Swift has now completed 18 years of mission, during which it discovered thousands of gamma-ray bursts 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.
Gamma-ray bursts (GRBs) associated with gravitational wave events are, and will likely continue to be, viewed at a larger inclination than GRBs without gravitational wave detections. As demonstrated ...by the afterglow of GW 170817A, this requires an extension of the common GRB afterglow models, which typically assume emission from an on-axis top-hat jet. We present a characterization of the afterglows arising from structured jets, providing a framework covering both successful and choked jets. We compute new closure relations for decelerating structured jets and compare them with the established relations for energy injection and refreshed shock models. The temporal slope before the jet break is found to be a simple function of the ratio between the viewing angle and effective opening angle of the jet. A numerical model to calculate synthetic light curves and spectra is publicly available as the open-source Python package afterglowpy.
ABSTRACT To date, the Burst Alert Telescope (BAT) onboard Swift has detected ∼1000 gamma-ray bursts (GRBs), of which ∼360 GRBs have redshift measurements, ranging from z = 0.03 to z = 9.38. We ...present the analyses of the BAT-detected GRBs for the past ∼11 years up through GRB 151027B. We report summaries of both the temporal and spectral analyses of the GRB characteristics using event data (i.e., data for each photon within approximately 250 s before and 950 s after the BAT trigger time), and discuss the instrumental sensitivity and selection effects of GRB detections. We also explore the GRB properties with redshift when possible. The result summaries and data products are available at http://swift.gsfc.nasa.gov/results/batgrbcat/index.html. In addition, we perform searches for GRB emissions before or after the event data using the BAT survey data. We estimate the false detection rate to be only one false detection in this sample. There are 15 ultra-long GRBs (∼2% of the BAT GRBs) in this search with confirmed emission beyond ∼1000 s of event data, and only two GRBs (GRB 100316D and GRB 101024A) with detections in the survey data prior to the starting of event data.
Recently, the optical counterpart of the gravitational-wave source GW170817 has been identified in the NGC 4993 galaxy. Together with evidence from observations in electromagnetic waves, the event ...has been suggested as a result of a merger of two neutron stars (NSs). We analyze the multi-wavelength data to characterize the host galaxy property and its distance to examine if the properties of NGC 4993 are consistent with this picture. Our analysis shows that NGC 4993 is a bulge-dominated galaxy with and a Sérsic index of for the bulge component. The spectral energy distribution from 0.15 to 24 m indicates that this galaxy has no significant ongoing star formation, a mean stellar mass of , a mean stellar age greater than ∼3 Gyr, and a metallicity of about 20%-100% of solar abundance. Optical images reveal dust lanes and extended features that suggest a past merging activity. Overall, NGC 4993 has characteristics of normal, but slightly disturbed elliptical galaxies. Furthermore, we derive the distance to NGC 4993 with the fundamental plane relation using 17 parameter sets of 7 different filters and the central stellar velocity dispersion from the literature, finding an angular diameter distance of 37.7 8.7 Mpc. NGC 4993 is similar to some host galaxies of short gamma-ray bursts (GRBs) but much different from those of long GRBs, supporting the picture of GW170817 as a result of the merger of two NSs.
Abstract
FRB 180916.J0158+65 is one of the nearest, periodically repeating, and actively bursting fast radio bursts (FRBs) that has been localized to the outskirts of a spiral galaxy. In this work we ...study the FRB with the hard X-ray 14–195 keV data from the Burst Alert Telescope (BAT) on board The Neil Gehrels Swift Observatory. BAT uses coded mask technology giving a localization of ≲3′ in the hard X-ray band, along with an accurate background estimation. BAT has been observing the source location in survey mode since 2020 February. The survey mode observations involve background subtracted spectra, integrated over a time span ranging 300–2000 s at the source location (from 2022 February–2022 January). We analyzed all the ∼230 survey mode observations from BAT and checked for any signal in any of the observations. We did not detect any signal at >5
σ
confidence level in any of the observations. We could estimate a 5
σ
upper limit on the 14–195 keV flux, which ranged between 4.5 × 10
−10
–7.6 × 10
−9
erg cm
−2
s
−1
. At the source distance this relates to a 5
σ
upper limit on a luminosity of 5.08 × 10
44
–8.5 × 10
45
erg s
−1
. With this estimate, we could rule out any persistent X-ray emission at the source location for these snapshots of BAT observations.
