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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 the first full release of a survey of the 150 MHz radio sky, observed with the Giant Metrewave Radio Telescope (GMRT) between April 2010 and March 2012 as part of the TIFR GMRT Sky Survey (TGSS) project. Aimed at producing a reliable compact source survey, our automated data reduction pipeline efficiently processed more than 2000 h of observations with minimal human interaction. Through application of innovative techniques such as image-based flagging, direction-dependent calibration of ionospheric phase errors, correcting for systematic offsets in antenna pointing, and improving the primary beam model, we created good quality images for over 95 percent of the 5336 pointings. Our data release covers 36 900 deg 2 (or 3.6 π steradians) of the sky between −53• and +90• declination (Dec), which is 90 percent of the total sky. The majority of pointing images have a noise level below 5 mJy beamwith an approximate resolution of 25 × 25 (or 25 × 25 /cos (Dec − 19 • ) for pointings south of 19 • declination). We have produced a catalog of 0.62 Million radio sources derived from an initial, high reliability source extraction at the seven sigma level. For the bulk of the survey, the measured overall astrometric accuracy is better than two arcseconds in right ascension and declination, while the flux density accuracy is estimated at approximately ten percent. Within the scope of the TGSS alternative data release (TGSS ADR) project, the source catalog, as well as 5336 mosaic images (5• × 5 • ) and an image cutout service, are made publicly available at the CDS as a service to the astronomical community. Next to enabling a wide range of different scientific investigations, we anticipate that these survey products will provide a solid reference for various new low-frequency radio aperture array telescopes (LOFAR, LWA, MWA, SKA-low), and can play an important role in characterizing the epoch-of-reionisation (EoR) foreground. The TGSS ADR project aims at continuously improving the quality of the survey data products. Near-future improvements include replacement of bright source snapshot images with archival targeted observations, using new observations to fill the holes in sky coverage and replace very poor quality observational data, and an improved flux calibration strategy for less severely affected observational data.

We report on the results of an extensive monitoring campaign of the radio afterglow of GRB 970508, lasting 450 days after the burst. The spectral and temporal radio behavior indicate that the fireball has undergone a transition to sub-relativistic expansion at t ∼ 100 days. This allows us to perform "calorimetry" of the explosion. The derived total energy, E 0 ∼ 5 × 10 50 erg is well below the ∼ 5 × 10 51 erg inferred under the assumption of spherical symmetry from γ-ray and early afterglow observations. A natural consequence of this result, which can also account for deviations at t < 100 days from the spherical relativistic fireball model predictions, is that the fireball was initially a wide-angle jet of opening angle ∼ 30 • .Our analysis also allows to determine the energy fractions carried by electrons and magnetic field, and the density of ambient medium surrounding the fireball. We find that during the sub-relativistic expansion electrons and magnetic field are close to equipartition, and that the density of the ambient medium is ∼ 1 cm −3 . The inferred density rules out the possibility that the fireball expands into a strongly non-uniform medium, as would be expected, e.g., in the case of a massive star progenitor.

Long-lived emission, known as afterglow, has now been detected from about a dozen g-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ,1 to 3. The energy required to produce these bright g-ray¯ashes is enormous: up to ,10 53 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z > > 1:6. This is to date the brightest g-ray burst with a well-localized position and if the g-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.1 Palomar Observatory 105-24,

We have commenced a multi-year program, the Caltech-NRAO Stripe 82 Survey (CNSS), to search for radio transients with the Jansky VLA in the Sloan Digital Sky Survey Stripe 82 region. The CNSS will deliver five epochs over the entire ∼270 deg 2 of Stripe 82, an eventual deep combined map with a rms noise of ∼40 µJy and catalogs at a frequency of 3 GHz, and having a spatial resolution of 3. This first paper presents the results from an initial pilot survey of a 50 deg 2 region of Stripe 82, involving four epochs spanning logarithmic timescales between one week and 1.5 years, with the combined map having a median rms noise of 35 µJy. This pilot survey enabled the development of the hardware and software for rapid data processing, as well as transient detection and follow-up, necessary for the full 270 deg 2 survey. Data editing, calibration, imaging, source extraction, cataloging and transient identification were completed in a semi-automated fashion within six hours of completion of each epoch of observations, using dedicated computational hardware at the NRAO in Socorro, and custom-developed data reduction and transient detection pipelines. Classification of variable and transient sources relied heavily on the wealth of multi-wavelength legacy survey data in the Stripe 82 region, supplemented by repeated mapping of the region by the Palomar Transient Factory (PTF). 3.9 +0.5 −0.9 % of the few thousand detected point sources were found to vary by greater than 30%, consistent with similar studies at 1.4 GHz and 5 GHz. Multi-wavelength photometric data and light curves suggest that the variability is mostly due to shock-induced flaring in the jets of AGN. Although this was only a pilot survey, we detected two bona fide transients, associated with an RS CVn binary and a dKe star. Comparison with existing legacy survey data (FIRST, VLA-Stripe 82) revealed additional highly variable and transient sources on timescales between 5-20 years, largely associated with renewed AGN activity. The rates of such AGN possibly imply episodes of enhanced accretion and jet activity occurring once every ∼40,000 years in these galaxies. We compile the revised radio transient rates and make recommendations for future transient surveys and joint radio-optical experiments.

We present late-time imaging and spectroscopic observations of the optical transient (OT) of gamma-ray burst (GRB) 970508. Imaging observations roughly 200 and 300 days after the burst provide unambiguous evidence for the flattening of the light-curve. The spectroscopic observations reveal two persistent features which we identify with [O II] 3727 Angstrom and [Ne III] 3869 Angstrom at a redshift of z= 0.835 -- the same redshift as the absorption system seen when the transient was bright. The OT was coincident with the underlying galaxy to better than 370 milliarcsec or a projected radial separation of less than 2.7 kpc. The luminosity of the [O II] line implies a minimum star-formation rate of > 1 M_solar per year. In our assumed cosmology, the implied restframe absolute magnitude is M_B = -18.55, or L_B = 0.12 L_*. This object, the likely host of GRB 970508, can thus be characterized as an actively star-forming dwarf galaxy. The close spatial connection between this dwarf galaxy and the GRB requires that at least some fraction of progenitors be not ejected in even the weakest galactic potentials.Comment: Submitted to the Astrophysical Journal Letters (30 July 1998). Five pages including one table and two figures, written in LaTeX using the emulateapj and psfig macro

We report the discovery of optical and radio afterglow emission from the extremely soft X-ray flash XRF 020903. Our spectroscopic observations provide the first redshift for an X-ray flash, thereby setting the distance scale for these events. At z ¼ 0:251, XRF 020903 is one of the nearest cosmic explosions ever detected, second only to the recent GRB 030329 and the unusual GRB 980425/SN 1998bw. Moreover, XRF 020903 is the first X-ray flash for which we detect an optical afterglow. The luminosity of the radio afterglow of XRF 020903 is 1000 times greater than that of Ibc supernovae but similar to those of GRB afterglows. From broadband afterglow modeling we show that the explosion energy of XRF 020903 is not dissimilar to values inferred for typical gamma-ray bursts, suggesting that these cosmological explosions may derive from a similar mechanism.