arXiv subjects

S. V. Vadawale

Publications and source records attributed to S. V. Vadawale.

33 records · Page 2Linked to original sources

arXiv2010

Comparative study of different scattering geometries for the proposed Indian X-ray polarization measurement experiment using Geant4

Polarization measurements in X-rays can provide unique opportunity to study the behavior of matter and radiation under extreme magnetic fields and extreme gravitational fields. Unfortunately, over past two decades, when X-ray astronomy witnessed multiple order of magnitude improvement in temporal, spatial and spectral sensitivities, there is no (or very little) progress in the field of polarization measurements of astrophysical X-rays. Recently, a proposal has been submitted to ISRO for a dedicated small satellite based experiment to carry out X-ray polarization measurement, which aims to provide the first X-ray polarization measurements since 1976. This experiment will be based on the well known principle of polarization measurement by Thomson scattering and employs the baseline design of a central low Z scatterer surrounded by X-ray detectors to measure the angular intensity distribution of the scattered X-rays. The sensitivity of such experiment is determined by the collecting area, scattering and detection efficiency, X-ray detector background, and the modulation factor. Therefore, it is necessary to carefully select the scattering geometry which can provide the highest modulation factor and thus highest sensitivity within the specified experimental constraints. The effective way to determine optimum scattering geometry is by studying various possible scattering geometries by means of Monte Carlo simulations. Here we present results of our detailed comparative study based on Geant4 simulations of five different scattering geometries which can be considered within the weight and size constraints of the proposed small satellite based X-ray polarization measurement experiment.

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arXiv2005

Detector and Telescope Development for ProtoEXIST and Fine Beam Measurements of Spectral Response of CZT Detectors

We outline our plan to develop ProtoEXIST, a balloon-borne prototype experiment for the Energetic X-ray Imaging Survey Telescope (EXIST) for the Black Hole Finder Probe. EXIST will consist of multiple wide-field hard X-ray coded-aperture telescopes. The current design of the EXIST mission employs two types of telescope systems: high energy telescopes (HETs) using CZT detectors, and low energy telescopes (LETs) using Si detectors. With ProtoEXIST, we will develop and demonstrate the technologies required for the EXIST HETs. As part of our development efforts, we also present recent laboratory measurements of the spectral response and efficiency variation of imaging CZT detectors on a fine scale (~0.5 mm). The preliminary results confirm the need for multi-pixel readouts and small inter-pixel gaps to achieve uniform spectral response and high detection efficiency across detectors.

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arXiv2005

Monte-Carlo simulations of the expected imaging performance of the EXIST high-energy telescope

EXIST is being studied as the Black Hole Finder Probe, one of the 3 Einstein Probe missions under NASA's Beyond Einstein program. The major science goals for EXIST include highly sensitive full-sky hard X-ray survey in a very wide energy band of 5-600 keV. The scientific requirements of wide energy band (10-600 keV for the High Energy Telescope considered for EXIST) and large field of view (approximately 130 deg x 60 deg in the current design, incorporating an array of 18 contiguous very large area coded aperture telescopes) presents significant imaging challenges. The requirement of achieving high imaging sensitivity puts stringent limits on the uniformity and knowledge of systematics for the detector plane. In order to accomplish the ambitious scientific requirements of EXIST, it is necessary to implement many novel techniques. Here we present the initial results of our extensive Monte-Carlo simulations of coded mask imaging for EXIST to estimate the performance degradation due to various factors affecting the imaging such as the non-ideal detector plane and bright partially coded sources.

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arXiv2005

GRS 1915+105: the distance, radiative processes and energy-dependent variability

We present an exhaustive analysis of five broad-band observations of GRS 1915+105 in two variability states, chi and omega, observed simultaneously by the PCA, HEXTE and OSSE. We find all the spectra well fitted by Comptonization of disc blackbody photons, with very strong evidence for the presence of a nonthermal electron component in the Comptonizing plasma. Both the energy and the power spectra in the chi state are typical to the very high/intermediate state of black-hole binaries. The spectrum of the omega state is characterized by a strong blackbody component Comptonized by thermal electrons and a weak nonthermal tail. We then calculate rms spectra (fractional variability as functions of energy) for the PCA data. We accurately model the rms spectra by coherent superposition of variability in the components implied by the spectral fits, namely a less variable blackbody and more variable Comptonization. The latter dominates at high energies, resulting in a flattening of the rms at high energies in most of the data. This is also the case for the spectra of the QPOs present in the chi state. Then, some of our data require a radial dependence of the rms of the disc blackbody. We also study the distance to the source, and find 11 kpc as the most likely value, contrary to a recent claim of a much lower value.

