Swift and NuSTAR observations of GW170817: Detection of a blue kilonova

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 of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading UV 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 ultrarelativistic, highly collimated ejecta (a γ-ray burst afterglow).

A new gamma-ray burst classification scheme from GRB 060614

Gamma-ray bursts (GRBs) are known to come in two duration classes, separated at approximately 2 s. Long-duration bursts originate from star-forming regions in galaxies, have accompanying supernovae when these are near enough to observe and are probably caused by massive-star collapsars. Recent observations show that short-duration bursts originate in regions within their host galaxies that have lower star-formation rates, consistent with binary neutron star or neutron star-black hole mergers. Moreover, although their hosts are predominantly nearby galaxies, no supernovae have been so far associated with short-duration GRBs. Here we report that the bright, nearby GRB 060614 does not fit into either class. Its approximately 102-s duration groups it with long-duration GRBs, while its temporal lag and peak luminosity fall entirely within the short-duration GRB subclass. Moreover, very deep optical observations exclude an accompanying supernova, similar to short-duration GRBs. This combination of a long-duration event without an accompanying supernova poses a challenge to both the collapsar and the merging-neutron-star interpretations and opens the door to a new GRB classification scheme that straddles both long- and short-duration bursts.

The association of GRB 060218 with a supernova and the evolution of the shock wave

Although the link between long gamma-ray bursts (GRBs) and supernovae has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a gamma-ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5) and its connection to supernova SN 2006aj (ref. 6). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor. We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf-Rayet star.

Relativistic ejecta from X-ray flash XRF 060218 and the rate of cosmic explosions

Over the past decade, long-duration gamma-ray bursts (GRBs)--including the subclass of X-ray flashes (XRFs)--have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10(48) erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

Gamma-ray bursts: huge explosion in the early Universe

Long gamma-ray bursts (GRBs) are bright flashes of high-energy photons that can last for tens of minutes; they are generally associated with galaxies that have a high rate of star formation and probably arise from the collapsing cores of massive stars, which produce highly relativistic jets (collapsar model). Here we describe gamma- and X-ray observations of the most distant GRB ever observed (GRB 050904): its redshift (z) of 6.29 means that this explosion happened 12.8 billion years ago, corresponding to a time when the Universe was just 890 million years old, close to the reionization era. This means that not only did stars form in this short period of time after the Big Bang, but also that enough time had elapsed for them to evolve and collapse into black holes.

An origin for short gamma-ray bursts unassociated with current star formation

Two short (< 2 s) gamma-ray bursts (GRBs) have recently been localized and fading afterglow counterparts detected. The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual gamma-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2-3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts--the coalescence of binary systems of neutron stars or a neutron star-black hole pair--are the progenitors of short bursts.

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z approximately 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10'') and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from--and the localization of--the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright x-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid x-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

An unexpectedly rapid decline in the X-ray afterglow emission of long γ-ray bursts

'Long' gamma-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the gamma-rays we see. As the jet travels further outward into the surrounding circumstellar medium, 'external' shocks create the afterglow emission seen in the X-ray, optical and radio bands. Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

A giant γ-ray flare from the magnetar SGR 1806–20

Two classes of rotating neutron stars-soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars-are magnetars, whose X-ray emission is powered by a very strong magnetic field (B approximately 10(15) G). SGRs occasionally become 'active', producing many short X-ray bursts. Extremely rarely, an SGR emits a giant flare with a total energy about a thousand times higher than in a typical burst. Here we report that SGR 1806-20 emitted a giant flare on 27 December 2004. The total (isotropic) flare energy is 2 x 10(46) erg, which is about a hundred times higher than the other two previously observed giant flares. The energy release probably occurred during a catastrophic reconfiguration of the neutron star's magnetic field. If the event had occurred at a larger distance, but within 40 megaparsecs, it would have resembled a short, hard gamma-ray burst, suggesting that flares from extragalactic SGRs may form a subclass of such bursts.

