arXiv subjects

R. Misra

Publications and source records attributed to R. Misra.

At least 73 records · Page 4Linked to original sources

arXiv2008

The spectral and temporal properties of an Ultra-Luminous X-ray source in NGC 6946

We report variability of the X-ray source, X-7, in NGC 6946, during a 60 ksec Chandra observation when the count rate decreased by a factor of ~1.5 in ~5000 secs. Spectral fitting of the high and low count rate segments of the light curve reveal that the simplest and most probable interpretation is that the X-ray spectra are due to disk black body emission with an absorbing hydrogen column density equal to the Galactic value of 2.1 X 10^{21} cm^{-2}. During the variation, the inner disk temperature decreased from ~0.29 to ~0.26 keV while the inner disk radius remained constant at ~6 X 10^8 cm. This translates into a luminosity variation from 3.8 to 2.8 X 10^{39} ergs cm^{-2} sec^{-1} and a black hole mass of ~400 solar masses. More complicated models like assuming intrinsic absorption and/or the addition of a power-law component imply a higher luminosity and a larger black hole mass. Even if the emission is beamed by a factor of ~5, the size of the emitting region would be > 2.7 X 10^8 cm implying a black hole mass > 180 solar masses. Thus, these spectral results provide strong evidence that the mass of the black hole in this source is definitely > 100 solar masses and more probably ~400 solar masses.

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arXiv2008

Damped harmonic oscillator interpretation of the soft-state power spectra of Cyg X-1

We develop a model of an accretion disc in which the variability induced at a given radius is governed by a damped harmonic oscillator at the corresponding epicyclic frequency. That variability induces both linear and non-linear responses in the locally emitted radiation. The total observed variability of a source is the sum of these contributions over the disc radius weighted by the energy dissipation rate at each radius. It is shown that this simple model, which effectively has only three parameters including the normalization, can explain the range of the power spectra observed from Cyg X-1 in the soft state. Although a degeneracy between the black hole mass and the strength of the damping does not allow a unique determination of the mass, we can still constrain it to <16--20 solar masses. We also show that our model preserves the observed linear rms-flux relationship even in the presence of the non-linear flux response.

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arXiv2007

The dependence of the estimated luminosities of ULX on spectral models

Data from {\it Chandra} observations of thirty nearby galaxies were analyzed and 365 X-ray point sources were chosen whose spectra were not contaminated by excessive diffuse emission and not affected by photon pile up. The spectra of these sources were fitted using two spectral models (an absorbed power-law and a disk blackbody) to ascertain the dependence of estimated parameters on the spectral model used. It was found that the cumulative luminosity function depends on the choice of the spectral model, especially for luminosities $> 10^{40}$ ergs/s. In accordance with previous results, a large number ($\sim 80$) of the sources have luminosities $> 10^{39}$ ergs/s (Ultra-Luminous X-ray sources) with indistinguishable average spectral parameters (inner disk temperature $\sim 1$ keV and/or photon index $Γ\sim 2$) with those of the lower luminosities ones. After considering foreground stars and known background AGN,we identify four sources whose minimum luminosity exceed $10^{40}$ ergs/s, and call them Extremely Luminous X-ray sources (ELX). The spectra of these sources are in general better represented by the disk black body model than the power-law one. These ELX can be grouped into two distinct spectral classes. Two of them have an inner disk temperature of $< 0.5$ keV and hence are called ``supersoft'' ELX, while the other two have temperatures $\gtrsim 1.3$ keV and are called ``hard'' ELX. The estimated inner disk temperatures of the supersoft ELX are compatible with the hypothesis that they harbor intermediate size black holes, which are accreting at $\sim 0.5$ times their Eddington Luminosity. The radiative mechanism for hard ELX, seems to be Inverse Comptonization, which in contrast to standard black holes systems, is probably saturated.

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arXiv2006

Self-consistent computation of gamma-ray spectra due to proton-proton interactions in black hole systems

In the inner regions of an accretion disk around a black hole, relativistic protons can interact with ambient matter to produce electrons, positrons and $γ$-rays. The resultant steady state electron and positron particle distributions are self-consistently computed taking into account Coulomb and Compton cooling, $e^-e^+$ pair production (due to $γ-γ$ annihilation) and pair annihilation. While earlier works used the diffusion approximation to obtain the particle distributions, here we solve a more general integro-differential equation that correctly takes into account the large change in particle energy that occur when the leptons Compton scatter off hard X-rays. Thus this formalism can also be applied to the hard state of black hole systems, where the dominant ambient photons are hard X-rays. The corresponding photon energy spectrum is calculated and compared with broadband data of black hole binaries in different spectral states. The results indicate that the $γ$-ray spectra ($E > 0.8$ MeV) of both the soft and hard spectral states and the entire hard X-ray/$γ$-ray spectrum of the ultra-soft state, could be due to $p-p$ interactions. These results are consistent with the hypothesis that there always exists in these systems a $γ$-ray spectral component due to $p-p$ interactions which can contribute between 0.5 to 10% of the total bolometric luminosty. The model predicts that {\it GLAST} would be able to detect black hole binaries and provide evidence for the presence of non-thermal protons which in turn would give insight into the energy dissipation process and jet formation in these systems.

