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Newly formed black holes of stellar mass launch collimated outflows (jets) of ionized matter that approach the speed of light. These outflows power prompt, brief and intense flashes of γ-rays known as γ-ray bursts (GRBs), followed by longer-lived afterglow radiation that is detected across the electromagnetic spectrum. Measuring the polarization of the observed GRB radiation provides a direct probe of the magnetic fields in the collimated jets. Rapid-response polarimetric observations of newly discovered bursts have probed the initial afterglow phase1, 2, 3, and show that, minutes after the prompt emission has ended, the degree of linear polarization can be as high as 30 per cent—consistent with the idea that a stable, globally ordered magnetic field permeates the jet at large distances from the central source3. By contrast, optical4, 5, 6 and γ-ray7, 8, 9 observations during the prompt phase have led to discordant and often controversial10, 11, 12 results, and no definitive conclusions have been reached regarding the origin of the prompt radiation or the configuration of the magnetic field. Here we report the detection of substantial (8.3 ± 0.8 per cent from our most conservative simulation), variable linear polarization of a prompt optical flash that accompanied the extremely energetic and long-lived prompt γ-ray emission from GRB 160625B. Our measurements probe the structure of the magnetic field at an early stage of the jet, closer to its central black hole, and show that the prompt phase is produced via fast-cooling synchrotron radiation in a large-scale magnetic field that is advected from the black hole and distorted by dissipation processes within the jet.

Short-duration γ-ray bursts are intense flashes of cosmic γ-rays, lasting less than about two seconds, whose origin is unclear1, 2. The favoured hypothesis is that they are produced by a relativistic jet created by the merger of two compact stellar objects (specifically two neutron stars or a neutron star and a black hole). This is supported by indirect evidence such as the properties of their host galaxies3, but unambiguous confirmation of the model is still lacking. Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species4, 5, whose decay should result in a faint transient, known as a ‘kilonova’, in the days following the burst6, 7, 8. Indeed, it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe5, 9. Recent calculations suggest that much of the kilonova energy should appear in the near-infrared spectral range, because of the high optical opacity created by these heavy r-process elements10, 11, 12, 13. Here we report optical and near-infrared observations that provide strong evidence for such an event accompanying the short-duration γ-ray burst GRB 130603B. If this, the simplest interpretation of the data, is correct, then it confirms that compact-object mergers are the progenitors of short-duration γ-ray bursts and the sites of significant production of r-process elements. It also suggests that kilonovae offer an alternative, unbeamed electromagnetic signature of the most promising sources for direct detection of gravitational waves.

Over the past five years evidence has mounted that long-duration (> 2 s) gamma-ray bursts (GRBs)--the most brilliant of all astronomical explosions--signal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova, but now includes the association of GRBs with regions of massive star formation in distant galaxies, the appearance of supernova-like 'bumps' in the optical afterglow light curves of several bursts and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that a very energetic supernova (a hypernova) was temporally and spatially coincident with a GRB at redshift z = 0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs, thereby favouring the 'collapsar' model., 19 pages, 3 figures

The ultimate fate of the universe, infinite expansion or a big crunch, can be determined by measuring the redshifts, apparent brightnesses, and intrinsic luminosities of very distant supernovae. Recent developments have provided tools that make such a program practicable: (1) Studies of relatively nearby Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be accurately determined; (2) New research techniques have made it possible to schedule the discovery and follow-up observations of distant supernovae, producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These distant supernovae provide a record of changes in the expansion rate over the past several billion years. By making precise measurements of supernovae at still greater distances, and thus extending this expansion history back far enough in time, we can distinguish the slowing caused by the gravitational attraction of the universe's mass density Omega_M from the effect of a possibly inflationary pressure caused by a cosmological constant Lambda. We report here the first such measurements, with our discovery of a Type Ia supernova (SN 1997ap) at z = 0.83. Measurements at the Keck II 10-m telescope make this the most distant spectroscopically confirmed supernova. Over two months of photometry of SN 1997ap with the Hubble Space Telescope and ground-based telescopes, when combined with previous measurements of nearer SNe Ia, suggests that we may live in a low mass-density universe. Further supernovae at comparable distances are currently scheduled for ground and space-based observations.

Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether X-ray flashes - analogues of GRBs, but with lower luminosities and fewer gamma-rays - can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB-supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB-supernovae., Final published version

Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z approximately 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified3. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.29^(+0.11)_(-0.12) (refs 5–7). Subsequently, it was measured spectroscopically to be z = 6.29 ± 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.

Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy. Here we report that GRB 090423 lies at a redshift of z approximately 8.2, implying that massive stars were being produced and dying as GRBs approximately 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.

THE accretion of gas from a companion star onto a compact object, such as a neutron star or black hole, can release large amounts of energy. Episodic accretion is generally thought to explain transient X-ray emission, such as that associated with the recently discovered1 Galactic superluminal source GRO J1655 - 40, and to be connected with the ejection of material at relativistic velocities2. The only other known Galactic superluminal source2 is difficult to study because it is obscured by intervening gas and dust. Here we report the discovery of the optical counterpart to GRO J1655 - 40, which we estimate to lie at a distance of only 3 kpc. We have identified the precursor on historical photographic plates; at the time of the X-ray burst in August 19941, it brightened by 3 mag in the V bandpass. Although the 12-day delay between the X-ray burst1 and the main radio outburst which marked the beginning of superluminal motion3 poses a puzzle that remains to be explained, our results show that at optical wavelengths the characteristics of GRO J1655 - 40 are similar to those of other accreting-black-hole candidates4-7.

WE HAVE discovered an eclipsing binary millisecond pulsar in the globular cluster Terzan 5. This, the second known eclipsing binary pulsar after PSR1957 + 20, has a pulse period of 11.56 ms and a very short orbital period of 1.8 hours. In contrast to PSR1957 + 20, where the eclipses occupy about 10 per cent of the orbital period1, the eclipse duration in this pulsar is very variable and never less than one-third of the orbital period. The pulsar is in a circular orbit of radius 0.11 light seconds, which implies a minimum companion mass of 0.089 solar masses, about four times the companion mass of PSR1957 + 20. Timing observations suggest an identification of the pulsar with a variable continuum source locate about 30 arcsec west of the cluster centre. These observations and the variable eclipse duration show that the eclipse is due to absorption or scattering in a tenuous wind which flows from the companion star. We have also detected a second pulsar in the direction of Terzan 5. This pulsar, which has a period of 442 ms,may also be a cluster member, but is more likely to be a foreground object.

The detection of soft X-rays of about 1 keV energy from the eclipsing pulsar PSR1957+20 is reported. This high-energy radiation should be a valuable diagnostic of the wind in this recycled pulsar system. Possible sources of the X-ray emission are the interstellar nebula driven by the pulsar wind, the interaction between the pulsar and its evaporating companion, and the pulsar itself. The small apparent size of the X-ray object argues against the first of these possibilities and suggests that the X-rays are produced within the binary.

Nature 391, 51– 54 (1998) The name of one of the authors (Ariel Goobar) of this Letter was inadvertently removed during page make-up. A. Goobar is in the Physics Department, Stockholm University, as indicated in the footnote addresses.

The recently discovered millisecond pulsar PSR1957 + 20 is eclipsed for ∼50 min out of each 9.16-h orbit1; several authors suggested1–4 that spin-down radiation from the pulsar is evaporating material from the surface of the ˜0.02Mṁ companion. Here we report observations intended to find the optical counterpart of the pulsar and to study luminosity variations with orbital phase. We show that the candidate optical counterpart5 is actually two stars, one the true counterpart, the other an unrelated background star. The luminosity of the system varies by at least a factor of five, with a maximum of magnitude 20.3 when the pulsar hides the companion. The phase of the variations implies that the light comes mostly from the companion star, and suggests that it is tidally locked, with the bright side constantly illuminated by the pulsar. The companion has a low colour temperature, ∼5,500 K, which with its magnitude and the pulsar's dispersion-measure distance indicates a radius of 0.15R⊙, about the size of a 0.02M⊙ hydrogen white dwarf.

