2015MNRAS.447.1984D (Published at MNRAS)
Title: Characterization of GX 339-4 outburst of 2010-11: analysis by XSPEC using two component advective flow model
Author: Dipak Debnath, Santanu Mondal, Sandip K. Chakrabarti
Key
findings: We study spectral properties of GX 339-4 during its 2010-11 outburst
with two component advective flow (TCAF) model after its inclusion in
XSPEC as a table model. We compare results fitted by TCAF model with
combined disc blackbody and power-law model. For a spectral fit, we use
2.5-25 keV spectral data of the Proportional Counter Array instrument
onboard RXTE satellite. From our fit, accretion flow parameters such as
Keplerian (disc) rate, sub-Keplerian (halo) rate, location and strength
of shock are extracted. We quantify how the disc and the halo rates vary
during the entire outburst. We study how the halo to disc accretion rate
ratio (ARR), quasi-periodic oscillations (QPOs), shock locations and its
strength vary when the system passes through hard, hard-intermediate,
soft-intermediate and soft states. We find pieces of evidence of
monotonically increasing and decreasing nature of QPO frequencies
depending on the variation of ARR during rising and declining phases.
Interestingly, on days of transition from hard state to
hard-intermediate spectral state (during the rising phase) or vice-versa
(during decline phase), ARR is observed to be locally maximum.
Non-constancy of ARR while obtaining reasonable fits points to the
presence of two independent components in the flow.
=========================================================2015ApJ...798...57M (Published at Astrophysical Journal)
Title: Is Compton Cooling Sufficient to Explain Evolution of Observed Quasi-periodic Oscillations in Outburst Sources?
Authors: Santanu Mondal; Sandip K. Chakrabarti, Dipak Debnath
Key Findings: In outburst sources, quasi-periodic oscillation (QPO) frequency is known
to evolve in a certain way: in the rising phase, it monotonically goes
up till a soft intermediate state is achieved. In the propagating
oscillatory shock model, oscillation of the Compton cloud is thought to
cause QPOs. Thus, in order to increase QPO frequency, Compton cloud must
collapse steadily in the rising phase. In decline phases, exactly
opposite should be true. We investigate cause of this evolution of the
Compton cloud. The same viscosity parameter which increases the
Keplerian disk rate, also moves the inner edge of the Keplerian
component, thereby reducing the size of the Compton cloud and reducing
the cooling time scale. We show that cooling of the Compton cloud by
inverse Comptonization is enough for it to collapse sufficiently so as
to explain the QPO evolution. In the Two Component Advective Flow (TCAF)
configuration of Chakrabarti-Titarchuk, centrifugal force induced shock
represents boundary of the Compton cloud. We take the rising phase of
2010 outburst of Galactic black hole candidate H~1743-322 and find an
estimation of variation of $\alpha$ parameter of the sub-Keplerian flow
to be monotonically rising from $0.0001$ to $0.02$, well within the
range suggested by magneto-rotational instability. We also estimate the
inward velocity of the Compton cloud to be a few meters/second which is
comparable to what is found in several earlier studies of our group by
empirically fitting the shock locations with the time of observations.
=========================================================
2014MNRAS.440L.121D (Published at MNRAS LETTERS)
Title: Implementation of two-component advective flow solution in XSPEC
Authors: Dipak Debnath, Sandip K. Chakrabarti, Santanu Mondal
Key Findings: Spectral and temporal properties of black hole candidates can be
explained reasonably well using Chakrabarti-Titarchuk solution of
two-component advective flow (TCAF). This model requires two accretion
rates, namely the Keplerian disc accretion rate and the halo accretion
rate, the latter being composed of a sub-Keplerian, low-angular-momentum
flow which may or may not develop a shock. In this solution, the
relevant parameter is the relative importance of the halo (which creates
the Compton cloud region) rate with respect to the Keplerian disc rate
(soft photon source). Though this model has been used earlier to
manually fit data of several black hole candidates quite satisfactorily,
for the first time, we made it user friendly by implementing it into
XSPEC software of Goddard Space Flight Center (GSFC)/NASA. This enables
any user to extract physical parameters of the accretion flows, such as
two accretion rates, the shock location, the shock strength, etc., for
any black hole candidate. We provide some examples of fitting a few
cases using this model. Most importantly, unlike any other model, we
show that TCAF is capable of predicting timing properties from the
spectral fits, since in TCAF, a shock is responsible for deciding
spectral slopes as well as quasi-periodic oscillation frequencies.
