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We have developed a method for the linear reconstruction of an image from undersampled, dithered data. The algorithm, known as Variable‐Pixel Linear Reconstruction, or informally as “Drizzle,” preserves photometry and resolution, can weight input images according to the statistical significance of each pixel, and removes the effects of geometric distortion on both image shape and photometry. This paper presents the method and its implementation. The photometric and astrometric accuracy and image fidelity of the algorithm as well as the noise characteristics of output images are discussed. In addition, we describe the use of drizzling to combine dithered images in the presence of cosmic rays.

X-Rated Supernova A link between long γ-ray bursts (GRBs) and supernovae has been established, but whether there is a similar relationship between the weaker and softer X-ray flashes and supernovae is unclear. GRB/XRF 060218, spotted by the Swift satellite on 18 February this year, may supply that missing link. In the first of four papers on this novel burster, Campana et al . report the sighting of the X-ray signature of a shock break-out, possible evidence of a supernova in progress. Pian et al . report the optical discovery of a type Ic supernova 2006aj associated with GRB/XRF 060218. Soderberg et al . report radio and X-ray observations that show that XRF 060218 is 100 times less energetic than, but of a type that is ten times more common than cosmological GRBs. Mazzali et al . modelled the spectra and light curve of SN 2006aj to show that it had a much smaller explosion energy and ejected much less mass than other GRB-supernovae, suggesting that it was produced by a star with a mass was only about 20 times that of the Sun, leaving behind a neutron star, rather than a black hole. A report of the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. SN 2006aj was intrinsically less luminous than the gamma-ray burst (GRB)–supernovae connection, but more luminous than many supernovae not accompanied by a GRB. Long-duration γ-ray bursts (GRBs) are associated with type Ic supernovae 1 that are more luminous than average 2 , 3 , 4 , 5 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 6 . Whether X-ray flashes—analogues of GRBs, but with lower luminosities and fewer γ-rays—can also be associated with supernovae, and whether they are intrinsically ‘weak’ events or typical GRBs viewed off the axis of the burst 7 , 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 8 and the supernova radio flux 9 . Our data, combined with radio and X-ray observations 8 , 9 , 10 , 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.

Short-duration gamma -ray bursts are intense flashes of cosmic gamma -rays, lasting less than about two seconds, whose origin is unclear. 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 galaxies, but unambiguous confirmation of the model is still lacking. Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species, whose decay should result in a faint transient, known as a 'kilonova', in the days following the burst. Indeed, it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe. 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 elements. Here we report optical and near-infrared observations that provide strong evidence for such an event accompanying the short-duration gamma -ray burst GRB130603B. If this, the simplest interpretation of the data, is correct, then it confirms that compact-object mergers are the progenitors of short-duration gamma -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.

Strong evidence that a kilonova — an event similar to a faint, short-lived supernova — accompanied the short-duration γ-ray burst GRB 130603B provides support for the hypothesis that such bursts are produced by the merger of two compact stellar objects. Merger of compacts identified as burst trigger Hubble Space Telescope observations of the location of the short-duration γ-ray burst SGRBH 130603B, which was detected by the Burst Alert Telescope on NASA's Swift satellite on 3 June 2013, provide support for the favoured model for the origin of such bursts — the merger of two compact stellar objects. Nial Tanvir et al . imaged the position at optical and near-infrared wavelengths at about 9 and 30 days after the bursts and observe signs of a faint, fast transient or 'kilonova'. The simplest interpretation of the data is that the burst was a compact object merger. The authors suggest that such mergers are probably sites of significant production of heavy elements through r-process nucleosynthesis. Short-duration γ-ray bursts are intense flashes of cosmic γ-rays, lasting less than about two seconds, whose origin is unclear 1 , 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 galaxies 3 , but unambiguous confirmation of the model is still lacking. Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species 4 , 5 , whose decay should result in a faint transient, known as a ‘kilonova’, in the days following the burst 6 , 7 , 8 . Indeed, it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe 5 , 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 elements 10 , 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.

