We investigated how gamma-ray burst (GRB) prompt emission measurements are affected by increasing distance to the source. We selected a sample of 26 bright GRBs with measured redshifts z < 1 observed by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory (Swift) and simulated what BAT would have observed if the GRBs were at larger redshifts. We measured the durations of the simulated gamma-ray signals using a Bayesian block approach and calculated the enclosed fluences and peak fluxes. As expected, we found that almost all durations (fluences) measured for simulated high-z GRBs were shorter (less) than their true durations (energies) due to low signal-to-noise ratio emission becoming completely dominated by background, i.e., the "tip-of-the-iceberg" effect. This effect strongly depends on the profile and intensity of the source light curve. Due to the uniqueness of GRB light curves, there is no common behavior in the evolution of measured durations with redshift. We compared our synthetic high-z (i.e., z > 3) GRBs to a sample of 72 observed high-z bursts and found that the two samples were not inconsistent with being drawn from the same underlying population. We conclude that (i) prompt emission durations (fluences) of high-z GRBs observed by Swift/BAT are most likely underestimations, sometimes by factors of about several tens (similar to 2), and (ii) changes in the average GRB prompt emission duration and fluence with increasing redshift are consistent with the tip-of-the-iceberg effect.

We investigated how Gamma-Ray Burst (GRB) prompt emission measurements are affected by increasing distance to the source. We selected a sample of 26 bright GRBs with measured redshifts z<1 observed by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory (Swift) and simulated what BAT would have observed if the GRBs were at larger redshifts. We measured the durations of the simulated gamma-ray signals using a Bayesian block approach and calculated the enclosed fluences and peak fluxes. As expected, we found that almost all durations (fluences) measured for simulated high-z GRBs were shorter (less) than their true durations (energies) due to low signal-to-noise ratio emission becoming completely dominated by background, i.e., the “tip-of-the-iceberg” effect. This effect strongly depends on the profile and intensity of the source light curve. Due to the uniqueness of GRB light curves, there is no common behavior in the evolution of measured durations with redshift. We compared our synthetic high-z (i.e., z>3) GRBs to a sample of 72 observed high-z bursts and found that the two samples were not inconsistent with being drawn from the same underlying population. We conclude that: (i) prompt emission durations (fluences) of high-z GRBs observed by Swift/BAT are most likely underestimations, sometimes by factors of ∼several tens (∼2), and (ii) changes in the average GRB prompt emission duration and fluence with increasing redshift are consistent with the tip-of-the-iceberg effect.

For over 25 yr, the origin of long-duration gamma-ray bursts (lGRBs) has been linked to the collapse of rotating massive stars. However, we have yet to pinpoint the stellar progenitor powering these transients. Moreover, the dominant engine powering the explosions remains open to debate. Observations of both lGRBs, supernovae associated with these GRBs, such as broad-line (BL) stripped-envelope (type Ic) supernovae (hereafter, Ic-BL), supernovae (SNe), and perhaps superluminous SNe, fast blue optical transients, and fast x-ray transients, may provide clues to both engines and progenitors. In this paper, we conduct a detailed study of the tight-binary formation scenario for lGRBs, comparing this scenario to other leading progenitor models. Combining this progenitor scenario with different lGRB engines, we can compare to existing data and make predictions for future observational tests. We find that the combination of the tight-binary progenitor scenario with the black hole accretion disk engine can explain lGRBs, low-luminosity GRBs, ultra-long GRBs, and Ic-BL. We discuss the various progenitor properties required for these different subclasses and note such systems would be future gravitational-wave merger sources. We show that the current literature on other progenitor-engine scenarios cannot explain all of these transient classes with a single origin, motivating additional work. We find that the tight-binary progenitor with a magnetar engine is excluded by existing observations. The observations can be used to constrain the properties of stellar evolution, the nature of the GRB, and the associated SN engines in lGRBs and Ic-BL. We discuss the future observations needed to constrain our understanding of these rare, but powerful, explosions.

The Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) is a coded aperture gamma-ray instrument with a large field of view that was designed to detect and localize transient events. When a transient is detected, either on board or externally, the BAT saves time-tagged event (TTE) data, which provide the highest-quality information of the locations of the photons on the detector plane and their energies. These data can be used to produce spectra, lightcurves, and sky images of a transient event. While these data products are produced by the Swift Data Center and can be produced by current software, they are often preset to certain time and energy intervals, which have limited their use in the current time domain and multimessenger environment. Here, we introduce a new capability for the BatAnalysis Python package to download and process TTE data under an open-source Python framework that allows for easy interfacing with other Python packages. The new capabilities of the BatAnalysis software allow for TTE data to be used by the community in a variety of advanced customized analyses of astrophysical sources which BAT may have TTE data for, such as fast radio bursts (FRBs), gamma-ray bursts (GRBs), low-mass X-ray binaries (LMXB), soft gamma repeaters, magnetars, and many other sources. We highlight the usefulness of the BatAnalysis package in analyzing TTE data produced by an onboard GRB trigger, an FRB external trigger, a subthreshold detection of the LMXB EXO 0748–676, and an external trigger of a GRB that BAT detected during a slew.

Extended regions of very high-energy γ -ray emission associated with middle-aged pulsars have been found by γ -ray observatories. These regions, called TeV halos or pulsar halos, are thought to be created when energetic electrons from a pulsar or pulsar wind nebula transport into the interstellar medium and undergo inverse Compton scattering with the cosmic microwave background radiation and the interstellar radiation field. The same electrons are expected to emit synchrotron emission in the X-ray band in the interstellar magnetic field. HESS J1813-126 is a pulsar halo candidate from which TeV γ -ray emission with extension 0 . ° 21 and a hard E −2 spectrum is observed. We searched for the synchrotron component of this pulsar halo with Swift-X-Ray Telescope (XRT). In particular, we observed two fields within the region covered by HESS J1813-126 for 35 ks each and a region nearby as a background reference for 10 ks. We also analyzed 21 ks of archival Swift-XRT observations of PSR J1813-1246 and archival Swift-XRT observations of the nearby Galactic X-ray background. We find no evidence for excess X-ray emission from the two observations near HESS J1813-126 and place an upper limit differential flux of 2.18 × 10 −3 and 2.02 × 10 −3 keV −1 cm −2 s −1 at 1 keV, assuming a power-law spectrum in Fields 1 and 2. The archival differential flux upper limit is 3.36 × 10 −3 keV −1 cm −2 s −1 at 1 keV. The nondetection implies that the magnetic field inside the halo is not significantly enhanced compared to the average Galactic magnetic field.

GRB 220831A is a gamma-ray burst (GRB) with a duration and spectral peak energy that places it at the interface between the distribution of long-soft and short-hard GRBs. In this paper, we present the multiwavelength follow-up campaign to GRB 220831A and its optical, near-infrared, X-ray and radio counterparts. Our deep optical and near-infrared observations do not reveal an underlying host galaxy, and establish that GRB 220831A is observationally hostless to depth, m(i )greater than or similar to 26.6 AB mag. Based on the Amati relation and the non-detection of an accompanying supernova, we find that this GRB is most likely to have originated from a collapsar at z>2, but it could also possibly be a compact object merger at z<0.4 with a large separation distance from its host galaxy. Regardless of its origin, we show that its optical and near-infrared counterpart departs from the evolution expected from a dominated synchrotron afterglow, exhibiting a steep post-break temporal power-law index of -3.83(-0.79)(+0.62), too steep to be the jet-break. By analysing a range of models, we find that the observed steep departure from forward shock closure relations is likely due to an internal process producing either a flare or a plateau.

The Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory has been serving as a survey instrument for the hard X-ray sky, and has detected thousands of X-ray sources (e.g., active galactic nuclei, X-ray binaries, etc). BAT monitors these X-ray sources and follows their light curves on timescales from minutes to years. In addition, BAT has discovered hundreds of new X-ray sources in survey images stacked throughout the mission lifetime. We present an updated BAT survey catalog since the last published BAT 105 month survey catalog with additional 4.5 yr of data until 2017 December. Data since 2007 are reprocessed to include updated instrumental calibrations. Analysis in this study shows that additional systematic noise can be seen in the 157 month mosaic images, resulting in decreases in the expected improvement in sensitivity and the number of new detections. The BAT 157 month survey reaches a sensitivity of 8.83 × 10 ^−12 erg s ^−1 cm ^−2 for 90% of the sky and 6.44 × 10 ^−12 erg s ^−1 cm ^−2 for 10% of the sky. This catalog includes spectra, and monthly and snapshot light curves in eight energy bands (14–20, 20–24, 24–35, 35–50, 50–75, 75–100, 100–150, and 150–195 keV) for 1888 sources, including 256 new detections above a detection threshold of 4.8 σ . The light curves, spectra, and tables that summarize the information of the detected-sources are available in the online journal and on the catalog web page: https://swift.gsfc.nasa.gov/results/bs157mon/ .

