Sonali Chakrabarti

A double component in GRB 090618: a proto-black hole and a genuinely long GRB

Context. The joint X-ray and gamma-ray observations of GRB 090618 by very many satellites offer an unprecedented possibility of testing crucial aspects of theoretical models. In particular, they allow us to test (a) in the process of gravitational collapse, the formation of an optically thick e + e − -baryon plasma self-accelerating to Lorentz factors in the range 200 < Γ < 3000; (b) its transparency condition with the emission of a component of 10 53−54 baryons in the TeV region and (c) the collision of these baryons with the circumburst medium (CBM) clouds, characterized by dimensions of 10 15−16 cm. In addition, these observations offer the possibility of testing a new understanding of the thermal and power-law components in the early phase of this GRB. Aims. We test the fireshell model of GRBs in one of the closest (z = 0.54) and most energetic (Eiso = 2.90 × 10 53 erg) GRBs, namely GRB 090618. It was observed at ideal conditions by several satellites, namely Fermi, Swift, Konus-WIND, AGILE, RT-2, and Suzaku, as well as from on-ground optical observatories. Methods. We analyzed the emission from GRB 090618 using several spectral models, with special attention to the thermal and powerlaw components. We determined the fundamental parameters of a canonical GRB within the context of the fireshell model, including the identification of the total energy of the e + e − plasma, E e + e− tot , the proper GRB (P-GRB), the baryon load, the density and structure of the CBM. Results. We find evidence of the existence of two different episodes in GRB 090618. The first episode lasts 50 s and is characterized by a spectrum consisting of a thermal component, which evolves between kT = 54 keV and kT = 12 keV, and a power law with an average index γ = 1.75 ± 0.04. The second episode, which lasts for ∼100 s, behaves as a canonical long GRB with a Lorentz gamma factor at transparency of Γ= 495, a temperature at transparency of 29.22 keV and with a characteristic size of the surrounding clouds of Rcl ∼ 10 15−16 cm and masses of ∼10 22−24 g. Conclusions. We support the recently proposed two-component nature of GRB 090618, namely, episode 1 and episode 2, with a specific theoretical analysis. We furthermore illustrate that episode 1 cannot be considered to be either a GRB or a part of a GRB event, but it appears to be related to the progenitor of the collapsing bare core, leading to the formation of the black hole, which we call a “proto-black hole”. Thus, for the first time, we are witnessing the process of formation of a black hole from the phases just preceding the gravitational collapse all the way up to the GRB emission.

Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

Aims. In the interstellar clouds, molecular hydrogens are formed from atomic hydrogen on grain surfaces. An atomic hydrogen hops around till it finds another one with which it combines. This necessa rily implies that the average recombination time, or equivalently, the effective grain surface area depends on the relative numbers of atomic hydrogen influx rate and the number of sites on the grain. Our a im is to discover this dependency. Methods. We perform a numerical simulation to study the recombination of hydrogen on grain surfaces in a variety of cloud conditions. We use a square lattice (with a periodic boundary condition) of various sizes on two types of grains, namely, amorphous carbon and olivine. Results. We find that the steady state results of our simulation match v ery well with those obtained from a simpler analytical consideration ,

Observational evidence for mass ejection during soft X-ray dips in GRS 1915+105

We investigate the connection between the X-ray and radio properties of the Galactic microquasar GRS 1915+105, by analyzing the X-ray data observed with RXTE, during the presence of a huge radio flare (~450 mJy). The X-ray lightcurve shows two dips of ~100 s duration. Detailed time resolved spectral analysis shows the existence of three spectral components: a multicolor disk-blackbody, a Comptonized component due to hot plasma and a power-law. We find that the Comptonized component is very weak during the dip. This is further confirmed by the PHA ratio of the raw data and ratio of the lightcurves in different energy bands. These results, combined with the fact that the 0.5-10 Hz QPO disappears during the dip and that the Comptonized component is responsible for the QPO lead to the conclusion that during the dips the matter emitting Comptonized spectrum is ejected away. This establishes a direct connection between the X-ray and radio properties of the source.

