Alak Ray

A relativistic type Ibc supernova without a detected γ-ray burst

Long duration γ-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of γ-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs. Independently, a second mildly relativistic supernova has been reported.

Detection of a radio counterpart to the 27 December 2004 giant flare from SGR 1806-20

It was established over a decade ago that the remarkable high-energy transients known as soft γ-ray repeaters (SGRs) are located in our Galaxy and originate from neutron stars with intense (≤ 1015G) magnetic fields—so-called ‘magnetars’. On 27 December 2004, a giant flare with a fluence exceeding 0.3 erg cm-2 was detected from SGR 1806–20. Here we report the detection of a fading radio counterpart to this event. We began a monitoring programme from 0.2 to 250 GHz and obtained a high-resolution 21-cm radio spectrum that traces the intervening interstellar neutral hydrogen clouds. Analysis of the spectrum yields the first direct distance measurement of SGR 1806 - 20: the source is located at a distance greater than 6.4 kpc and we argue that it is nearer than 9.8 kpc. If correct, our distance estimate lowers the total energy of the explosion and relaxes the demands on theoretical models. The energetics and the rapid decay of the radio source are not compatible with the afterglow model that is usually invoked for γ-ray bursts. Instead, we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.

Results from an extensive simultaneous broadband campaign on the underluminous active nucleus M81*: Further evidence for mass-scaling accretion in black holes

We present the results of a broadband simultaneous campaign on the nearby low-luminosity active galactic nucleus M81*. From 2005 February through August, we observed M81* five times using the Chandra X-Ray Observatory with the HETGS, complemented by ground-based observations with the Giant Meterwave Radio Telescope, the Very Large Array and Very Large Baseline Array, the Plateau de Bure Interferometer at IRAM, the Submillimeter Array, and Lick Observatory. We discuss how the resulting spectra vary over short and longer timescales compared to previous results, especially in the X-rays where this is the first ever longer term campaign at spatial resolution high enough to nearly isolate the nucleus (17 pc). We compare the spectrum to our Galactic center weakly active nucleus Sgr A*, which has undergone similar campaigns, as well as to weakly accreting X-ray binaries in the context of outflow-dominated models. In agreement with recent results suggesting that the physics of weakly accreting black holes scales predictably with mass, we find that the exact same model that successfully describes hard-state X-ray binaries applies to M81*, with very similar physical parameters.

The Late-Time Radio Emission from SN 1993J at Meter Wavelengths

This paper presents the investigations of SN 1993J using low-frequency observations with the Giant Metrewave Radio Telescope (GMRT). We analyze the light curves of SN 1993J at 1420, 610, 325, and 243 MHz during 7.5 10 yr after the explosion. The supernova has become optically thin early on in the 1420 and 610 MHz bands, while it has only recently entered the optically thin phase in the 325 MHz band. The radio light curve in the 235 MHz band is more or less flat. This indicates that the supernova is undergoing a transition from an optically thick to optically thin limit in this frequency band. In addition, we analyze the supernova radio spectra at five epochs on days 3000, 3200, 3266, 3460, and 3730 after the explosion. The spectral break in the day 3200 composite spectrum from the GMRT and Very Large Array implies that the plasma is dominated by a magnetic field, and the latter is far from being in equipartition with relativistic particles. SN 1993J is the only young supernova for which the magnetic field and the size of the radio-emitting region are determined through unrelated methods. Thus, the mechanism that controls the evolution of the radio spectra can be identified. We suggest that at all epochs, the synchrotron self-absorption mechanism is primarily responsible for the turnover in the spectra. Light-curve models based on free-free absorption in homogeneous or inhomogeneous media at high frequencies overpredict the flux densities at low frequencies. The discrepancy is increasingly larger at lower and lower frequencies. We suggest that an extra opacity, sensitively dependent on frequency, is likely to account for the difference at lower frequencies. The evolution of the magnetic field (determined from synchrotron self-absorption turnover) is roughly consistent with

Type IIP supernova SN 2004et: a multiwavelength study in X-ray, optical and radio

B \propto t^{-1}

Ultra-high-energy cosmic ray acceleration in engine-driven relativistic supernovae

. The radio spectral index in the optically thin part evolves from \alpha \sim 0.8 1.0 at a few tens of days to 0.6 in about 10 yr.

X-RAYS FROM THE EXPLOSION SITE: 15 YEARS OF LIGHT CURVES OF SN 1993J

We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as L x oc t -0.4 . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of ∼2 x 10 -6 M ⊙ yr -1 . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of 56 Ni to be 0.06 ± 0.03 M ⊙ . Using the expressions of Litvinova & Nadezhin we estimate an explosion energy of (0.98 ± 0.25) x 10 51 erg. We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of ∼20 M ⊙ for SN 2004et.

SN 2013ej - A type IIL supernova with weak signs of interaction

The origin of ultra-high-energy cosmic rays (UHECRs) remains an enigma. They offer a window to new physics, including tests of physical laws at energies unattainable by terrestrial accelerators. They must be accelerated locally, otherwise, background radiations would severely suppress the flux of protons and nuclei, at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit. Nearby, gamma ray bursts (GRBs), hypernovae, active galactic nuclei and their flares have all been suggested and debated as possible sources. A local sub-population of type Ibc supernovae (SNe) with mildly relativistic outflows have been detected as sub-energetic GRBs, X-ray flashes and recently as radio afterglows without detected GRB counterparts. Here, we measure the size-magnetic field evolution, baryon loading and energetics, using the observed radio spectra of SN 2009bb. We place such engine-driven SNe above the Hillas line and establish that they can readily explain the post-GZK UHECRs.

A MISSING-LINK IN THE SUPERNOVA–GRB CONNECTION: THE CASE OF SN 2012ap

We present a comprehensive analysis of the X-ray light curves of SN 1993J in a nearby galaxy M81. This is the only supernova other than SN 1987A, which is so extensively followed in the X-ray bands. Here we report on SN 1993J observations with the Chandra in the year 2005 and 2008, and Swift observations in 2005, 2006 and 2008. We combined these observations with all available archival data of SN 1993J, which includes ROSAT, ASCA, Chandra, and XMM-Newton, observations from 1993 April to 2006 August. In this paper we report the X-ray light curves of SN 1993J, extending up to fifteen years, in the soft (0.3-2.4 keV), hard (2-8 keV) and combined (0.3-8 keV) bands. The hard and soft-band fluxes decline at different rates initially, but after about 5 years they both undergo a t(sup -1) decline. The soft X-rays, which are initially low, start dominating after a few hundred days. We interpret that most of the emission below 8 keV is coming from the reverse shock which is radiative initially for around first 1000-2000 days and then turn into adiabatic shock. Our hydrodynamic simulation also confirms the reverse shock origin of the observed light curves. We also compare the Ha line luminosity of SN 1993J with its X-ray light curve and note that the Ha line luminosity has a fairly high fraction of the X-ray emission, indicating presence of clumps in the emitting plasma.

ELEVEN YEARS OF RADIO MONITORING OF THE TYPE IIn SUPERNOVA SN 1995N

We present optical photometric and spectroscopic observations of supernova 2013ej. It is one of the brightest type II supernovae exploded in a nearby (