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.

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ∼10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the

With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW 170817. The bright, rapidly fading ultraviolet emission indicates a high mass (≈ 0.03 solar masses) wind-driven outflow with moderate electron fraction (Y e ≈ 0.27). Combined with the X-ray limits, we favor an observer viewing angle of ≈ 30• away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow). One-sentence summaryWe report X-ray and UV observations of the first binary neutron star merger detected via gravitational waves. Main TextAt 12:41:04.45 on 2017 August 17 (UT times are used throughout this work), the Laser Interferometric GravitationalWave Observatory (LIGO) and Virgo Consortium (LVC) registered a strong gravitational wave (GW) signal (LVC trigger G298048; (1)), later named GW 170817 (2). Unlike previous GW sources reported by LIGO, which involved only black holes (3), the gravitational strain waveforms indicated a merger of two neutron stars. Binary neutron star mergers have long been considered a promising candidate for the detection of an electromagnetic counterpart associated with a gravitational wave source. Two seconds later, the Gamma-Ray Burst Monitor (GBM) on the Fermi spacecraft triggered on a short (duration ≈ 2 s) gamma-ray signal consistent with the GW localization, GRB 170817A (4, 5). 330°00'00" 300°00'00" 270°00'00" 240°00'00" 210°00'00" 180°00'00" 150°00'00" 120°00'00" 90°00'00" 30°0°Figure 1: Skymap of Swift XRT observations, in equatorial (J2000) coordinates. The grey probability area is the GW localization (13), the blue region shows the Fermi-GBM localization, and the red circles are Swift-XRT fields of view. UVOT fields are colocated with a field of view 60% of the XRT. The location of the counterpart, EM 170817, is marked with a large yellow cross. The early 37-point mosaic can be seen, centred on the GBM probability. The widely scattered points are from the first uploaded observing plan, which was based on the singledetector GW skymap. The final observed plan was based on the first 3-detector map (11), however we show here the higher-quality map (13) so that our coverage can be compared to the final probability map (which was not available at the time of our planning; (7)).Swift satellite (6) in its low-Earth orbit meant that the GW and gamma-ray burst (GRB) localizations were occulted by the Earth (7) and so not visible to its Burst Alert Telescope (BAT). These discoveries triggered a world-wide effort to find, localize and characterize the EM counterpart (8). We present UV and X-ray observations conducted as part of t...

We present the calibration of the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray satellite. We used the Crab as the primary effective area calibrator and constructed a piece-wise linear spline function to modify the vignetting response. The achieved residuals for all off-axis angles and energies, compared to the assumed spectrum, are typically better than ±2% up to 40 keV and 5-10 % above due to limited counting statistics. An empirical adjustment to the theoretical 2D point spread function (PSF) was found using several strong point sources, and no increase of the PSF half power diameter (HPD) has been observed since the beginning of the mission. We report on the detector gain calibration, good to 60 eV for all grades, and discuss the timing capabilities of the observatory, which has an absolute timing of ± 3 ms. Finally we present cross-calibration results from two campaigns between all the major concurrent X-ray observatories (Chandra, Swift, Suzaku and XMM-Newton), conducted in 2012 and 2013 on the sources 3C 273 and PKS 2155-304, and show that the differences in measured flux is within ∼10% for all instruments with respect to NuSTAR. Subject headings: space vehicles: instruments -X-rays: individual (3C 273) -X-rays: individual (PKS 2155-304)

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive (44)Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. Cassiopeia A is a young, nearby, core-collapse remnant from which (44)Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed (44)Ti emission to estimated (56)Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the (44)Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the (44)Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.