=========================================================
2014ApJ...786....4M (Published at Astrophysical Journal)
Title: Inference on Accretion Flow Dynamics Using TCAF Solution from the
Analysis of Spectral Evolution of H 1743-322 during the 2010 Outburst
Authors: Santanu Mondal, Dipak Debnath, Sandip K. Chakrabarti
Key Findings: We study accretion flow dynamics of the Galactic transient black hole
candidate (BHC) H 1743-322 during its 2010 outburst by analyzing
spectral data using the two-component advective flow (TCAF; Keplerian
and sub-Keplerian) solution after its inclusion in XSPEC as a local
model. We compare our TCAF solution fitted results with combined disk
blackbody (DBB) and power-law (PL) model fitted results and find a
similar smooth variation of thermal (Keplerian or DBB) and non-thermal
(PL or sub-Keplerian) fluxes/rates in two types of model fits. For a
spectral analysis, 2.5-25 keV spectral data from the Rossi X-Ray Timing
Explorer Proportional Counter Array instrument are used. From the TCAF
solution fit, accretion flow parameters, such as Keplerian rate,
sub-Keplerian rate, location of centrifugal pressure-supported shock,
and strength of the shock, are extracted, providing a deeper
understanding of the accretion process and properties of accretion disks
around BHC H 1743-322 during its X-ray outburst. Based on the halo to
disk accretion rate ratio, shock properties, accretion rates, and the
nature of the quasi-periodic oscillations' (if observed) entire outburst
is classified into four different spectral states: hard,
hard-intermediate, soft-intermediate, and soft. From the time variation
of intrinsic flow parameters, it appears that their evolutions in the
declining phase do not retrace the path of the rising phase. Since our
current model does not include magnetic fields, spectral turnover at
energies beyond 500-600 keV cannot be explained.
=========================================================
2013AdSpR..52.2143D (Published at Advances in Space Research)
Title: Evolution of the temporal and the spectral properties in 2010 and 2011 outbursts of H 1743-322
Authors: Dipak Debnath, Sandip K. Chakrabarti, Anuj Nandi
Key Findings: The Galactic black hole candidate H 1743-322 exhibited two X-ray outbursts in rapid succession: one in August 2010 and the other in April 2011. We analyze archival data of this object from the PCA instrument on board RXTE (2–25 keV energy band) to study the evolution of its temporal and spectral characteristics during both the outbursts, and hence to understand the behavioral change of the accretion flow dynamics associated with the evolution of the various X-ray features. We study the evolution of QPO frequencies during the rising and the declining phases of both the outbursts. We successfully fit the variation of QPO frequency using the Propagating Oscillatory Shock (POS) model in each of the outbursts and obtain the accretion flow parameters such as the instantaneous shock locations, the shock velocity and the shock strength. Based on the degree of importance of the thermal (disk black body) and the non-thermal (power-law) components of the spectral fit and properties of the QPO (if present), the entire profiles of the 2010 and 2011 outbursts are subdivided into four different spectral states: hard, hard-intermediate, soft-intermediate and soft. We attempt to explain the nature of the outburst profile (i.e., hardness-intensity diagram) with two different types of mass accretion flow.
=========================================================
2013arXiv1309.3602D (Thirteenth Marcel Grossmann (MG13) Conference Proceeding)
Title: Properties of the propagating oscillatory shock wave in the accretion flows around few transient black hole candidates during their outbursts
Authors: Dipak Debnath, Sandip K. Chakrabarti
Key findings: In our study of the timing properties of few Galactic black hole candidates evolutions of the low and intermediate frequency quasi-periodic oscillations (LIFQPOs) are observed. In 2005, for explaining evolution of QPO frequency during rising phase of 2005 GRO J1655-40 outburst, Chakrabarti and his students introduced a new model, namely propagating oscillatory shock (POS) model. Here we present the results obtained from the same POS model fitted QPO evolutions during both the rising and declining phases of the outbursts of 2005 GRO J165540, 2010-11 GX 339-4, and 2010 & 2011 H 1743-322.
