests that a population of relativistic electrons with index $p \sim 2.7$ produces the optical-NIR emission via synchrotron radiation in an adiabatically expanding blastwave, with negligible host galaxy extinction, and the X-rays via Inverse Compton scattering off lower-frequency afterglow photons

ALMA and GMRT Constraints on the Off-axis Gamma-Ray Burst 170817A from the Binary Neutron Star Merger GW170817

Binary neutron-star mergers (BNSMs) are among the most readily detectable gravitational-wave (GW) sources with the Laser Interferometer Gravitational-wave Observatory (LIGO). They are also thought to produce short γ-ray bursts (SGRBs) and kilonovae that are powered by r-process nuclei. Detecting these phenomena simultaneously would provide an unprecedented view of the physics during and after the merger of two compact objects. Such a Rosetta Stone event was detected by LIGO/Virgo on 2017 August 17 at a distance of ∼44 Mpc. We monitored the position of the BNSM with Atacama Large Millimeter/submillimeter Array (ALMA) at 338.5 GHz and the Giant Metrewave Radio Telescope (GMRT) at 1.4 GHz, from 1.4 to 44 days after the merger. Our observations rule out any afterglow more luminous than ´ - - 3 10 erg s Hz 26 1 1 in these bands, probing >2–4 dex fainter than previous SGRB limits. We match these limits, in conjunction with public data announcing the appearance of X-ray and radio emission in the weeks after the GW event, to templates of off-axis afterglows. Our broadband modeling suggests that GW170817 was accompanied by an SGRB and that the γ-ray burst (GRB) jet, powered by EAG,iso ~ 1050 erg, had a half-opening angle of ~20, and was misaligned by ~41 from our line of sight. The data are also consistent with a more collimated jet: EAG,iso ~ 1051 erg, q1 2,jet obs ~ ~ 5 , 17 q . This is the most conclusive detection of an off-axis GRB afterglow and the first associated with a BNSM-GW event to date. We use the viewing angle estimates to infer the initial bulk Lorentz factor and true energy release of the burst

Discovery of the nearby long, soft GRB100316D with an associated supernova

We report the Swift discovery of the nearby long, soft gamma-ray burst GRB 100316D, and the subsequent unveiling of its low-redshift host galaxy and associated supernova. We derive the redshift of the event to be z= 0.0591 ± 0.0001 and provide accurate astrometry for the gamma-ray burst (GRB) supernova (SN). We study the extremely unusual prompt emission with time-resolved γ-ray to X-ray spectroscopy and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini-South, Very Large Telescope and Hubble Space Telescope observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB 100316D to a subsample of GRB-SNe. GRB 100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB 060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB 100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB-SN connection can be gleaned

The Swift gamma-ray burst mission

The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr-1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z > 10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector that will detect bursts, calculate 1'-4' positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5'' positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 0farcs3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of ~1 mcrab (~2 × 10-11 ergs cm-2 s-1 in the 15-150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of ~8 yr