Optical transient surveys continue to generate increasingly large data sets, prompting the introduction of machine-learning algorithms to search for quality transient candidates efficiently. Existing machine-learning infrastructure can be leveraged in novel ways to search these data sets for new classes of transients. We present a machine-learning accelerated search pipeline for the Deeper, Wider, Faster (DWF) programme designed to identify high-quality astrophysical transient candidates that contain a single detection. Given the rapid observing cadence of the DWF programme, these single-detection transient candidates have durations on sub-minute time-scales. This work marks the first time optical transients have been systematically explored on these time-scales, to a depth of m similar to 23. We report the discovery of two high-quality sub-minute transient candidates from a pilot study of 671 761 light curves and investigate their potential origins with multiwavelength data. We discuss, in detail, possible non-astrophysical false positives, confidently reject electronic artefacts and asteroids, ruling out glints from satellites below 800 km and strongly disfavouring those at higher altitudes. We calculate a rate on the sky of 4 . 72(-3.28 )(+ 6 . 39)x 10(5) per day for these sub-minute transient candidates.

We present the results of our multiwavelength (X-ray to radio) follow-up campaign of the Einstein Probe transient EP240408a. The initial 10 s trigger displayed bright soft X-ray (0.5–4 keV) radiation with peak luminosity L X ≳ 10 49 (10 50 ) erg s −1 for an assumed redshift z ≳ 0.5 (2.0). The Neil Gehrels Swift Observatory and Neutron star Interior Composition ExploreR discovered a fading X-ray counterpart lasting for ∼5 days (observer frame), which showed a long-lived (∼4 days) plateau-like emission ( t −0.5 ) before a sharp power-law decline ( t −7 ). The plateau emission was in excess of L X ≳ 10 46 (10 47 ) erg s −1 at z ≳ 0.5 (2.0). Deep optical and radio observations resulted in nondetections of the transient. Our observations with Gemini South revealed a faint potential host galaxy ( r ≈ 24 AB mag) near the edge of the X-ray localization. The faint candidate host, and lack of other potential hosts ( r ≳ 26 AB mag; J ≳ 23 AB mag), imply a higher redshift origin ( z ≳ 0.5), which produces extreme X-ray properties that are inconsistent with many known extragalactic transient classes. In particular, the lack of a bright gamma-ray counterpart, with the isotropic-equivalent energy (10–10,000 keV) constrained by GECam and Konus-Wind to E γ ,iso ≲ 4 × 10 50 (6 × 10 51 ) erg at z ≈ 0.5 (2.0), conflicts with known gamma-ray bursts of similar X-ray luminosities. We therefore favor a jetted tidal disruption event as the progenitor of EP240408a at z ≳ 1.0, possibly caused by the disruption of a white dwarf by an intermediate-mass black hole. The alternative is that EP240408a may represent a new, previously unknown class of transient.

Compact binary mergers detectable in gravitational waves can be accompanied by a kilonova, an electromagnetic transient powered by radioactive decay of newly synthesised r-process elements. A few kilonova candidates have been observed during short gamma-ray burst follow-up, and one found associated with a gravitational wave detection, GW170817. However, robust kilonova candidates are yet to be found in un-triggered, wide-field optical surveys; a search not requiring an initial gravitational wave or gamma-ray burst trigger. Here we present the first observing run for the Kilonova and Transients Programme (KNTraP) using the Dark Energy Camera. The first KNTraP run ran for 11 nights, covering 31 fields at a nightly cadence in two filters. The programme is non-disruptive, can detect transients beyond the LIGO/Virgo/KAGRA horizon, is agnostic to the merger orientation, avoids the Sun and/or Galactic plane, and produces high cadence multiwavelength light curves. The data were processed nightly in real-time for rapid identification of transient candidates, allowing for follow-up of interesting candidates before they faded away. Three fast-rising candidates were identified in real-time, however none had the characteristics of the kilonova AT2017gfo associated with GW170817 or with the expected evolution for kilonovae from our fade-rate models. After the run, the data were reprocessed, then subjected to stringent filtering and model fitting to search for kilonovae offline. Multiple KNTraP runs (3+) are expected to detect kilonovae via this optical-only search method. No kilonovae were detected in this first KNTraP run using our selection criteria, constraining the KN rate to R < 1.8x10(5 )Gpc(-3 )yr(-1)

The gamma-ray burst (GRB) GRB 211211A is believed to have occurred due to the merger of two neutron stars or a neutron star and a black hole, despite its duration of more than a minute. Subsequent analysis has revealed numerous interesting properties including the possible presence of a ∼22 Hz quasiperiodic oscillation (QPO) during precursor emission. Here we perform timing analysis of Fermi and Swift gamma-ray data on GRB 211211A and, although we do not find a strong QPO during the precursor, we do find an extremely significant 19.5 Hz flux oscillation, which has higher fractional amplitude at higher energies, in a ∼0.2 s segment beginning ∼1.6 s after the start of the burst. After presenting our analysis we discuss possible mechanisms for the oscillation.