=========================================================
2013arXiv1309.3600D (Thirteenth Marcel Grossmann (MG13) Conference Proceeding)
Title: A comparative study of the timing and the spectral properties during two recent outbursts (2010 & 2011) of H 1743-322
Author: Dipak Debnath, Sandip K. Chakrabarti, Anuj Nandi
Key findings: The Galactic black hole candidate (BHC) H~1743-322 recently exhibited two outbursts in X-rays in August 2010 & April 2011. The nature (outburst profile, evolution of quasi-periodic oscillation (QPO) frequency and spectral states, etc.) of these two successive outbursts, which continued for around two months each, are very similar. We present the results obtained from a comparative study on the temporal and the spectral properties of the source during these two outbursts. The evolutions of QPOs observed in both the outbursts were well fitted with propagating oscillatory shock (POS) model. During both the outbursts, the observed spectral states (i.e, hard, hard-intermediate, soft-intermediate and soft) follow the `standard' type of hysteresis-loop, which could be explained with two component advective flow (TCAF) model.
=========================================================
2013AdSpR..52..740P (15 August 2013) (http://www.sciencedirect.com/science/article/pii/S0273117713002287)
Title: Comptonization efficiencies of the variability classes of GRS 1915 + 105
Author: Partha Sarathi Pal, Sandip K. Chakrabarti, Anuj Nandi
Key findings: The Galactic microquasar GRS 1915+105 exhibits at least seventeen types of variability classes. Intra and inter class transitions are reported to be observed within seconds to hours. Since the observation was not continuous, these classes appeared to be exhibited in a random order. Our goal is to predict a sequence of these classes. In this paper, we compute the ratio of the photon counts obtained from the power-law component and the blackbody component of each class and call this ratio as the ‘Comptonizing efficiency’ (CE) of that class. We sequence the classes in the ascending order of CE and find that this sequence matches with a few class transitions observed by RXTE satellite and IXAE instruments on board IRS-P3. A change in CE corresponds to a change in the optical depth of the Compton cloud. Our result implies that the optical depth of the Compton cloud gradually rises as the variability class becomes harder.
=========================================================
2013ASInC...8...85D (Recent Trends in the Study of Compact Objects: Theory and Observation
ASI Conference Series, 2013, Vol. 8, pp 85–88)
(http://bulletin.astron-soc.in/asics_vol008/085_dipak.pdf)
Title: Extracting flow parameters of H 1743-322 during early phase of its 2010 outburst using two component advective flow model
Author: Dipak Debnath, Sandip K. Chakrabarti, Santanu Mondal
Key findings: We study the spectral properties of Galactic transient black hole candidate H 1743-322 during its early phase of 2010 outburst with Two Component Advective Flow (TCAF) model, after its inclusion in spectral analysis software package XSPEC as a local model. For the analysis, spectral data from RXTE/PCA instrument in 2.5-25 keV energy band are used. From the spectral fit, accretion flow parameters such as Keplerian (disk) rate, sub-Keplerian (halo) rate, location of the shock and strength of the shock are directly extracted. QPO frequencies are predicted from the TCAF model spectral fitted shock parameters, ‘closely’ matches with the observed frequencies.
=========================================================
2013ASInC...8...55P (Recent Trends in the Study of Compact Objects: Theory and Observation
ASI Conference Series, 2013)
Author: P. S. Pal, S. K. Chakrabarti
Key findings: The Temporal and Spectral variations of black hole candidates during outbursts have been reported in several publications. It is well known that during an outburst, the source becomes soft in the first few days, and then returns to the hard state after a few weeks or months. In the present paper, we show the variation of Comptonization Efficiency (CE), obtained from the ratio of the black body photon number to the power-law photon number, as a function of time in several outbursts. Since the power-law photons are generated through inverse-Comptonization of the intercepted soft photons, the CE is a measure of the geometry of the Compton cloud. Our investigation indicated that all the outbursts starts with a large CE and becomes very small after a few days, when the Compton cloud becomes very small to intercept any significant number of soft photons. CE returns back to a larger value at the end of the outburst.
