We report Chandra observations of GW170817, the first neutron star-neutron star merger discovered by the joint LIGO-Virgo Collaboration, and the first direct detection of gravitational radiation ...associated with an electromagnetic counterpart, Fermi short γ-ray burst GRB 170817A. The event occurred on 2017 August 17 and subsequent observations identified an optical counterpart, SSS17a, coincident with NGC 4993 (∼10″ separation). Early Chandra ( days) and Swift ( days) observations yielded non-detections at the optical position, but ∼9 days post-trigger Chandra monitoring revealed an X-ray point source coincident with SSS17a. We present two deep Chandra observations totaling ∼95 ks, collected on 2017 September 01-02 ( days). We detect X-ray emission from SSS17a with erg s−1, and a power law spectrum of . We find that the X-ray light curve from a binary NS coalescence associated with this source is consistent with the afterglow from an off-axis short γ-ray burst, with a jet angled 23° from the line of sight. This event marks both the first electromagnetic counterpart to a LIGO-Virgo gravitational-wave source and the first identification of an off-axis short GRB. We also confirm extended X-ray emission from NGC 4993 ( erg s−1) consistent with its E/S0 galaxy classification, and report two new Chandra point sources in this field, CXOU J130948 and CXOU J130946.
We introduce a new capability of the Neil Gehrels Swift Observatory to provide event-level data from the Burst Alert Telescope (BAT) on demand in response to transients detected by other instruments. ...We show that the availability of these data can effectively increase the rate of detections and arcminute localizations of gamma-ray bursts (GRB) like GRB 170817 by >400%. We describe an autonomous spacecraft-commanding pipeline purpose built to enable this science; to our knowledge, this is the first fully autonomous extremely low-latency commanding of a space telescope for scientific purposes. This pipeline has been successfully run in its complete form since 2020 January, and has resulted in the recovery of BAT event data for >800 externally triggered events to date (gravitational waves, GWs; neutrinos; GRBs triggered by other facilities; fast radio bursts; and very high-energy detections), now running with a success rate of ∼90%. We exemplify the utility of this new capability by using the resultant data to (1) set the most sensitive upper limits on prompt 1 s duration short GRB-like emission within 15 s around the unmodeled GW burst candidate S200114f, and (2) provide an arcminute localization for short GRB 200325A and other bursts. We also show that using data from GUANO to localize GRBs discovered by other instruments, we can increase the net rate of arcminute-localized GRBs by 10%-20% per year. Along with the scientific yield of more sensitive searches for subthreshold GRBs, the new capabilities designed for this project will serve as the foundation for further automation and rapid target of opportunity capabilities for the Swift mission, and have implications for the design of future rapid-response space telescopes.
Approximately one-third of the gamma-ray sources in the third Fermi-LAT catalog are unidentified or unassociated with objects at other wavelengths. Observations with the X-Ray Telescope on the Neil ...Gehrels Swift Observatory (Swift-XRT) have yielded possible counterparts in ∼30% of these source regions. The objective of this work is to identify the nature of these possible counterparts, utilizing their gamma-ray properties coupled with the Swift derived X-ray properties. The majority of the known sources in the Fermi catalogs are blazars, which constitute the bulk of the extragalactic gamma-ray source population. The galactic population on the other hand is dominated by pulsars. Overall, these two categories constitute the majority of all gamma-ray objects. Blazars and pulsars occupy different parameter space when X-ray fluxes are compared with various gamma-ray properties. In this work, we utilize the X-ray observations performed with the Swift-XRT for the unknown Fermi sources and compare their X-ray and gamma-ray properties to differentiate between the two source classes. We employ two machine-learning algorithms, decision tree and random forest (RF) classifier, to our high signal-to-noise ratio sample of 217 sources, each of which corresponds to Fermi unassociated regions. The accuracy scores for both methods were found to be 97% and 99%, respectively. The RF classifier, which is based on the application of a multitude of decision trees, associated a probability value (Pbzr) for each source to be a blazar. This yielded 173 blazar candidates from this source sample, with Pbzr ≥ 90% for each of these sources, and 134 of these possible blazar source associations had Pbzr ≥ 99%. The results yielded 13 sources with Pbzr ≤ 10%, which we deemed as reasonable candidates for pulsars, seven of which result with Pbzr ≤ 1%. There were 31 sources that exhibited intermediate probabilities and were termed ambiguous due to their unclear characterization as a pulsar or a blazar.
The Radio to GeV Afterglow of GRB 221009A Laskar, Tanmoy; Alexander, Kate D.; Margutti, Raffaella ...
Astrophysical journal. Letters,
03/2023, Volume:
946, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
GRB 221009A (
z
= 0.151) is one of the closest known long
γ
-ray bursts (GRBs). Its extreme brightness across all electromagnetic wavelengths provides an unprecedented opportunity to study a ...member of this still-mysterious class of transients in exquisite detail. We present multiwavelength observations of this extraordinary event, spanning 15 orders of magnitude in photon energy from radio to
γ
-rays. We find that the data can be partially explained by a forward shock (FS) from a highly collimated relativistic jet interacting with a low-density, wind-like medium. Under this model, the jet’s beaming-corrected kinetic energy (
E
K
∼ 4 × 10
50
erg) is typical for the GRB population. The radio and millimeter data provide strong limiting constraints on the FS model, but require the presence of an additional emission component. From equipartition arguments, we find that the radio emission is likely produced by a small amount of mass (≲6 × 10
−7
M
⊙
) moving relativistically (Γ ≳ 9) with a large kinetic energy (≳10
49
erg). However, the temporal evolution of this component does not follow prescriptions for synchrotron radiation from a single power-law distribution of electrons (e.g., in a reverse shock or two-component jet), or a thermal-electron population, perhaps suggesting that one of the standard assumptions of afterglow theory is violated. GRB 221009A will likely remain detectable with radio telescopes for years to come, providing a valuable opportunity to track the full lifecycle of a powerful relativistic jet.
