Aritra Basu

The Discovery and Nature of the Optical Transient CSS100217:102913+404220

We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed that this transient was coincident with a galaxy at redshift z = 0.147 and reached an apparent magnitude of V ~ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be M_V = –22.7 approximately 45 days after maximum light. Over a period of 287 rest-frame days, this event had an integrated bolometric luminosity of 1.3 × 10^(52) erg based on time-averaged bolometric corrections of ~15 from V- and R-band observations. Analysis of the pre-outburst Sloan Digital Sky Survey (SDSS) spectrum of the source shows features consistent with a narrow-line Seyfert 1 galaxy. High-resolution Hubble Space Telescope and Keck follow-up observations show that the event occurred within 150 pc of the nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival Catalina Sky Survey NEO search and SDSS data to investigate the three most likely sources of such an event: (1) an extremely luminous supernova, (2) the tidal disruption of a star by the massive nuclear black hole, and (3) variability of the central active galactic nucleus (AGN). We find that CSS100217 was likely an extremely luminous Type IIn supernova and occurred within the range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.

Magnetic fields in nearby normal galaxies: energy equipartition

We present maps of total magnetic field using ‘equipartition’ assumptions for five nearby normal galaxies at sub-kpc spatial resolution. The mean magnetic field is found to be ∼11 μG. The field is strongest near the central regions where mean values are ∼20−25 μG and fall to ∼15 μG in disc and ∼10 μG in the outer parts. There is little variation in the field strength between arm and interarm regions, such that, in the interarms, the field is 20 per cent weaker than in the arms. There is no indication of variation in magnetic field as one moves along arm or interarm after correcting for the radial variation of magnetic field. We also studied the energy densities in gaseous and ionized phases of the interstellar medium and compared to the energy density in the magnetic field. The energy density in the magnetic field was found to be similar to that of the gas within a factor of 2 at sub-kpc scales in the arms, and thus magnetic field plays an important role in pressure balance of the interstellar medium. Magnetic field energy density is seen to dominate over the kinetic energy density of gas in the interarm regions and outer parts of the galaxies and thereby helps in maintaining the large-scale ordered fields seen in those regions.

Multiwavelength characterization of faint ultra steep spectrum radio sources: A search for high-redshift radio galaxies

Context. Ultra steep spectrum (USS) radio sources are one of the efficient tracers of powerful high-z radio galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts. Aims. Using our 325 MHz GMRT observations (5σ ∼ 800 μJy) and 1.4 GHz VLA observations (5σ ∼ 80−100 μJy) available in two subfields (VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature. Methods. The optical and IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Results. Redshift estimates are available for 86/116 (∼74%) USS sources in the VLA-VVDS field and for 39/44 (∼87%) USS sources in the SXDF fields with median values (zmedian) ∼1.18 and ∼1.57, respectively, which are higher than estimates for non-USS radio sources (zmedian non−USS ∼ 0.99 and ∼0.96), in the two subfields. The MIR color–color diagnostic and radio luminosities are consistent with most of our USS sample sources at higher redshifts (z > 0.5) being AGN. The flux ratio of radio to mid-IR (S 1.4 GHz/S 3.6 μm) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ∼ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (∼26% in the VLA-VVDS and ∼13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and so, can be considered as potential HzRG candidates. Conclusions. Our study shows that the criterion of ultra steep spectral index remains a reasonably efficient method to select high-z sources even at sub-mJy flux densities. In addition to powerful HzRG candidates, our faint USS sample also contains populations of weaker radio-loud AGNs potentially hosted in obscured environments.

New insights into the interstellar medium of the dwarf galaxy IC 10 : connection between magnetic fields, the radio--infrared correlation and star formation

We present the highest sensitivity and angular resolution study at 0.32 GHz of the dwarf irregular galaxy IC\,10, observed using the Giant Metrewave Radio Telescope, probing

GMRT radio continuum study of Wolf–Rayet galaxies – I. NGC 4214 and NGC 4449

\sim45

CHANG-ES X: Spatially Resolved Separation of Thermal Contribution from Radio Continuum Emission in Edge-on Galaxies

pc spatial scales. We find the galaxy-averaged radio continuum spectrum to be relatively flat, with a spectral index

Broadband radio spectro-polarimetric observations of high-Faraday-rotation-measure AGN

\alpha = -0.34\pm0.01

RADIO–FAR-INFRARED CORRELATION IN “BLUE CLOUD” GALAXIES WITH 0 < z < 1.2

(

GMRT 333-MHz observations of six nearby normal galaxies

S_\nu \propto \nu^\alpha

From the NVSS RM Catalogue to Future Polarisation Surveys

), mainly due to a high contribution from free--free emission. At 0.32 GHz, some of the H{\sc ii} regions show evidence of free--free absorption as they become optically thick below