Spectral differences between the radio-loud and radio-quiet low-hard states of GRS 1915+105: Possible detection of synchrotron radiation in X-rays

The Galactic microquasar GRS 1915+105 exhibits several episodes of steady X-ray emission character- ized by a hard power-law spectrum and intense Quasi Periodic Oscillations. It is known that there are two types of such low-hard states, one with steady radio emission and the other without any signicant radio emission. We present the results of a detailed X-ray spectroscopic study of GRS 1915+105, using data from the Rossi X-ray Timing Explorer obtained during various episodes of the low-hard states of the source. We show that there are distinct X-ray spectral dierences between the radio-quiet and radio-loud low-hard states of the source. The X-ray spectra of the radio-quiet low-hard state is best described by a model consisting of a multicolor disk-blackbody and a Comptonized component, whereas the X-ray spectra of radio-loud low-hard state requires a model consisting of three components: a multicolor disk-blackbody, a Comptonized component and a power-law, for statistically and physically acceptable ts. We attempt to model the presence of this additional power-law component as due to synchrotron radiation which is responsible for the radio and infrared radiation from the source. We show that a simple adiabatically expanding jet model for the synchrotron radiation can account for the observed X-ray flux for reasonable values of the magnetic eld and the mass outflow rate. This is the rst report of detection of the synchrotron radiation in the X-ray band for this source.

Study of the chemical evolution and spectral signatures of some interstellar precursor molecules of adenine, glycine & alanine

Abstract We carry out a quantum chemical calculation to obtain the infrared and electronic absorption spectra of several complex molecules of the interstellar medium (ISM). These molecules are the precursors of adenine, glycine & alanine. They could be produced in the gas phase as well as in the ice phase. We carried out a hydro-chemical simulation to predict the abundances of these species in the gas as well as in the ice phase. Gas and grains are assumed to be interacting through the accretion of various species from the gas phase onto the grain surface and desorption (thermal evaporation and photo-evaporation) from the grain surface to the gas phase. Depending on the physical properties of the cloud, the calculated abundances varies. The influence of ice on vibrational frequencies of different pre-biotic molecules was obtained using Polarizable Continuum Model (PCM) model with the integral equation formalism variant (IEFPCM) as default SCRF method with a dielectric constant of 78.5. Time dependent density functional theory (TDDFT) is used to study the electronic absorption spectrum of complex molecules which are biologically important such as, formamide and precursors of adenine, alanine and glycine. We notice a significant difference between the spectra of the gas and ice phase (water ice). The ice could be mixed instead of simple water ice. We have varied the ice composition to find out the effects of solvent on the spectrum. We expect that our study could set the guidelines for observing the precursor of some bio-molecules in the interstellar space.

Formation of Cyanoformaldehyde in the interstellar space

Cyanoformaldehyde (HCOCN) molecule has recently been suspected towards the Sagittarius B2(N) by the Green Bank telescope, though a confirmation of this observation has not yet been made. In and around a star forming region, this molecule could be formed by the exothermic reaction between two abundant interstellar species, H2CO and CN. Till date, the reaction rate coefficient for the formation ofthis molecule is unknown. Educated guesses were used to explain the abundance of this molecule by chemical modeling. In this paper, we carried out quantum chemical calculations to find out empirical rate coefficients for the formation of HCOCN and different chemical properties during the formation of HCOCN molecules. Though HCOCN is stable against unimolecular decomposition, this gas phase molecule could be destroyed by many other means, like: ion-molecular reactions or by the effect of cosmic rays. Ionmolecular reaction rates are computed by using the capture theories. We have also included the obtained rate coefficients into our large gas-grain chemical network to study the chemical evolution of these species in various interstellar conditions. Formation of one of the isotopologue(DCOCN) of HCOCN is also studied. Our study predicts the possibility of finding HCOCN and DCOCN in the ice phase with a reasonably high abundance. In order to detect HCOCN or DCOCN in various interstellar environments, it is necessary to know the spectroscopic properties of these molecules. To this effect, we carried out quantum chemical calculations to find out different spectral parameters of HCOCN for the transition in electronic, infrared and rotational modes. We clearly show how the isotopic substitution (DCOCN) plays a part in the vibrational progressions of HCOCN.