Abstract
Electromagnetic observations of gravitational-wave and high-energy neutrino events are crucial in understanding the physics of their astrophysical sources. X-ray counterparts are especially ...useful in studying the physics of the jet, the energy of the outflow, and the particle acceleration mechanisms in the system. Ultraviolet and optical observations can help us constrain the mass and velocity of the outflow and provide hints on the viewing angle. We present the Neil Gehrels Swift Observatory prompt searches for X-ray and UV/optical counterparts to the joint gravitational-wave and high-energy neutrino coincident events that happened during the third observing run of LIGO/Virgo. Swift observed the overlap between gravitational-wave and neutrino error regions for three of the considerable (
p
-value < 1%) joint gravitational-wave and high-energy neutrino coincident alerts, which were generated by the IceCube Neutrino Observatory in real time after triggering by the LIGO/Virgo gravitational-wave public alerts. The searches did not associate any X-ray or UV/optical counterparts with any of the joint gravitational-wave and high-energy neutrino coincident events; however, the follow-up of these alerts significantly improved the tiling techniques covering regions between the gravitational-wave sky maps and the neutrino’s error regions, making the real-time system ready for future potential discoveries. We discuss the details of each follow-up procedure, the results of each search, and the plans for future searches.
We report the discovery by the Swift hard X-ray monitor of the transient source Swift J2058.4+0516 (Sw J2058+05). Our multi-wavelength follow-up campaign uncovered a long-lived (duration > ~ months), ...luminous X-ray (L sub(X,iso) approximate 3 x 10 super(47) erg s super(-1)) and radio (nuL sub(nu,iso) approximate 10 super(42) erg s super(-1)) counterpart. The associated optical emission, however, from which we measure a redshift of 1.1853, is relatively faint, and this is not due to a large amount of dust extinction in the host galaxy. Based on numerous similarities with the recently discovered GRB 110328A/Swift J164449.3+573451 (Sw J1644+57), we suggest that Sw J2058+05 may be the second member of a new class of relativistic outbursts resulting from the tidal disruption of a star by a supermassive black hole. If so, the relative rarity of these sources (compared with the expected rate of tidal disruptions) implies that either these outflows are extremely narrowly collimated (straighttheta < 1degrees) or only a small fraction of tidal disruptions generate relativistic ejecta. Analogous to the case of long-duration gamma-ray bursts and core-collapse supernovae, we speculate that rapid spin of the black hole may be a necessary condition to generate the relativistic component. Alternatively, if powered by gas accretion (i.e., an active galactic nucleus (AGN)), Sw J2058+05 would seem to represent a new mode of variability in these sources, as the observed properties appear largely inconsistent with known classes of AGNs capable of generating relativistic jets (blazars, narrow-line Seyfert 1 galaxies).
Abstract We report on the discovery of Swift J010902.6-723710, a rare eclipsing Be/X-ray Binary system by the Swift SMC Survey (S-CUBED). Swift J010902.6-723710 was discovered via weekly S-CUBED ...monitoring observations when it was observed to enter a state of X-ray outburst on 2023 October 10. X-ray emission was found to be modulated by a 182 s period. Optical spectroscopy is used to confirm the presence of a highly inclined circumstellar disk surrounding a B0-0.5Ve optical companion. Historical UV and IR photometry is then used to identify strong eclipse-like features reoccurring in both light curves with a 60.623 day period, which is adopted as the orbital period of the system. Eclipsing behavior is found to be the result of a large accretion disk surrounding the neutron star. Eclipses are produced when the disk passes in front of the OBe companion, blocking light from both the stellar surface and circumstellar disk. This is only the third Be/X-ray binary to have confirmed eclipses. We note that this rare behavior provides an important opportunity to constrain the physical parameters of a Be/X-ray binary with greater accuracy than is possible in noneclipsing systems.
We report on the X-ray properties of the new transient Swift J0840.7−3516, discovered with Swift/BAT in 2020February, using extensive data from Swift, MAXI, NICER, and NuSTAR. The source flux ...increased for∼103safter the discovery, decayed rapidly over∼5 orders of magnitude in five days, and then remained almost constant over nine months. Large-amplitude short-term variations on timescales of 1–104s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law-shaped spectrum with a photon index of∼1.0extending up to∼30 keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became∼2 at the end of the decay. A spectral absorption feature at 3–4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low-mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star.
Abstract
The Fermi-LAT unassociated sources represent some of the most enigmatic gamma-ray sources in the sky. Observations with the Swift-XRT and -UVOT telescopes have identified hundreds of likely ...X-ray and UV/optical counterparts in the uncertainty ellipses of the unassociated sources. In this work we present spectral fitting results for 205 possible X-ray/UV/optical counterparts to 4FGL unassociated targets. Assuming that the unassociated sources contain mostly pulsars and blazars, we develop a neural network classifier approach that applies gamma-ray, X-ray, and UV/optical spectral parameters to yield a descriptive classification of unassociated spectra into pulsars and blazars. From our primary sample of 174 Fermi sources with a single X-ray/UV/optical counterpart, we present 132
P
bzr
> 0.99 likely blazars and 14
P
bzr
< 0.01 likely pulsars, with 28 remaining ambiguous. These subsets of the unassociated sources suggest a systematic expansion to catalogs of gamma-ray pulsars and blazars. Compared to previous classification approaches our neural network classifier achieves significantly higher validation accuracy and returns more bifurcated
P
bzr
values, suggesting that multiwavelength analysis is a valuable tool for confident classification of Fermi unassociated sources.
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