Discovery of Narrow X-Ray Absorption Lines from NGC 3783 with the Chandra High Energy Transmission Grating Spectrometer

We present the first grating-resolution X-ray spectra of the Seyfert 1 galaxy NGC 3783, obtained with the High Energy Transmission Grating Spectrometer on the Chandra X-Ray Observatory. These spectra reveal many narrow absorption lines from the H-like and He-like ions of O, Ne, Mg, Si, S, and Ar as well as Fe xvii-Fe xxi L-shell lines. We have also identified several weak emission lines, mainly from O and Ne. The absorption lines are blueshifted by a mean velocity of approximately 440+/-200 km s-1 and are not resolved, indicating a velocity dispersion within the absorbing gas of a few hundred kilometers per second or less. We measure the lines' equivalent widths and compare them with the predictions of photoionization models. The best-fitting model has a microturbulence velocity of 150 km s-1 and a hydrogen column density of 1.3x1022 cm-2. The measured blueshifts and inferred velocity dispersions of the X-ray absorption lines are consistent with those of the strongest UV absorption lines observed in this object. However, simple models that propose to strictly unify the X-ray and UV absorbers have difficulty explaining simultaneously the X-ray and UV absorption-line strengths.

Mitigating Charge Transfer Inefficiency in the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer

The ACIS front-illuminated CCDs on board the Chandra X-Ray Observatory were damaged in the extreme environment of the Earth's radiation belts, resulting in enhanced charge transfer inefficiency (CTI). This produces a row dependence in gain, event grade, and energy resolution. We model the CTI as a function of input photon energy, including the effects of detrapping (charge trailing), shielding within an event (charge in the leading pixels of the 3x3 event island protects the rest of the island by filling traps), and nonuniform spatial distribution of traps. This technique cannot fully recover the degraded energy resolution, but it reduces the position dependence of gain and grade distributions. By correcting the grade distributions as well as the event amplitudes, we can improve the instrument's quantum efficiency. We outline our model for CTI correction and discuss how the corrector can improve astrophysical results derived from ACIS data.

Chandra Uncovers a Hidden Low-Luminosity Active Galactic Nucleus in the Radio Galaxy Hydra A (3C 218)

We report the detection with Chandra of a low-luminosity active galactic nucleus (LLAGN) in the low-ionization nuclear emission-line region (LINER) hosted by Hydra A, a nearby (z=0.0537) powerful FR I radio galaxy with complex radio and optical morphology. In a 20 ks ACIS-S exposure during the calibration phase of the instrument, a point source is detected at energies greater, similar2 keV at the position of the compact radio core, embedded in diffuse thermal X-ray emission (kT approximately 1 keV) at softer energies. The spectrum of the point source is well fitted by a heavily absorbed power law with intrinsic column density NintH approximately 3x1022 cm-2 and photon index Gamma approximately 1.7. The intrinsic (absorption-corrected) luminosity is L2-10keV approximately 1.3x1042 ergs s-1. These results provide strong evidence that an obscured AGN is present in the nuclear region of Hydra A. We infer that the optical/UV emission of the AGN is mostly hidden by the heavy intrinsic reddening. In order to balance the photon budget of the nebula, we must either postulate that the ionizing spectrum includes a UV bump or invoke and additional power source (shocks in the cooling flow or interaction with the radio jets). Using an indirect estimate of the black hole mass and the X-ray luminosity, we infer that the accretion rate is low, suggesting that the accretion flow is advection dominated. Finally, our results support current unification schemes for radio-loud sources, in particular the presence of the putative molecular torus in FR I galaxies. These observations underscore the power of the X-rays and of Chandra in the quest for black holes.

Diamond-turned lacquer-coated soft x-ray telescope mirrors

X-ray astronomy has reached sufficient maturity to demand at least moderate angular resolution lightgathering telescopes to accompany detector development. Keeping the cost of such telescopes within the budget of low-cost flight opportunities such as sounding rockets and SPARTAN missions is a substantial challenge. We have developed a program of precision diamond mirror turning, mechanical polishing, lacquer coating, and metal deposition which produces x-ray telescopes with minute of arc angular resolution at moderate cost. We describe the process and report calibration results for a 80 cm (31.4 in.) diam Wolter I telescope flown aboard an Aries sounding rocket.