BurstCube is a 6U (10 x 20 x 30 cm) CubeSat designed to detect gamma-ray bursts (GRBs) and enable multimessenger observations, scheduled to launch in early 2024. BurstCube science is informed by the coincident detection of GRB 170817A and gravitational wave (GW) 170817, which confirmed compact binary mergers as progenitors for GRBs. Future coincident detections will also provide important context to the GW measurements - namely constraining the neutron star equation of state and testing fundamental physics, while also probing the origin of GRB prompt emission. Full sky coverage in the gamma-ray regime is needed to increase the likelihood of such measurements. Once in orbit, BurstCube will expand sky coverage while rapidly providing public alerts and localization information to the community using the Tracking and Data Relay Satellite (TDRS) and General Coordinates Network (GCN). This work will describe the current status of the mission, as well as an outline of post-launch operations, performance, and science goals.

We present the discovery of the radio afterglow of the short gamma-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of z ∼ 2.4. While radio observations commenced ≲1 day after the burst, no radio emission was detected until ∼11 days. The radio afterglow subsequently brightened by a factor of ∼3 in the span of a week, followed by a rapid decay (a “radio flare”). We find that a forward shock afterglow model cannot self-consistently describe the multiwavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of ≈5. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of ≈4, or a reverse shock from a shell collision are viable solutions to match the broadband behavior. At z ∼ 2.4, GRB 210726A is among the highest-redshift short GRBs discovered to date, as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by ≲10 days after the burst, potentially missing these late-rising, luminous radio afterglows.

We present the results of our X-ray, ultraviolet, and optical follow-up campaigns of 1RXS J165424.6-433758, an X-ray source detected with the Swift Deep Galactic Plane Survey. The source X-ray spectrum (Swift and NuSTAR) is described by thermal bremsstrahlung radiation with a temperature of kT = 10.1 ± 1.2 keV, yielding an X-ray (0.3–10 keV8) luminosity L X = (6.5 ± 0.8) × 10 31 erg s −1 at a Gaia distance of 460 pc. Spectroscopy with the Southern African Large Telescope revealed a flat continuum dominated by emission features, demonstrating an inverse Balmer decrement, the λ 4640 Bowen blend, almost a dozen He i lines, and He ii λ 4541, λ 4686, and λ 5411. Our high-speed photometry demonstrates a preponderance of flickering and flaring episodes, and revealed the orbital period of the system, P orb = 2.87 hr, which fell well within the cataclysmic variable (CV) period gap between 2 and 3 hr. These features classify 1RXS J165424.6-433758 as a nearby polar magnetic CV.

The Swift Burst Alert Telescope (BAT) is a coded aperture gamma-ray instrument with a large field of view that primarily operates in survey mode when it is not triggering on transient events. The survey data consists of eighty-channel detector plane histograms that accumulate photon counts over time periods of at least 5 minutes. These histograms are processed on the ground and are used to produce the survey dataset between $14$ and $195$ keV. Survey data comprises $> 90\%$ of all BAT data by volume and allows for the tracking of long term light curves and spectral properties of cataloged and uncataloged hard X-ray sources. Until now, the survey dataset has not been used to its full potential due to the complexity associated with its analysis and the lack of easily usable pipelines. Here, we introduce the BatAnalysis python package , a wrapper for HEASoftpy, which provides a modern, open-source pipeline to process and analyze BAT survey data. BatAnalysis allows members of the community to use BAT survey data in more advanced analyses of astrophysical sources including pulsars, pulsar wind nebula, active galactic nuclei, and other known/unknown transient events that may be detected in the hard X-ray band. We outline the steps taken by the python code and exemplify its usefulness and accuracy by analyzing survey data from the Crab Pulsar, NGC 2992, and a previously uncataloged MAXI Transient. The BatAnalysis package allows for $\sim$ 18 years of BAT survey to be used in a systematic way to study a large variety of astrophysical sources.