GRB 130925A is a peculiar event characterized by an extremely long gamma-ray duration (approximate7 ks), as well as dramatic flaring in the X-rays for approximate20 ks. After this period, its X-ray ...afterglow shows an atypical soft spectrum with photon index Gamma ~ 4, as observed by Swift and Chandra, until approximate10 super(7) s, when XMM-Newton observations uncover a harder spectral shape with Gamma ~ 2.5, commonly observed in gamma-ray burst (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.
We report observations of the optical counterpart of the long gamma-ray burst GRB 221009A. Due to the extreme rarity of being both nearby (z = 0.151) and highly energetic (Eϒ,iso≥ 1054erg), GRB ...221009A offers a unique opportunity to probe the connection between massive star core collapse and relativistic jet formation across a very broad range of γ-ray properties. Adopting a phenomenological power-law model for the afterglow and host galaxy estimates from high-resolution Hubble Space Telescope imaging, we use Bayesian model comparison techniques to determine the likelihood of an associated supernova (SN) contributing excess flux to the optical light curve. Though not conclusive, we find moderate evidence (KBayes=101.2for the presence of an additional component arising from an associated SN, SN 2022xiw, and find that it must be substantially fainter (<67% as bright at the 99% confidence interval) than SN 1998bw. Given the large and uncertain line-of-sight extinction, we attempt to constrain the SN parameters (MNi, Mejand EKE) under several different assumptions with respect to the host galaxy's extinction. We find properties that are broadly consistent with previous GRB-associated SNe: MNi = 0.05–0.25 M⊙, Mej = 3.5–11.1 M⊙, and EKE = (1.6–5.2) × 1052 erg. We note that these properties are weakly constrained due to the faintness of the SN with respect to the afterglow and host emission, but we do find a robust upper limit on MNi of MNi < 0.36 M⊙. Given the tremendous range in isotropic gamma-ray energy release exhibited by GRBs (seven orders of magnitude), the SN emission appears to be decoupled from the central engine in these systems.
The origin and progenitors of short-hard gamma-ray bursts (GRBs) remain a puzzle and a highly debated topic. Recent Swift observations suggest that these GRBs may be related to catastrophic ...explosions in degenerate compact stars, denoted as 'Type I' GRBs. The most popular models include the merger of two compact stellar objects (NS-NS or NS-BH). We utilize a Monte Carlo approach to determine whether a merger progenitor model can self-consistently account for all the observations of short-hard GRBs, including a sample with redshift measurements in the Swift era (z-known sample) and the CGRO/BATSE sample. We apply various merger time delay distributions invoked in compact star merger models to derive the redshift distributions of these Type I GRBs, and then constrain the unknown luminosity function of Type I GRBs using the observed luminosity-redshift (L-z) distributions of the z-known sample. The best luminosity function model, together with the adopted merger delay model, is then applied to confront the peak flux distribution (log N-log P distribution) of the BATSE and Swift samples. We find that for all the merger models invoking a range of merger delay timescales (including those invoking a large fraction of 'prompt mergers'), it is difficult to reconcile the models with all the data. The data are instead statistically consistent with the following two possible scenarios. First, that short/hard GRBs are a superposition of compact-star-merger-origin (Type I) GRBs and a population of GRBs that track the star formation history, which are probably related to the deaths of massive stars (Type II GRBs). Second, the entire short/hard GRB population is consistent with a typical delay of 2 Gyr with respect to the star formation history with modest scatter. This may point toward a different Type I progenitor than the traditional compact star merger models.
We have recently witnessed the first multi-messenger detection of colliding neutron stars through gravitational waves (GWs) and electromagnetic (EM) waves (GW 170817) thanks to the joint efforts of ...LIGO/Virgo and Space/Ground-based telescopes. In this paper, we report on the RATIR follow-up observation strategies and show the results for the trigger G194575. This trigger is not of astrophysical interest; however, it is of great interest to the robust design of a follow-up engine to explore large sky-error regions. We discuss the development of an image-subtraction pipeline for the six-color, optical/NIR imaging camera RATIR. Considering a two-band (i and r) campaign in the fall of 2015, we find that the requirement of simultaneous detection in both bands leads to a factor ∼10 reduction in false alarm rate, which can be further reduced using additional bands. We also show that the performance of our proposed algorithm is robust to fluctuating observing conditions, maintaining a low false alarm rate with a modest decrease in system efficiency that can be overcome utilizing repeat visits. Expanding our pipeline to search for either optical or NIR detections (three or more bands), considering separately the optical riZ and NIR YJH bands, should result in a false alarm rate 1% and an efficiency 90%. RATIR's simultaneous optical/NIR observations are expected to yield about one candidate transient in the vast 100 deg2 LIGO error region for prioritized follow-up with larger aperture telescopes.
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