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arXiv2006

The non-linear behavior of the black hole system GRS 1915+105

Using non-linear time series analysis, along with surrogate data analysis, it is shown that the various types of long term variability exhibited by the black hole system GRS 1915+105, can be explained in terms of a deterministic non-linear system with some inherent stochastic noise. Evidence is provided for a non-linear limit cycle origin of one of the low frequency QPO detected in the source, while some other types of variability could be due to an underlying low dimensional chaotic system. These results imply that the partial differential equations which govern the magneto-hydrodynamic flow of the inner accretion disk, can be approximated by a small number ($\approx 3 -5$) of non-linear but {\it ordinary} differential equations. While this analysis does not reveal the exact nature of these approximate equations, they may be obtained in the future, after results of magneto-hydrodynamic simulation of realistic accretion disks become available.

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arXiv2006

A non subjective approach to the GP algorithm for analysing noisy time series

We present an adaptation of the standard Grassberger-Proccacia (GP) algorithm for estimating the Correlation Dimension of a time series in a non subjective manner. The validity and accuracy of this approach is tested using different types of time series, such as, those from standard chaotic systems, pure white and colored noise and chaotic systems added with noise. The effectiveness of the scheme in analysing noisy time series, particularly those involving colored noise, is investigated. An interesting result we have obtained is that, for the same percentage of noise addition, data with colored noise is more distinguishable from the corresponding surrogates, than data with white noise. As examples for real life applications, analysis of data from an astrophysical X-ray object and human brain EEG, are presented.

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arXiv2006

The Unusual Spectrum of the Ultra Luminous X-ray Source, M82 X-1

The results of a spectral analysis, using {\it XMM-Newton} and {\it Chandra} data of the brightest ultra luminous X-ray source in the nearby galaxy M82, are presented. The spectrum of M82 X-1, was found to be unusually hard (photon spectral index $Γ\approx 1$) with a sharp cutoff at $\approx 6$ keV. Disk black body emission model requires a nonphysically high temperature. Instead, the spectrum is better described, with a lower reduced $χ^2$, as emission due to nearly saturated Comptonization of photons in an optically thick ($τ\approx 10-30$, depending on the geometry) plasma having a temperature $kT \approx 2$ keV. This is in contrast to the high energy spectra of other black hole systems, which are relatively steeper ($Γ> 1.5$) and hence are modeled as un-saturated thermal and/or non-thermal Comptonization of soft photons, in an optically thin ($τ\approx 1$) high temperature plasma. An Iron line emission which is marginally resolved ($σ\sim 0.2$ keV) is required to fit the data. We argue that the standard geometries for the X-ray producing region, which are an optically thin inner disk or an uniform/patchy corona on top of a cold disk, are not applicable to this source. Alternatively, the geometry of the X-ray producing region could be a large sphere surrounding a cold accretion disk or an optically thick inner disk region which cools by bremsstrahlung self-Comptonization. For the latter scenario, such an inner disk region, whose effective optical depth to absorption is less than unity, is expected in the standard accretion disk theory for near Eddington accretion rates.

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arXiv2005

Interpretation of the Radio/X-ray Knots of AGN Jets within the Internal Shock Model Framework

The dynamics of relativistically moving blobs ejected out of a central AGN, are considered. It is assumed that the collision between two blobs are completely inelastic, such that the bulk kinetic energy lost in the collision is used to energize electrons to relativistic energies via acceleration in internal shocks which are formed due to the collision. These high energy electrons which are produced on a time-scale corresponding to the collision time-scale, cool by radiative losses due to synchrotron and Inverse Compton processes. The model is applied to the radio/X-ray knots of several AGN. For three of these sources we have analyzed long (> 40 ksec) Chandra observations and report on constrains on the X-ray spectral indices. In the framework of this model the AGN are inferred to sporadically eject relativistic blobs on time-scales ranging from 10^{11} to 10^{12} secs for different sources. It is shown that the collision time-scales can be longer than the age of the knot and, hence, non-thermal electrons are continuously being injected into the system. This continuous injection, in contrast to an instantaneous one time injection, gives rise to a characteristic spectral break, rather than a high-energy cutoff in the spectrum.