=========================================================
(http://www.aanda.org/articles/aa/pdf/2012/07/aa17436-11.pdf)
Title: A double component in GRB 090618: a proto-black hole and a genuinely long gamma-ray burst
Author: Izzo, L.; Ruffini, R.; Penacchioni, A. V.; Bianco, C. L.; Caito, L.; Chakrabarti, S. K.; Rueda, J. A.; Nandi, A.; Patricelli, B.
Key findings: 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. We find evidence of the existence of two different episodes in GRB 090618. 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.
=========================================================
(http://www.aanda.org/articles/aa/pdf/2012/06/aa17844-11.pdf)
Title: Accretion flow dynamics during the evolution of timing and spectral properties of GX 339-4 during its 2010-11 outburst
Author: Nandi, A.; Debnath, D.; Mandal, S.; Chakrabarti, S. K.
Key findings: The Galactic transient black hole candidate (BHC) GX 339-4 exhibited several outbursts at regular intervals of about two to three years in the Rossi X-ray Timing Explorer (RXTE) era. After remaining in an almost quiescent state for three long years, it again became X-ray active in January, 2010, continuing to be so over the next ~14 months. We found that the QPO frequency variation could be explained by the propagating oscillatory shock model (POS) and the hardness versus intensity variation can be reproduced if we assume that higher viscosity causes the conversion of a low angular momentum disk component into a Keplerian component during the outburst phase. The decline phase starts because of the reduction in the viscosity.
=========================================================
(http://www.worldscientific.com/doi/abs/10.1142/S0218271811020329)
Title: Evidence of Variation of the Accretion Flow Geometry in GRS 1915 + 105 from Ixae and RXTE Data
Author: Pal, Partha Sarathi; Chakrabarti, Sandip K., Nandi, Anuj
Key findings: The Galactic microquasar GRS 1915 + 105 exhibits various types of light curves. There is, however, no understanding of when a certain type of light curve will be exhibited and only in a handful of cases, the transitions from one type to another have actually been observed. We study the detailed spectral properties in these cases to show that different classes have different ratio of the power-law photon and the blackbody photon. Since the power-law photons are from the Compton cloud, and the intensity of the power-law photon component depends on the degree of interception of the soft photons by the Compton cloud, we conclude that not only the accretion rate, but the accretion flow geometry must also change during a class transition.
=========================================================
(http://www.aanda.org/articles/aa/pdf/2010/12/aa14990-10.pdf)
Title: Properties of the propagating shock wave in the accretion flow around GX 339-4 in the 2010 outburst
Author: Debnath, D.; Chakrabarti, S. K., Nandi, A.
Key findings: The black hole candidate GX 339-4 exhibited an X-ray outburst in January 2010, which is still continuing. We here discuss the timing and the spectral properties of the outburst using RXTE data. This source shows the tendency of a rapidly increasing QPO frequency (ν_QPO) on a viscous time scale, which can be modeled quite accurately. In this case, the shock seems to have disappeared at about ~172 Schwarzschild radii, unlike in the 2005 outburst of GRO J1655-40, where the shock disappeared behind the horizon.
=========================================================
(http://mnras.oxfordjournals.org/content/404/4/2136.full.pdf+html)
Title: Evidence for two-component flows around the black hole candidate XTEJ1550-564 from spectral features during its 1998-1999 outburst
Author: Dutta, Broja G., Chakrabarti, Sandip K.
Key findings: We study the spectral properties of the accretion disc in the Galactic black hole candidate XTEJ1550-564 during the 1998-1999 outburst when the source exhibited double-peaked eruptions. This outburst lasted for 250d and the 2.5-25.0keV spectral state varied smoothly from one to another several times. We show that the spectral features of the 1998-1999 outburst could be clearly understood by a two-component (Keplerian and sub-Keplerian) advective flow (TCAF). We concentrate on the spectral data from a Proportional Counter Array instrument on the RXTE satellite for the black hole XTEJ1550-564 and fit them quite satisfactorily using TCAF model. From the spectral fit we calculate the disc parameters, such as the Keplerian rate, the sub-Keplerian rate (halo rate), the shock location and the inner edge of the Keplerian disc. This observation points to the presence of two independent components in the accretion flow and that the accretion rate at all radii need not be constant in an evolving disc.