The Interplanetary Network (IPN) is a detection, localization and alert system that utilizes the arrival time of transient signals in gamma-ray detectors on spacecraft separated by planetary baselines to geometrically locate the origin of these transients. Due to the changing astrophysical landscape and the new emphasis on time domain and multi-messenger astrophysics (TDAMM) from the Pathways to Discovery in Astronomy and Astrophysics for the 2020s, this Gamma-ray Transient Network Science Analysis Group was tasked to understand the role of the IPN and high-energy monitors in this new era. The charge includes describing the science made possible with these facilities, tracing the corresponding requirements and capabilities, and highlighting where improved operations of existing instruments and the IPN would enhance TDAMM science. While this study considers the full multiwavelength and multimessenger context, the findings are specific to space-based high-energy monitors. These facilities are important both for full characterization of these transients as well as facilitating follow-up observations through discovery and localization. The full document reports a brief history of this field, followed by our detailed analyses and findings in some 68 pages, providing a holistic overview of the role of the IPN and high-energy monitors in the coming decades.

Short gamma-ray bursts (GRBs) are associated with binary neutron star mergers, which are multimessenger astronomical events that have been observed both in gravitational waves and in the multiband electromagnetic spectrum(1). Depending on the masses of the stars in the binary and on details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may be formed after the merger, existing for approximately 10-300 ms before collapsing to a black hole(2,3). Numerical relativity simulations across different groups consistently show broad power spectral features in the 1-5-kHz range in the post-merger gravitational-wave signal(4-14), which is inaccessible by current gravitational-wave detectors but could be seen by future third-generation ground-based detectors in the next decade(15-17). This implies the possibility of quasiperiodic modulation of the emitted gamma rays in a subset of events in which a neutron star is formed shortly before the final collapse to a black hole(18-21). Here we present two such signals identified in the short bursts GRB 910711 and GRB 931101B from archival Burst and Transient Source Experiment (BATSE) data, which are compatible with the predictions from numerical relativity.

γ -ray observations of the Cygnus Cocoon, an extended source surrounding the Cygnus X star-forming region, suggest the presence of a cosmic-ray accelerator reaching energies up to a few PeV. The very-high-energy (VHE; 0.1–100 TeV) γ -ray emission may be explained by the interaction of cosmic-ray hadrons with matter inside the Cocoon, but an origin of inverse Compton radiation by relativistic electrons cannot be ruled out. Inverse Compton γ -rays at VHE are accompanied by synchrotron radiation peaked in X-rays. Hence, X-ray observations may probe the electron population and magnetic field of the source. We observed 11 fields in or near the Cygnus Cocoon with the Neil Gehrels Swift Observatory’s X-Ray Telescope (Swift-XRT) totaling 110 ks. We fit the fields to a Galactic and extragalactic background model and performed a log-likelihood ratio test for an additional diffuse component. We found no significant additional emission and established upper limits in each field. By assuming that the X-ray intensity traces the TeV intensity and follows a dN / dE ∝ E − 2.5 spectrum, we obtained a 90% upper limit of F X < 8.7 × 10 −11 erg cm −2 s −1 or <5.2 × 10 −11 erg cm −2 s −1 on the X-ray flux of the entire Cygnus Cocoon between 2 and 10 keV depending on the choice of hydrogen column density model for the absorption. The obtained upper limits suggest that no more than one-quarter of the γ -ray flux at 1 TeV is produced by inverse Compton scattering, when assuming an equipartition magnetic field of ∼20 μ G.

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime.

The observed durations of prompt gamma-ray emission from gamma-ray bursts (GRBs) are often used to infer the progenitors and energetics of the sources. Inaccurate duration measurements will have a significant impact on constraining the processes powering the bursts. The “tip-of-the-iceberg” effect describes how the observed signal is lost into background noise; lower instrument sensitivity leads to higher measurement bias. In this study, we investigate how observing conditions, such as the number of enabled detectors, background level, and incident angle of the source relative to the detector plane, affect the measured duration of GRB prompt emission observed with the Burst Alert Telescope on board the Neil Gehrels Swift Observatory (Swift/BAT). We generate “simple-pulse” light curves from an analytical fast rise exponential decay function and from a sample of eight real GRB light curves. We fold these through the Swift/BAT instrument response function to simulate light curves Swift/BAT would have observed for specific observing conditions. We find duration measurements are highly sensitive to observing conditions and the incident angle of the source has the highest impact on measurement bias. In most cases duration measurements of synthetic light curves are significantly shorter than the true burst duration. For the majority of our sample, the percentage of duration measurements consistent with the true duration is as low as ∼25%–45%. In this article, we provide quantification of the tip-of-the-iceberg effect on GRB light curves due to Swift/BAT instrumental effects for several unique light curves.