The binary neutron star merger GW170817 was the first multi-messenger event observed in both gravitational and electromagnetic waves 1 , 2 . The electromagnetic signal began approximately two seconds post-merger with a weak, short burst of gamma rays 3 , which was followed over the next hours and days by the ultraviolet, optical and near-infrared emission from a radioactively powered kilonova 4 – 11 . Later, non-thermal rising X-ray and radio emission was observed 12 , 13 . The low luminosity of the gamma rays and the rising non-thermal flux from the source at late times could indicate that we are outside the opening angle of the beamed relativistic jet. Alternatively, the emission could be arising from a cocoon of material formed from the interaction between a jet and the merger ejecta 13 – 15 . Here we present late-time optical detections and deep near-infrared limits on the emission from GW170817 at 110 days post-merger. Our new observations are at odds with expectations of late-time emission from kilonova models, being too bright and blue 16 , 17 . Instead, the emission arises from the interaction between the relativistic ejecta of GW170817 and the interstellar medium. We show that this emission matches the expectations of a Gaussian-structured relativistic jet, which would have launched a high-luminosity, short gamma-ray burst to an aligned observer. However, other jet structure or cocoon models can also match current data—the future evolution of the afterglow will directly distinguish the origin of the emission. Late-time optical and near-infrared observations of the binary neutron star merger GW170817 are at odds with kilonova models but match a Gaussian-structured relativistic jet, which would have launched a high-luminosity short gamma-ray burst to an aligned observer.

When massive stars exhaust their fuel, they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration γ-ray burst. One would then expect that these long γ-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the γ-ray bursts are far more concentrated in the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long γ-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration γ-ray bursts are associated with the most extremely massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long γ-ray bursts are relatively rare in galaxies such as our own Milky Way. Not in our back yard In their death throes massive stars often produce supernovae, and occasionally a long-duration γ-ray burst (GRB). That suggests that GRBs and supernovae should be found in similar environments, but work based on more than a thousand hours of Hubble Space Telescope observation time shows that expectation to be wrong. Most long GRBs are found in small, faint, irregular galaxies. Supernovae appear equally divided between spiral and irregular galaxies. GRBs are concentrated in the brightest regions of their host galaxies whereas supernovae occur throughout their host galaxies. A happy conclusion of this finding is that GRBs, which would cause havoc here on Earth if exploding nearby, should be relatively rare in the Milky Way. γ-ray bursts are more concentrated in the very brightest regions of their host galaxies than are supernovae — in addition, the host galaxies of the γ-ray bursts are significantly fainter and more irregular than the hosts of the supernovae.

We present the late-time Hubble Space Telescope observations of two Gamma Ray Burst (GRB) associated supernovae (SNe), GRB 030329/SN 2003dh and XRF 060218/SN 2006aj. Using the multi-color data up to ~320 days after the burst, we constrain the late-time decay nature of these SNe. The decay rates of SN 2003dh are steeper than SN 2006aj. A comparison with two other GRB SNe, GRB 980425/SN 1998bw and the SN associated with XRF 020903, shows that the decay rates of SN 2003dh are similar to XRF 020903 and those of SN 2006aj are similar to SN 1998bw. The late-time decay rates are steeper than the 56Co→56Fe radioactive decay rate indicating that there is some leakage of gamma-rays. We also compare the late-time decay rates of nine type Ic SNe, including the SNe of long GRBs, Ic broad lined and normal Ics. The decay rates of the SNe sample show a remarkable similarity in I band at late-times with a scatter of ~10%.

A new pulsar was discovered moving in a nearly circular orbit around a low-mass companion star. The occurrence of eclipse, orbital period and size suggest a low companion mass. This phenomena is discussed.

Strong evidence that a kilonova -- an event similar to a faint, short-lived supernova -- accompanied the short-duration [gamma]-ray burst GRB 130603B provides support for the hypothesis that such bursts are produced by the merger of two compact stellar objects.

When massive stars exhaust their fuel, they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration [gamma]-ray burst. One would then expect that these long [gamma]-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the [gamma]-ray bursts are far more concentrated in the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long [gamma]-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration [gamma]-ray bursts are associated with the most extremely massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long [gamma]-ray bursts are relatively rare in galaxies such as our own Milky Way.