=========================================================
2013ASInC...8...85D (Recent Trends in the Study of Compact Objects: Theory and Observation
ASI Conference Series, 2013, Vol. 8, pp 85–88)
(http://bulletin.astron-soc.in/asics_vol008/085_dipak.pdf)
Title: Extracting flow parameters of H 1743-322 during early phase of its 2010 outburst using two component advective flow model
Author: Dipak Debnath, Sandip K. Chakrabarti, Santanu Mondal
Key findings: We study the spectral properties of Galactic transient black hole candidate H 1743-322 during its early phase of 2010 outburst with Two Component Advective Flow (TCAF) model, after its inclusion in spectral analysis software package XSPEC as a local model. For the analysis, spectral data from RXTE/PCA instrument in 2.5-25 keV energy band are used. From the spectral fit, accretion flow parameters such as Keplerian (disk) rate, sub-Keplerian (halo) rate, location of the shock and strength of the shock are directly extracted. QPO frequencies are predicted from the
TCAF model spectral fitted shock parameters, ‘closely’ matches with the observed frequencies.
=========================================================
2013ASInC...8...55P (Recent Trends in the Study of Compact Objects: Theory and Observation
2013ASInC...8...55P (Recent Trends in the Study of Compact Objects: Theory and Observation
ASI Conference Series, 2013)
Author: P. S. Pal, S. K. Chakrabarti
Key findings: The Temporal and Spectral variations of black hole candidates during outbursts have been reported in several publications. It is well known that during an outburst, the source becomes soft in the first few days, and then returns to the hard state after a few weeks or months. In the present paper, we show the variation of Comptonization Efficiency (CE), obtained from the ratio of the black body photon number to the power-law photon number, as a function of time in several outbursts. Since the power-law photons are generated through inverse-Comptonization of the intercepted soft photons, the CE is a measure of the geometry of the Compton cloud. Our investigation indicated that all the outbursts starts with a large CE and becomes very small after a few days, when the Compton cloud becomes very small to intercept any significant number of soft photons. CE returns back to a larger value at the end of the outburst.
=========================================================
(http://www.aanda.org/articles/aa/pdf/2012/07/aa17436-11.pdf)
Title: A double component in GRB 090618: a proto-black hole and a genuinely long gamma-ray burst
Author: Izzo, L.; Ruffini, R.; Penacchioni, A. V.; Bianco, C. L.; Caito, L.; Chakrabarti, S. K.; Rueda, J. A.; Nandi, A.; Patricelli, B.
Key findings: 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. We find evidence of the existence of two different episodes in GRB 090618. 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.
=========================================================
(http://www.aanda.org/articles/aa/pdf/2012/06/aa17844-11.pdf)
Title: Accretion flow dynamics during the evolution of timing and spectral properties of GX 339-4 during its 2010-11 outburst
Author: Nandi, A.; Debnath, D.; Mandal, S.; Chakrabarti, S. K.
Key findings: The Galactic transient black hole candidate (BHC) GX 339-4 exhibited several outbursts at regular intervals of about two to three years in the Rossi X-ray Timing Explorer (RXTE) era. After remaining in an almost quiescent state for three long years, it again became X-ray active in January, 2010, continuing to be so over the next ~14 months. We found that the QPO frequency variation could be explained by the propagating oscillatory shock model (POS) and the hardness versus intensity variation can be reproduced if we assume that higher viscosity causes the conversion of a low angular momentum disk component into a Keplerian component during the outburst phase. The decline phase starts because of the reduction in the viscosity.
=========================================================
(http://www.worldscientific.com/doi/abs/10.1142/S0218271811020329)
Title: Evidence of Variation of the Accretion Flow Geometry in GRS 1915 + 105 from Ixae and RXTE Data
Author: Pal, Partha Sarathi; Chakrabarti, Sandip K., Nandi, Anuj
Key findings: The Galactic microquasar GRS 1915 + 105 exhibits various types of light curves. There is, however, no understanding of when a certain type of light curve will be exhibited and only in a handful of cases, the transitions from one type to another have actually been observed. We study the detailed spectral properties in these cases to show that different classes have different ratio of the power-law photon and the blackbody photon. Since the power-law photons are from the Compton cloud, and the intensity of the power-law photon component depends on the degree of interception of the soft photons by the Compton cloud, we conclude that not only the accretion rate, but the accretion flow geometry must also change during a class transition.