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arXiv2004

Multipixel characterization of imaging CZT detectors for hard X-ray imaging and spectroscopy

We report our in-depth study of Cd-Zn-Te (CZT) crystals to determine an optimum pixel and guard band configuration for Hard X-ray imaging and spectroscopy. We tested 20x20x5mm crystals with 8x8 pixels on a 2.46mm pitch. We have studied different types of cathode / anode contacts and different pixel pad sizes. We present the measurements of leakage current as well as spectral response for each pixel. Our I-V measurement setup is custom designed to allow automated measurements of the I-V curves sequentially for all 64 pixels, whereas the radiation properties measurement setup allows for interchangeable crystals with the same XAIM3.2 ASIC readout from IDEAS. We have tested multiple crystals of each type, and each crystal in different positions to measure the variation between individual crystals and variation among the ASIC channels. We also compare the same crystals with and without a grounded guard band deposited on the crystal side walls vs. a floating guard band and compare results to simulations. This study was carried out to find the optimum CZT crystal configuration for prototype detectors for the proposed Black-Hole Finder mission, EXIST.

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arXiv2004

Laboratory coded aperture imaging experiments: radial hole coded masks and depth-sensitive CZT detectors

The proposed black-hole finder mission EXIST will consist of multiple wide-field hard X-ray coded-aperture telescopes. The high science goals set for the mission require innovations in telescope design. In particular, wide energy band coverage and fine angular resolution require relatively thick coded masks and thick detectors compared to their pixel size, which may introduce mask self-collimation and depth-induced image blurring with conventional design approaches. Previously we proposed relatively simple solutions to these potential problems: radial hole for mask selfcollimation and cathode depth sensing detector for image blurring. We have now performed laboratory experiments to explore the potential of these two techniques. The experimental results show that the radial hole mask greatly alleviates mask self-collimation and a ~1 mm resolution depth-sensitive detector scheme can be relatively easily achieved for the large scale required for EXIST.

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arXiv2004

Binary corrected X-ray light curve of Cygnus X-3 : implications for the timing properties of the compact binary system

We study the binary corrected X-ray light curve of Cygnus X-3 using the RXTE-ASM data and RXTE-PCA data. We find that the X-ray template derived from the EXOSAT observations adequately explains the binary variations. Using the recently determined quadratic binary ephemeris we correct the RXTE-PCA lightcurve and obtain the power spectrum which shows distinct features of shifting towards the low frequency regime vis-a-vis the Galactic X-ray binaries. The power spectrum doesn't vary from the hard to the soft state. We also obtain the binary corrected RXTE-ASM monitoring lightcurve and examine the previously obtained correlation between soft X-ray (2-12 keV from RXTE-ASM), hard X-ray (20-100 keV from CGRO-BATSE) and radio (2.2 GHz from GBI) after the binary correction. We find that the correlation time scale is less than a day.

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arXiv2004

Ejection of the inner accretion disk in GRS 1915+105: the magnetic rubber-band effect

We examine theoretically the behaviour of the inner accretion disk in GRS 1915+105 when soft X-ray dips are present in the X-ray light curve. We assume the presence of a radial shock in the accretion disk, as in some of the Two Component Advective Flow (TCAF) solutions. We discuss the behaviour of the flux tubes inside a TCAF (which we name Magnetized TCAF or MTCAF model for brevity) and compare various competing forces on the flux tubes. In this MTCAF model, we find that the magnetic tension is the strongest force in a hot plasma of temperature $\gsim 10^{10}$K and as a result, magnetic flux tubes entering in this region collapse catastrophically, thereby occasionally evacuating the inner disk. We postulate that this magnetic `rubber-band' effect induced evacuated disk matter produces the blobby components of outflows and IR/radio jets. We derive the size of the post-shock region by equating the time scale of the Quasi-Periodic Oscillations to the infall time of accreting matter in the post-shock region and found the shock location to be $\sim 45-66 r_g$. We calculate the transition radius $r_{tr}$, where the Keplerian disk deviates into a sub-Keplerian flow, to be $\sim 320r_g$. Based on the derived X-ray spectral parameters, we calculate the mass of this region to be $\sim10^{18}$g. We conclude that during the X-ray dips the matter in the post-shock region, which manifests itself as the thermal-Compton component in the X-ray spectrum, is ejected, along with some sub-Keplerian matter in the pre-shock region.

