Over the six years since the discovery of the γ-ray burst GRB 980425, which was associated with the nearby (distance ∼40 Mpc) supernova 1998bw, astronomers have debated fiercely the nature of this ...event. Relative to bursts located at cosmological distance (redshift z 1), GRB 980425 was under-luminous in γ-rays by three orders of magnitude. Radio calorimetry showed that the explosion was sub-energetic by a factor of 10. Here we report observations of the radio and X-ray afterglow of the recent GRB 031203 (refs 5-7), which has a redshift of z = 0.105. We demonstrate that it too is sub-energetic which, when taken together with the low γ-ray luminosity, suggests that GRB 031203 is the first cosmic analogue to GRB 980425. We find no evidence that this event was a highly collimated explosion viewed off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically sub-energetic γ-ray burst. Such sub-energetic events have faint afterglows. We expect intensive follow-up of faint bursts with smooth γ-ray light curves (common to both GRB 031203 and 980425) to reveal a large population of such events.
ABSTRACT
From an on-going survey of the Galactic bulge, we have discovered a number of compact, steep spectrum radio sources. In this study, we have carried out more detailed observations for two of ...these sources, located 43 arcmin and 12${_{.}^{\circ}}$7 from the Galactic Centre. Both sources have a very steep spectrum (α ≃ − 3) and are compact, with upper limits on the angular size of 1–2 arcsec. Their flux densities appear to be relatively steady on time-scales of years, months, and hours, with no indications of rapid variability or transient behaviour. We detect significant circularly polarized emission from both sources, but only weak or upper limits on linear polarization. Neither source has a counterpart at other wavelengths and deep, high-frequency searches fail to find pulsations. We compare their source properties with other known compact, non-thermal source populations in the bulge (e.g. X-ray binaries, magnetars, the Burper, and cataclysmic variables). Our existing data support the hypothesis that they are scatter broadened millisecond or recycled pulsars, either at the bulge or along the line of sight. We also consider the possibility that they may be a new population of Galactic radio sources which share similar properties as pulsars but lack pulsations; a hypothesis that can be tested by future large-scale synoptic surveys.
We apply a newly developed on-the-fly mosaicing technique on the Jansky Very Large Array (VLA) at 3 GHz in order to carry out a sensitive search for an afterglow from the Advanced LIGO binary black ...hole merger event GW151226. In three epochs between 1.5 and 6 months post-merger, we observed a 100 deg2 region, with more than 80% of the survey region having an rms sensitivity of better than 150 Jy/beam, in the northern hemisphere with a merger containment probability of 10%. The data were processed in near real time and analyzed to search for transients and variables. No transients were found but we have demonstrated the ability to conduct blind searches in a time-frequency phase space where the predicted afterglow signals are strongest. If the gravitational wave event is contained within our survey region, the upper limit on any late-time radio afterglow from the merger event at an assumed mean distance of 440 Mpc is about 1029 erg s−1 Hz−1. Approximately 1.5% of the radio sources in the field showed variability at a level of 30%, and can be attributed to normal activity from active galactic nuclei. The low rate of false positives in the radio sky suggests that wide-field imaging searches at a few Gigahertz can be an efficient and competitive search strategy. We discuss our search method in the context of the recent afterglow detection from GW170817 and radio follow-up in future gravitational wave observing runs.
We present extensive radio observations of SN 2003L, the most luminous and energetic Type Ibc radio supernova with the exception of SN 1998bw (associated with GRB 980425). Observations from the Very ...Large Array are well described by fitting a synchrotron self-absorption model to the emission spectrum. This model implies a subrelativistic ejecta velocity, u - 0.2c, and a size of r - 4.3 x 10 super(15) cm at t - 10 days. The circumstellar density is suitably fitted with a stellar wind profile, n sub(e) 8 r super(-2) cm super(-3) and a constant mass-loss rate of M - 7.5 x 10 super(-6) M sub( )yr super(-1). Moreover, the magnetic field follows B 8 r super(-1) and the kinetic energy of the radio-bright ejecta is roughly E - 10 super(48) ergs assuming equipartition of energy between relativistic electrons and magnetic fields. Furthermore, we show that free-free absorption does not contribute significantly to the radio spectrum, since it implies ejecta velocities that are inconsistent with size constraints derived from Very Long Baseline Array observations. In conclusion, we find that although SN 2003L has a radio luminosity comparable to that seen in SN 1998bw, it shows no evidence for a significant amount of energy coupled to relativistic ejecta. Using SN 2003L as an example, we comment briefly on the coupling of ejecta velocity and energy in Type Ibc supernovae.
Over the past decade, long-duration γ-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 1048 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.
On 2006 May 5, a 4 s duration, low-energy, similar to 10 super(49) erg, gamma-ray burst (GRB) was observed, spatially associated with a z = 0.0804 galaxy. Here we report the discovery of the GRB ...optical afterglow and observations of its environment using Gemini South, the Hubble Space Telescope (HST), Chandra, Swift, and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star-forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star-forming region suggests that the progenitor delay time, from birth to explosion, is smaller than similar to 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in the case of maximal extinction) at about 2 weeks after the burst and limits the ejected mass of radioactive super(56)Ni to be less than about 2 x 10 super(-4) M unk (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as that for short-duration GRBs.
We present the VLA-COSMOS 3 GHz Large Project based on 384 h of observations with the Karl G. Jansky Very Large Array (VLA) at 3 GHz (10 cm) toward the two square degree Cosmic Evolution Survey ...(COSMOS) field. The final mosaic reaches a median rms of 2.3 μJy beam-1 over the two square degrees at an angular resolution of 0.75″. To fully account for the spectral shape and resolution variations across the broad (2 GHz) band, we image all data with a multiscale, multifrequency synthesis algorithm. We present a catalog of 10 830 radio sources down to 5σ, out of which 67 are combined from multiple components.Comparing the positions of our 3 GHz sources with those from the Very Long Baseline Array (VLBA)-COSMOS survey, we estimate that the astrometry is accurate to 0.01″ at the bright end (signal-to-noise ratio, S/N3 GHz > 20). Survival analysis on our data combined with the VLA-COSMOS 1.4 GHz Joint Project catalog yields an expected median radio spectral index of α = −0.7. We compute completeness corrections via Monte Carlo simulations to derive the corrected 3 GHz source counts. Our counts are in agreement with previously derived 3 GHz counts based on single-pointing (0.087 square degrees) VLA data. In summary, the VLA-COSMOS 3 GHz Large Project simultaneously provides the largest and deepest radio continuum survey at high (0.75″) angular resolution to date, bridging the gap between last-generation and next-generation surveys.
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