=========================================================
(http://www.aanda.org/articles/aa/pdf/2010/12/aa14990-10.pdf)
Title: Properties of the propagating shock wave in the accretion flow around GX 339-4 in the 2010 outburst
Author: Debnath, D.; Chakrabarti, S. K., Nandi, A.
Key findings: The black hole candidate GX 339-4 exhibited an X-ray outburst in January 2010, which is still continuing. We here discuss the timing and the spectral properties of the outburst using RXTE data. This source shows the tendency of a rapidly increasing QPO frequency (ν_QPO) on a viscous time scale, which can be modeled quite accurately. In this case, the shock seems to have disappeared at about ~172 Schwarzschild radii, unlike in the 2005 outburst of GRO J1655-40, where the shock disappeared behind the horizon.
=========================================================
(http://mnras.oxfordjournals.org/content/404/4/2136.full.pdf+html)
Title: Evidence for two-component flows around the black hole candidate XTEJ1550-564 from spectral features during its 1998-1999 outburst
Author: Dutta, Broja G., Chakrabarti, Sandip K.
Key findings: We study the spectral properties of the accretion disc in the Galactic black hole candidate XTEJ1550-564 during the 1998-1999 outburst when the source exhibited double-peaked eruptions. This outburst lasted for 250d and the 2.5-25.0keV spectral state varied smoothly from one to another several times. We show that the spectral features of the 1998-1999 outburst could be clearly understood by a two-component (Keplerian and sub-Keplerian) advective flow (TCAF). We concentrate on the spectral data from a Proportional Counter Array instrument on the RXTE satellite for the black hole XTEJ1550-564 and fit them quite satisfactorily using TCAF model. From the spectral fit we calculate the disc parameters, such as the Keplerian rate, the sub-Keplerian rate (halo rate), the shock location and the inner edge of the Keplerian disc. This observation points to the presence of two independent components in the accretion flow and that the accretion rate at all radii need not be constant in an evolving disc
=========================================================
(http://mnras.oxfordjournals.org/content/394/3/1463.full.pdf+html)
Title: Accretion flow behaviour during the evolution of the quasi-periodic oscillation frequency of XTE J1550-564 in 1998 outburst
Author: Chakrabarti, Sandip K.; Dutta, Broja G.; Pal, P. S.
Key findings: Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550-564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81mHz to 13.1Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655-40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called `outburst' may not be a full-fledged outburst.
=========================================================
(http://www.astron-soc.in/bulletin/08December/361512008.PDF)
Title: Spectral and timing evolution of GRO J1655-40 during its outburst of 2005
Author: Debnath, D.; Chakrabarti, Sandip K.; Nandi, A.; Mandal, S.
Key findings: In a recent outburst which lasted for 260 days, the black hole candidate GRO J1655-40 exhibited a behaviour similar to its last outburst observed almost eight years ago. We analyze a total of 150 observational spells in 122 days of data spreaded over the entire outburst phase of Feb. 2005 to Oct. 2005. From our study, a comprehensive understanding of the detailed behaviour of this black hole candidate has emerged. Based on the degree of importance of the black body and the power-law components we divide the entire episode in four spectral states, namely, it hard, soft, very soft and it intermediate. Quasi-Periodic oscillations (QPOs) were found in two out of these four states, namely, in the hard and the intermediate states. In the hard state, at the rising phase of the outburst, QPO frequency ranged from 0.034 - 17.78Hz and the spectra was fitted by a disk black body, power-law and iron emission line at 6.2 - 6.5 keV. In the intermediate state, QPOs vary from 13.17Hz to 19.04Hz and the QPO frequency modulation in this state was not significant. The spectra in this state are well fitted by the disk black body and the power-law components. In the hard state of the declining phase of the outburst, we found QPOs of decreasing frequency from 13.14 Hz to 0.034 Hz. The spectra of this state were fitted by a disk black body and power-law components, but in the initial few days a cooler Comptonized component was required for a better fit. In the soft/very soft states, the spectral states are mostly dominated by the strong disk black body component.