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arXiv2004

On the incidence of KHz quasi-periodic oscillations in the neutron star system 4U 1636-53

Nearly $1.32 \times 10^6$ sec of non-zero count rate data were obtained from five years of {\rm RXTE} observations of the atoll source 4U 1636-53. The data was divided into 10309 segments of 128 sec each. The histogram of the number of segments as a function of count rate shows that the system can naturally be classified into four flux states. For each segment an automated search for kHz QPO was undertaken and the histograms of the maximum Leahy normalised power $P_{max}$ for the four flux states were created. These were fitted by probability distribution functions under the assumption that the intrinsic amplitude of the QPO has a normal distribution. The best fit distribution functions, showed that the fraction $f$ of the time the system exhibits a kHz QPO, decreases from near unity for the lowest flux state ($\approx 1400$ c/s) to zero for the highest one ($> 2550$ c/s) while the average amplitude remains nearly constant. Based on the best fit probability distribution functions, a threshold $P_{thre}$ was defined, such that 95% of the segments with $P_{max} > P_{thre}$ correspond to a real QPO signal. For segments selected by this criterion, the frequency versus count rate plot reproduced the earlier known ``parallel track'' variation with the tracks coinciding roughly with the flux states. It is shown that the gaps between the tracks are not caused by uneven sampling, but rather the QPO phenomenon is absent or weak when the system's flux level is intermediate between two flux states.

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arXiv2004

The chaotic behavior of the black hole system GRS 1915+105

A modified non-linear time series analysis technique, which computes the correlation dimension $D_2$, is used to analyze the X-ray light curves of the black hole system GRS 1915+105 in all twelve temporal classes. For four of these temporal classes $D_2 $ saturates to $\approx 4-5$ which indicates that the underlying dynamical mechanism is a low dimensional chaotic system. Of the other eight classes, three show stochastic behavior while five show deviation from randomness. The light curves for four classes which depict chaotic behavior have the smallest ratio of the expected Poisson noise to the variability ($ < 0.05$) while those for the three classes which depict stochastic behavior is the highest ($ > 0.2$). This suggests that the temporal behavior of the black hole system is governed by a low dimensional chaotic system, whose nature is detectable only when the Poisson fluctuations are much smaller than the variability.

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arXiv2003

The Effect of Non-Thermal Protons on the High Energy Spectra of Black Hole Binaries

In the inner regions of an accretion disk around a black hole, the presence of non-thermal protons would via p-p collisions produce electrons, positrons and $γ$-rays. For such a scenario, the steady state electron-positron distribution is computed taking into account Compton cooling, $e^{-} e^{+}$ pair production (due to $γ-γ$ interactions) and pair annihilation. The resultant spectra has a broad feature around $1 - 10$ MeV which may be tested with observations by INTEGRAL and GLAST. Using the available OSSE data for the black hole system, GRS 1915+115 we illustrate that an upper limit on the non-thermal proton fraction can be obtained, which may put constraints on the acceleration process active in this system.

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arXiv2003

A Continuous Injection Plasma Model for the X-Ray/Radio Knots in Kpc-Scale Jets of AGN

We consider the evolution of a spherically expanding plasma cloud, where there is continuous injection of non-thermal electrons. We compute the time dependent electron distribution and resultant photon spectra taking into account synchrotron, adiabatic and inverse Compton cooling. This model is different from previous works where, instead of a continuous injection of particles, a short injection period was assumed. We apply this model to the radio/optical knots in the large scale jets of AGN, detected in X-rays by {\it Chandra} and find that the overall broadband spectral features can be reproduced. It is shown that for some sources, constraints on the X-ray spectral index (by a longer {\it Chandra} observation) will be able to differentiate between the different models. This in turn will put a strong constraint on the acceleration mechanism active in these sources.

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arXiv2002

Flux enhancement in the inner region of a geometrically and optically thick accretion disk

The surface flux (and the corresponding observed flux) of a geometrically thick ``funnel'' shaped disk is computed taking into account the radiation impinging on the surface from other parts of the disk. It is found that the ratio of the maximum apparent luminosity to the real luminosity of the disk is only a factor $\approx 5$ even when the opening angle of the disk is small ($\approx 15^o$). Thus, geometrically beamed emission from ``funnel'' shaped sub-Eddington disks around stellar mass black holes, cannot explain the Ultra-Luminous X-ray sources detected in nearby galaxies.

