On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB170817A and GW170817 occurring by chance is 5.0 10 8 -. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of 1.74 0.05 s +  () between GRB170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between 3 10 15 -´and 7 10 16 +´times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1-1.4 per year during the 2018-2019 observing run and 0.3-1.7 per year at design sensitivity.

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Soft ␥-ray repeaters (SGRs) emit multiple, brief (ϳ0.1-s), intense outbursts of low-energy ␥-rays. They are extremely rare 1 -three 2-4 are known in our Galaxy and one 5 in the Large Magellanic Cloud. Two SGRs are associated 5-7 with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806 ؊ 20, with a period of 7.47 s and a spindown rate of 2:6 ؋ 10 ؊ 3 s yr ؊ 1 . We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are ϳ1,500 years and 8 ؋ 10 14 gauss, respectively. Our observations demonstrate the existence of 'magnetars' , neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions 8,9 that SGR bursts are caused by neutron-star 'crustquakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millennium-a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.SGR1806 Ϫ 20 became extremely active between October 1996 and November 1997, when over 40 intense bursts and numerous weaker ones were detected 10 with the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO). We observed SGR1806 Ϫ 20 with the Rossi X-Ray Timing Explorer (RXTE) five times between 5 and 18 November 1996, starting five days after the first triggered burst detection with BATSE. (Information on the archival data from RXTE/PCA and ASCA is available at http://heasarc.gsfc.nasa.gov.) During these observations 11 , the source emitted series of outbursts in a 'bunching' mode, never seen before. The intensity of the outbursts, as well as the 'bunching' mode, varied significantly: mini-outbursts were interlaced with very intense ones and the rate of bursts varied from bunch to bunch (S. Dieters et al., manuscript in preparation).We made a period search of the data after excluding all bursts from the time series. The data were then energy-selected for 2-24 keV X-rays, background subtracted and binned at 0.5-s resolution. The resulting light curve was searched for periodicities between 0.03 and 1 Hz, by calculating a fast-Fourier-transform power spectrum (Fig. 1). The peaks in the spectrum are centred on the fundamental frequency of 0.13375 Hz (period of 7.47655 s) and its first harmonic at 0.26750 Hz. We find no significant power in any other frequency in the searched range. The probability that we detect a signal at the fundamental frequency this strong by chance coincidence is 1 ϫ 10 Ϫ 13 (taking into account the number of trials, 1:9 ϫ 10 6 , and the probability per trial, 5 ϫ 10 Ϫ 20 ).To determine the fundamental period, all data sets were then corrected to the Solar System barycentre and separately folded at the longest detected period of 7.47655 s, and sub-harmonics thereof. These sub-harmonic folds showed...

We present a systematic spectral analysis of 350 bright GRBs observed with BATSE, with high spectral and temporal resolution. Our sample was selected from the complete set of 2704 BATSE GRBs, and included 17 short GRBs. To obtain well-constrained spectral parameters, four different photon models were fitted and the spectral parameters that best represent each spectrum were statistically determined. A thorough analysis was performed on 350 time-integrated and 8459 time-resolved burst spectra. Using the results, we compared time-integrated and time-resolved spectral parameters, and also studied correlations among the parameters and their evolution within each burst. The resulting catalog is the most comprehensive study of spectral properties of GRB prompt emission to date, and provides constraints with exceptional statistics on particle acceleration and emission mechanisms in GRBs.

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