Tata Institute of Fundamental Research

About: Tata Institute of Fundamental Research is a education organization based out in Mumbai, Maharashtra, India. It is known for research contribution in the topics: Magnetization & Large Hadron Collider. The organization has 7786 authors who have published 21742 publications receiving 622368 citations. The organization is also known as: TIFR.

Probing radio source environments via H i and OH absorption

Abstract: We present the results of H i and OH absorption measurements towards a sample of radio sources using the Arecibo 305-m telescope and the Giant Metrewave Radio Telescope (GMRT). In total, 27 radio sources were searched for associated 21-cm H i absorption. One totally new H i absorption system was detected against the radio galaxy 3C258, while five previously known H i absorption systems, and one galaxy detected in emission, were studied with improved frequency resolution and/or sensitivity. Our sample included 17 gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) objects, four of which exhibit H i absorption. This detection rate of ∼25 per cent compares with a value of ∼40 per cent by Vermeulen et al. for similar sources. We detected neither OH emission nor absorption towards any of the sources that were observed at Arecibo. We are, however, able to estimate a limit on the abundance ratio of N(H i)/N(OH) ≳ 4 × 106 for 3C258. We have combined our results with those from other available H i searches to compile a heterogeneous sample of 96 radio sources consisting of 27 GPS, 35 CSS, 13 compact flat spectrum (CFS) and 21 large (LRG) sources. The H i absorption detection rate is highest (∼45 per cent) for the compact GPS sources and least for the LRG sources. We find H i column density to be anticorrelated with source size, as reported earlier by Pihlstrom et al., a trend which is consistent with the results of optical spectroscopy. The H i column density shows no significant dependence on either redshift or luminosity, which are themselves strongly correlated. These results suggest that the environments of radio sources on GPS/CSS scales are similar at different redshifts. Further, in accordance with the unification scheme, the GPS/CSS galaxies have an H i detection rate of ∼40 per cent which is higher than the detection rate (∼20 per cent) towards the GPS/CSS quasars. Also, the principal (strongest) absorption component detected towards GPS sources appears blueshifted in ∼65 per cent of the cases. This is in agreement with the growing evidence for jet–cloud interactions playing an important role in determining the ionization and kinematical properties of the ambient gas.

Zero-Energy Rotating Accretion Flows near a Black Hole

Abstract: We characterize the nature of thin, axisymmetric, inviscid accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hole by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum (λ λb), most of the material is accreted into the black hole, forming a quasi-spherical flow or a simple disklike structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound value, λ λmb), almost no accretion into the black hole occurs. Instead, the flow produces a stable standing shock with one or more vortices behind it and is deflected away at the shock as a conical, outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than λmb (λu λ λmb), a fraction (typically 5%-10%) of the incoming material is accreted into the black hole, but the flow structure formed is similar to that for λ λmb. Some of the deflected material is accreted back into the black hole while the rest is blown away as an outgoing wind. These two cases with λ λu correspond those studied in the previous works by Molteni, Lanzafame, & Chakrabarti and Ryu et al. However, the flow with angular momentum close to the marginally stable value (λms) is found to be unstable. More specifically, if λb λ ~ λms λu, the flow displays a distinct periodicity in the sense that the inner part of the disk is built and destroyed regularly. The period is roughly equal to (4-6) × 103Rg/c, depending on the angular momentum of the flow. In this case, the internal energy of the flow around the black hole becomes maximum when the structure with the accretion shock and vortices is fully developed. But the mass accretion rate into the black hole reaches a maximum value when the structure collapses. Averaged over periods, more than half the incoming material is accreted into the black hole. We suggest the physical origin of these separate regimes from a global perspective. Then we discuss the possible relevance of the instability work to quasi-periodic oscillations.

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

Abstract: It was established over a decade ago that the remarkable high-energy transients known as soft $\gamma$-ray repeaters (SGRs) are located in our Galaxy and originate from neutron stars with intense $(\leq10^{15}G)$ magnetic fields—so-called ‘magnetars’. On 27 December 2004, a giant flare with a fluence exceeding $0.3\hspace{1mm} erg \hspace{1mm} cm^{-2}$ 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 $\gamma$-ray bursts. Instead, we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.