Hydro-chemical study of the evolution of interstellar pre-biotic molecules during the collapse of molecular clouds

One of the stumbling blocks for studying the evolution of interstellar molecules is the lack of adequate knowledge about the rate coefficients of various reactions which take place in the interstellar medium and molecular clouds. Some theoretical models of rate coefficients do exist in the literature for computing abundances of complex pre-biotic molecules. So far these have been used to study the abundances of these molecules in space. However, in order to obtain more accurate final compositions in these media, we have calculated the rate coefficients for the formation of some of the most important interstellar pre-biotic molecules by using quantum chemical theory. We use these rates inside our hydro-chemical model to examine the chemical evolution and final abundances of pre-biotic species during the collapsing phase of a proto-star. We find that a significant amount of various pre-biotic molecules could be produced during the collapse phase of a proto-star. We thoroughly study the formation of these molecules via successive neutral-neutral and radical-radical/radical-molecular reactions. We present the time evolution of the chemical species with an emphasis on how the production of these molecules varies with the depth of a cloud. We compare the formation of adenine in interstellar space using our rate-coefficients and using those obtained from existing theoretical models. Formation routes of the pre-biotic molecules are found to be highly dependent on the abundances of the reactive species and the rate coefficients involved in the reactions. The presence of grains strongly affects the abundances of the gas phase species. We also carry out a comparative study between different pathways available for the synthesis of adenine, alanine, glycine and other molecules considered in our network. Despite the huge abundances of the neutral reactive species, production of adenine is found to be strongly dominated by the radical-radical/radical-molecular reaction pathways. If all the reactions considered here contribute to the production of alanine and glycine, then neutral-neutral and radical-radical/radical-molecular pathways are both found to have a significant part in the production of alanine. Moreover, radical-radical/radical-molecular pathways also play a major role in the production of glycine.

Chemical evolution during the process of proto-star formation by considering a two dimensional hydrodynamic model

Abstract Chemical composition of a molecular cloud is highly sensitive to the physical properties of the cloud. In order to obtain the chemical composition around a star forming region, we carry out a two dimensional hydrodynamical simulation of the collapsing phase of a proto-star. A total variation diminishing scheme (TVD) is used to solve the set of equations governing hydrodynamics. This hydrodynamic code is capable of mimicking evolution of the physical properties during the formation of a proto-star. We couple our reasonably large gas-grain chemical network to study the chemical evolution during the collapsing phase of a proto-star. To have a realistic estimate of the abundances of bio-molecules in the interstellar medium, we include the recently calculated rate coefficients for the formation of several interstellar bio-molecules into our gas phase network. Chemical evolution is studied in detail by keeping grain at the constant temperature throughout the simulation as well as by using the temperature variation obtained from the hydrodynamical model. By considering a large gas-grain network with the sophisticated hydrodynamic model more realistic abundances are predicted. We find that the chemical composition are highly sensitive to the dynamic behavior of the collapsing cloud, specifically on the density and temperature distribution.

Molecular hydrogen formation during dense interstellar cloud collapse

We study the evolution of molecular hydrogen on the grain surfaces and in the gas phase using both the rate equation (which tracks the average number of various species) and the master equation (which tracks the expectation values of various species). We show that above a certain critical accretion rate of H on the grains, the results from these two methods become identical. We used this result to follow the collapse of a dense interstellar cloud and studied the formation of molecular hydrogen for two different temperatures (T = 10 and 12 K) and two different masses (1 and 10 M ○. ) of the cloud when olivine grains were used. Because at higher temperatures, the recombination is very small for these grains, we also studied similar hydrodynamic processes at higher temperatures (T = 20 and 25 K) when amorphous carbon grains were used. We find that generally, for olivine grains, more than 90 per cent H is converted to H 2 within ∼10 5-7 yr whereas for amorphous grains it takes ∼ 10 6-7 yr. H 2 formed in this manner can be adequate to produce the observed complex molecules.

Studies of the Correlation Between Ionospheric Anomalies and Seismic Activities in the Indian Subcontinent

The VLF (Very Low Frequency) signals are long thought to give away important information about the Lithosphere‐Ionosphere coupling. It is recently established that the ionosphere may be perturbed due to seismic activities. The effects of this perturbation can be detected through the VLF wave amplitude. There are several methods to find this correlations and these methods can be used for the prediction of these seismic events. In this paper, first we present a brief history of the use of VLF propagation method for the study of seismo‐ionospheric correlations. Then we present different methods proposed by us to find out the seismo‐ionospheric correlations. At the Indian Centre for Space Physics, Kolkata we have been monitoring the VTX station at Vijayanarayanam from 2002. In the initial stage, we received 17 kHz signal and latter we received 18.2 kHz signal. In this paper, first we present the results for the 17 kHz signal during Sumatra earthquake in 2004 obtained from the terminator time analysis method. ...