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arXiv2001

A Compton Up-scattering Model for Soft Lags in the Lower Kilohertz QPO in 4U1608-52

An empirical Compton up-scattering model is described which reproduces both the fractional amplitude (RMS) vs. energy and the soft time lags in the 830 Hz QPO observed in 4U1608-52 on Mar. 3, 1996. A combination of two coherent variations in the coronal and soft photon temperatures (with their relative contributions determined by enforcing energy conservation) gives rise to the QPO's energy dependent characteristics. All input parameters to the model, save a characteristic plasma size and the fraction of Comptonized photons impinging on the soft photon source, are derived from the time-averaged photon energy spectrum of the same observation. Fits to the fractional RMS and phase lag data for this kilohertz QPO imply that the spatial extent of the plasma is in the range from 4 to 15 km.

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arXiv2000

Hydrodynamical winds from two-temperature plasma in X-ray binaries

Hydrodynamical winds from a spherical two-temperature plasma surrounding a compact object are constructed. The mass-loss rate is computed as a function of electron temperature, optical depth and luminosity of the sphere, the values of which can be constrained by the fitting of the spectral energy distributions for known X-ray binary systems. The sensitive dependence of the mass loss rate with these parameters leads to the identification of two distinct regions in the parameter space separating wind-dominated from non wind dominated systems. A critical optical depth, tau_c, as a function of luminosity and electron temperature, is defined which differentiates these two regions. Systems with optical depths significantly smaller than tau_c are wind-dominated. The results are applied to black hole candidate X-ray binary systems in the hard spectral state (Cyg X-1, GX 339-4 and Nova Muscae), and it is found that the inferred optical depth (tau) is similar to tau_c suggesting that they are wind regulated systems. On the other hand, for X-ray binary systems containing a neutron star (e.g., Cyg X-2) tau is much larger than tau_c indicating the absence of significant hydrodynamical winds.

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arXiv2000

Temperature Profiles of Accretion Disks around Rapidly Rotating Neutron Stars in General Relativity and Implications for Cygnus X-2

We calculate the temperature profiles of (thin) accretion disks around rapidly rotating neutron stars (with low surface magnetic fields), taking into account the full effects of general relativity. We then consider a model for the spectrum of the X-ray emission from the disk, parameterized by the mass accretion rate, the color temperature and the rotation rate of the neutron star. We derive constraints on these parameters for the X-ray source Cygnus X-2 using the estimates of the maximum temperature in the disk along with the disk and boundary layer luminosities, using the spectrum inferred from the EXOSAT data. Our calculations suggest that the neutron star in Cygnus X-2 rotates close to the centrifugal mass-shed limit. Possible constraints on the neutron star equation of state are also discussed.

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arXiv2000

General relativistic spectra of accretion disks around rotating neutron stars

General relativistic spectra from accretion disks around rotating neutron stars in the appropriate space-time geometry for several different equation of state, spin rates and mass of the compact object have been computed. The analysis involves the computation of the relativistically corrected radial temperature profiles and the effect of Doppler and gravitational red-shifts on the spectra. Light bending effects have been omitted for simplicity. The relativistic spectrum is compared with the Newtonian one and it is shown that the difference between the two is primarily due to the different radial temperature profile for the relativistic and Newtonian disk solutions. To facilitate direct comparison with observations, a simple empirical function has been presented which describes the numerically computed relativistic spectra well. This empirical function (which has three parameters including normalization) also describes the Newtonian spectrum adequately. Thus the function can in principle be used to distinguish between the two. In particular, the best fit value of one of the parameters ($β$-parameter) $\approx 0.4$ for the Newtonian case, while it ranges from 0.1 to 0.35 for relativistic case depending upon the inclination angle, EOS, spin rate and mass of the neutron star. Constraining this parameter by fits to future observational data of X-ray binaries will indicate the effect of strong gravity in the observed spectrum.

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arXiv1999

Comparisons of various model fits to the Iron line profile in MCG-6-30-15

The broad Iron line in MCG-6-30-15 is fitted to the Comptonization model where line broadening occurs due to Compton down-scattering in a highly ionized optically thick cloud. These results are compared to the disk line model where the broadening is due to Gravitational/Doppler effects in the vicinity of a black hole. We find that both models fit the data well and it is not possible to differentiate between them by fitting only the ASCA data. The best fit temperature and optical depth of the cloud are found to be kT = 0.54 keV and $τ= 4.0$ from the Comptonization model. This model further suggests that while the temperature can be assumed to be constant, the optical depth varies during the observation period. We emphasis an earlier conclusion that simultaneous broad band data ($3 - 50$ keV) can rule out (or confirm) the Comptonization model.

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