=========================================================
(http://www.aanda.org/index.php?option=com_article&access=bibcode&Itemid=129&bibcode=2008A%2526A...489L..41CFUL)
Title: Evolution of the quasi-periodic oscillation frequency in GRO J1655-40 - Implications for accretion disk dynamics
Author: Chakrabarti, S. K.; Debnath, D.; Nandi, A.; Pal, P. S.
Key findings: Context: Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot blobs embedded in Keplerian disks. Any movement of these perturbations is expected systematically to change the QPO frequency.
Aims: Our goal is to find systems where such a systematic drifts have been observed. We also try to find the real cause of such drifts and whether they shed some light on the accretion disk dynamics.
Methods: Using archival data of the recent outburst of GRO J1655-40, we report the presence of such systematic drifts not only during the rising phase from the 25th of February 2005 to the 12th March 2005, when the QPO frequency monotonically increased from 82 mHz to 17.78 Hz but also in the decline phase from the 15th September 2005 to the 5th of October 2005, when the QPO frequency decreased from 13.14 Hz to 34 mHz.
Results: We fitted the frequency drifts with the propagatory oscillating shock solution. In the shock-oscillation solution, the frequency is inversely proportional to the infall time scale from the shock location. We obtained the shock location and strength through such a fit.
Conclusions: The astonishing smoothness of the variation of the QPO frequency over a period of weeks directly supports the view that it may due to the drift of an oscillating shock rather than the movements of a blob inside a differentially rotating disk.
Aims: Our goal is to find systems where such a systematic drifts have been observed. We also try to find the real cause of such drifts and whether they shed some light on the accretion disk dynamics.
Methods: Using archival data of the recent outburst of GRO J1655-40, we report the presence of such systematic drifts not only during the rising phase from the 25th of February 2005 to the 12th March 2005, when the QPO frequency monotonically increased from 82 mHz to 17.78 Hz but also in the decline phase from the 15th September 2005 to the 5th of October 2005, when the QPO frequency decreased from 13.14 Hz to 34 mHz.
Results: We fitted the frequency drifts with the propagatory oscillating shock solution. In the shock-oscillation solution, the frequency is inversely proportional to the infall time scale from the shock location. We obtained the shock location and strength through such a fit.
Conclusions: The astonishing smoothness of the variation of the QPO frequency over a period of weeks directly supports the view that it may due to the drift of an oscillating shock rather than the movements of a blob inside a differentially rotating disk.
=========================================================
(http://link.springer.com/article/10.1007%2Fs10509-007-9438-3)
Title: Spectral fit of Cygnus X-1 in high energy— a self-consistent study
Author: Mandal, Samir; Chakrabarti, Sandip K.
Key findings:
=========================================================
(http://www.worldscientific.com/doi/abs/10.1142/9789812704030_0121)
Title: Spectral Signatures of Winds from Accretion Disks Around Black Holes
Author: Chakrabarti, Sandip K.; Nandi, A.; Rao, A. R.
Key findings: We show that with the wind/jet activity, the spectral index of hard X-ray is changed in galactic microquasars. When mass loss takes place, the spectrum becomes softer and when mass gain takes place, the spectrum becomes harder. We present examples of such changes in GRS1915+105.
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(http://pos.sissa.it//archive/conferences/033/103/MQW6_103.pdf)
Title: Spectral and QPO Properties of GRO J1655-40 in the 2005 Outburst
Author: Chakrabarti, Sandip K.; Nandi, A.; Debnath, D.; Sarkar, R.; Dutta, B. G.
Key findings: GRO J1655-40 showed renewed X-ray activity in late February, 2005. We study the spectral properties and the Quasi-Periodic Oscillations (QPO) properties of this object for the first two weeks of RXTE observations after the outburst started. We show that as the spectrum becomes softer with time, the QPO frequency goes up. We analysed the QPOs for the period 25th Feb. 2005 to 12th of March, 2005 and showed that the frequency of QPO increased monotonically from 0.082Hz to 15.01Hz. This agrees well if the oscillating shock is assumed to propagate with a constant velocity of about 1.9km s-1 s.
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