A fast pulsar with period of 39.5 milliseconds has been discovered in the peculiar combination nebula CTB80. This nebula has been proposed as the remnant of a possible historical supernova in ad 1408 (refs 1–3), but we argue that neither the nebula nor the new pulsar is linked to this supernova, which is in any case of doubtful veracity4. We suggest instead that the pulsar is at least 10,000 years old and that the observed features in the nebula are the result of the pulsar ploughing through the medium at ˜ 300 km s−1. In contrast to the other young pulsars, this pulsar has a smaller magnetic field strength and is consequently underluminous. We argue that this pulsar is more typical of young pulsars in general, a hypothesis that raises fundamental questions about the birth and evolution of pulsars.

Following the discovery of an eclipsing, millisecond pulsar 1957+20 (ref. 1) we conducted a series of optical observations to identify the optical counterpart of the system. We report the detection of a probably variable object (hereafter star X) with a mean apparent V magnitude of ∼20 within the astrometric error circle of the equatorial radio position of the pulsar. Star X appears fainter at orbital phase 0.21, the onset of the pulsar eclipse, compared with the quadrature position, suggesting that it is probably the optical counterpart. We argue that the secondary heated by the pulsar wind and not the pulsar itself makes the dominant contribution to the visible radiation from this system. The absolute magnitude of star X is Mv ≈ 10.5, assuming a distance of ∼ 0.8 kpc derived from the dispersion measure. The optical luminosity of the counterpart directly measures the fraction of the pulsar wind power responsible for heating the secondary. Future optical observations may be able to constrain the nature of the pulsar wind.

We report the discovery of a remarkable pulsar with period 1.6 ms, moving in a nearly circular 9.17 hour orbit around a low mass companion star. At an observing frequency of 430 MHz, the pulsar, PSR 1957+20, is eclipsed once each orbit for about 50 minutes. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds — presumably as a result of propagation through plasma surrounding the companion. The pulsar’s orbit about the system barycenter has a radius of 0.089 light seconds projected onto the line of sight. The observed orbital period and size, together with the fact that eclipses occur, imply a surprisingly low companion mass, only a few percent the mass of the sun. The eclipsing mass extends well beyond the Roche lobe of the companion, and appears to possess a comet-like tail, strongly uggesting that the pulsar is evaporating its companion.

The discovery of a 110-ms pulsar, PSR2127+11, in the globular cluster M15, is reported. The results of nine months of timing measurements place the new pulsar about 2 arcsec from the center of the cluster, and indicate that it is not a member of a close binary system. The measured negative value of the period derivative is probably the result of the pulsar being bodily accelerated in our direction by the gravitational field of the collapsed core of M15. This apparently overwhelms a positive contribution to the period derivative due to magnetic braking. Although the pulsar has an unexpectedly long period, it is argued that it belongs to the class of 'recycled' pulsars, which have been spun up by accretion in a binary system. The subsequent loss of the pulsar's companion is probably due to disruption of the system by close encounters with other stars.

Shortly after the discovery1 of a 39.5-ms pulsar in the supernova remnant CTB 80, we began a series of timing measurements at the Arecibo Observatory to obtain more accurate values for the period and dispersion measure and to determine the period derivative and scintillation parameters. Data acquired over a span of 11 days yielded a surprisingly small period derivative, Ṗ = (5.92 ± 0.06) x 10− 15 s s− 1. If the pulsar slows by magnetic dipole radiation, this spin-down rate implies a surface field strength of only 5 x 1011 gauss yet the pulsar is believed to be much too young for its magnetic field to have decayed. Scintillation observations reveal unusually rapid fluctuations, indicating a large pulsar transverse velocity. Here we describe the timing and scintillation measurements and their results, and briefly explore some astrophysical consequences.