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arXiv2003

On the origin of the various types of radio emission in GRS 1915+105

We investigate the association between the radio ``plateau'' states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. We identify several candidate superluminal flare events from available monitoring data on this source and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the ``plateau'' state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are observed due to the interaction of the matter blob ejected during the X-ray soft dips, with the steady jet already established during the ``plateau'' state. This picture can explain all types of radio emission observed from this source in terms of its X-ray emission characteristics.

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arXiv2003

Correlated radio:X-ray emission in the hard states of Galactic microquasars

We present results of our study of correlated radio and X-ray emission in two black hole candidates and Galactic microquasars GRS 1915+105 and Cygnus X-1 in their steady long term hard states, along with Cygnus X-3 (using data obtained from RXTE-ASM, CGRO-BATSE and GBI). We detect a pivotal behavior in the X-ray spectrum of GRS 1915+105, correlated to the radio emission. Similar to the results obtained for Cygnus X-3, the flux of X-rays softer than the pivoting point correlates with the radio emission, while the corresponding harder X-ray flux anti-correlates with both the radio and the softer X-ray emission, in this state. We examine all the previously reported correlations of X-ray properties with the radio emission in Galactic microquasars and argue that these are consistent with a general picture where a spectral pivoting is a common feature in these sources with the shape of the spectrum determining the flux of radio emission, during the hard states. We also detect a general monotonic increase in the radio emission of these sources with the soft X-ray emission spanning about 5 orders of magnitude. We qualitatively explain these findings with a Two Component Advective Flow model where the location of a boundary layer between the thin disk and the Comptonizing region determines the spectral shape and also the amount of outflow.

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arXiv2002

Disk-Jet Connection in Cygnus X-3

We present the results of a detailed correlation study between the soft X-ray, hard X-ray, and radio emission (obtained from RXTE ASM, BATSE, and GBI observations, respectively) of the bright radio emitting Galactic X-ray binary Cygnus X-3. We detect a very strong positive correlation between the soft X-ray and radio emission during the low-hard and minor flaring periods of the source, and an anti-correlation between the soft and hard X-ray emissions. We present statistical arguments to suggest that the anti-correlation between the radio and hard X-ray emission, reported earlier, is primarily due to their correlation and anti-correlation, respectively, with the soft X-ray emission. We make a wide band X-ray spectral study using the pointed RXTE observations and detect a pivotal behaviour in the X-ray spectrum. We argue that this X-ray spectral pivoting is responsible for the anti-correlation between the soft and hard X-ray emissions. The strong correlation between the soft X-ray and radio emission suggests a close link between the accreting mechanism, plasma cloud surrounding the compact object and the radio emission.

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arXiv2001

OSSE and RXTE Observations of GRS 1915+105: Evidence for Non-thermal Comptonization

GRS 1915+105 was observed by the CGRO/OSSE 9 times in 1995-2000, and 8 of those observations were simultaneous with those by RXTE. We present an analysis of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law--like spectrum extending up to ~600 keV without any break. We interpret this emission as strong evidence for the presence of non-thermal electrons in the source. The broad-band spectra cannot be described by either thermal or bulk-motion Comptonization, whereas they are well described by Comptonization in hybrid thermal/ non-thermal plasmas.

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arXiv2001

Observational evidence for mass ejection during soft X-ray dips in GRS1915+105

We investigate the connection between the X-ray and radio properties of the Galactic microquasar GRS1915+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 second 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.

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arXiv2001

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 characterized 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 significant 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 differences 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 ofa 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 fits. We attempt to model the presence of this additional power-law component as due to synchrotronradiation which is responsible for the radio and infrared radiation from thesource. 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 field and the mass outflow rate. This is the first report of detection of the synchrotron radiation in the X-ray band for this source.

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arXiv2000

X-ray spectral components in the hard state of GRS 1915+105: origin of the 0.5 - 10 Hz QPO

We investigate the origin of the ubiquitous 0.5 - 10 Hz QPO in the Galactic microquasar GRS 1915+105. Using the archival X-ray data from RXTE, we make a wide band X-ray spectral fitting to the source during a low-hard state observed in 1999 June. We resolve the X-ray spectra into three components, namely a multi-color disk component, a Comptonised component and a power-law at higher energies. This spectral description is favored compared to other normally used spectra like a cut-off power law, hard components with reflection etc. We find that the 0.5 - 10 Hz QPO is predominantly due to variations in the Comptonised component. We use this result to constrain the location of the various spectral components in the source.

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