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A. Nandi

Publications and source records attributed to A. Nandi.

At least 37 records · Page 2Linked to original sources
arXiv2009

Quasi Periodic Oscillations due to Axisymmetric and Non-Axisymmetric Shock Oscillations in Black Hole Accretion

Quasi-Periodic Oscillations (QPOs) are very puzzling since they remain totally unexplained by popular earlier models of accretion disks. The significant rms value in power density spectrum implies that the oscillation involves in the dynamical and non-linear variation of certain region of the accretion disk itself. The nature of the energy dependence implies that the region which produces Comptonized hard tail is also responsible for QPOs. Similarly, the occurrences of the QPOs are strongly related to the jet formation and the spectral states. These features are the natural consequences of the advective disk paradigm that we are advocating. In the mid 90s, some of the present authors first pointed out that the QPOs in all possible types of black holes may be simply due to the oscillations of the CENBOL, the CENtrifugal pressure supported BOundary Layer which is formed in the sub-Keplerian flows around a black hole. This CENBOL could be axi-symmetric as well as non-axisymteric in nature since its boundary, namely, the centrifugally driven shocks could be axi-symmetric or non-axisymmetric. In addition, we pointed out that the transition radius where the flow becomes Keplerian to sub-Keplerian, as well as the location of the inner sonic point can also oscillate and produce the QPOs. Since the shock locations are functions of the specific angular momentum ($λ$) and specific energy (${\cal E}$) of the flow, our model naturally predicts that the QPO frequency should vary with mass, spin, $λ$ and ${\cal E}$. The QPO frequencies with specific ratios, such as, 2:3 must be due to non-axisymmetric effects when the shock switches between the two-armed and the three-armed spirals. We also discuss the possible effects that the disk inclination might have with the line of sight.

astro-ph.HE↗
arXiv2009

QPO Evolution in 2005 Outburst of the Galactic Nano Quasar GRO J1655-40

GRO J1655-40 showed significant X-ray activity in the last week of February, 2005 and remained active for the next 260 days. The rising and the decline phases of this particular outburst show evidence for systematic movements of the Comptonizing region, assumed to be a CENBOL, which causes the Quasi-periodic Oscillations or QPOs. We present both the spectral and the timing results of the RXTE/PCA data taken from these two hard spectral states. Assuming that the QPOs originate from an oscillating shock CENBOL, we show how the shock slowly moves in through the accretion flow during the rising phase at a constant velocity and accelerate away outward during the later part of the decline phase. By fitting the observed frequencies with our solution, we extract time variation of various disk parameters such as the shock locations, velocity etc.

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arXiv2009

Fresnel zone plates for Achromatic Imaging Survey of X-ray sources

A telescope with Fresnel Zone Plates has been contemplated to be an excellent imaging mask in X-rays and gamma-rays for quite some time. With a proper choice of zone plate material, spacing and an appropriate readout system it is possible to achieve any theoretical angular resolution. We provide the results of numerical simulations of how a large number of X-ray sources could be imaged at a high resolution. We believe that such an imager would be an excellent tool for a future survey mission for X-ray and gamma-ray sources which we propose.

astro-ph.IM↗
arXiv2009

CSPOB-Continuous Spectrophotometry of Black Holes

The goal of a small and dedicated satellite called the "Continuous Spectro-Photometry of Black Holes" or CSPOB is to provide the essential tool for the theoretical understanding of the hydrodynamic and magneto-hydrodynamic flows around black holes. In its life time of about three to four years, only a half a dozen black holes will be observed continuously with a pair of CSPOBs. Changes in the spectral and temporal variability properties of the high-energy emission would be caught as they happen. Several important questions are expected to be answered and many puzzles would be sorted out with this mission.

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arXiv2009

Spectral and timing evolution of GRO J1655-40 during its outburst of 2005

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, hard, soft, very soft and 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 and very soft states, the spectral states are mostly dominated by the strong disk black body component.

astro-ph.GA↗
arXiv2005

Propagating Oscillatory Shock Model for QPOS in GRO J1655-40 During the March 2005 Outburst

GRO 1655-40, a well known black hole candidate, showed renewed X-ray activity in March 2005 after being dormant for almost eight years. It showed very prominent quasi-periodic oscillations. We analysed the data of two observations in this {\it Rapid Communication}, one taken on March 2nd, 2005 and the other taken on the March 11th, 2005. On March 2nd, 2005 the shock was weak and the QPO was seen in roughly all energies. On March 11th, 2005 the power density spectra showed that quasi-periodic oscillations (QPOs) were exhibited in harder X-rays. On the first day, the QPO was seen at 0.13Hz and on the second day, the QPO was seen at $\sim 6.5$Hz with a spectral break at $\sim 0.1$Hz. 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 0.088Hz to 15.01Hz. This agrees well if the oscillating shock is assumed to propagate with a constant velocity. On several days we also noticed the presence of very high frequency QPOs and for the first time we detected QPOs in the 600-700Hz range, the highest frequency range so far reported for any black hole candidate.

astro-ph↗
arXiv2005

Results of Recent Multi-wavelength Campaign of SS433

We conducted multi wavelength campaign on SS433 in Sept. 2002 using X-ray, B, Infra-Red and radio telescopes. We observed variabilities on a time-scale of few minutes in all the wavelengths. We interpret them to be due to bullet-like features from the accretion disk. We also present X-ray properties as obtained by RXTE.

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arXiv2005

Evidence of Class Transitions in GRS 1915+105 from IXAE Data

GRS 1915+105 shows at least twelve distinct classes of light curves. By analysing the data obtained from Indian X-ray Astronomy Experiment (IXAE) instrument aboard IRS-P3 satellite, we show that in at least two days, transitions between one class to another were observed. In these days the so-called $κ$ class went to $ρ$ class and $χ$ class went to $ρ$ class. In the frequency-time plane such transitions exhibited change in quasi-periodic oscillation (QPO) frequency. We could detect that low-frequency QPOs can occur in anticipation of a class transition several hundred minutes before the actual transition

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arXiv2005

X-ray Observation of SS 433 with RXTE

Apart from regular monitoring by ASM, the compact object SS 433 was observed with RXTE several times last two/three years. We present the first analysis of these observations. We also include the results of the recent exciting TOO campaign made during donour inferior (orbital phase $ϕ=0$) and superior ($ϕ=0.5$) conjunctions which took place on Oct. 2nd, 2003, and on March 13th, 2004 respectively, when the jet itself was directly pointing towards us (i.e., precessional phase $ψ\sim 0$). Generally, we found that two distinct lines fit the spectra taken on all these days. We present some of the light-curves and the X-ray spectra, and show that the Doppler shifts of the emitted lines roughly match those predicted by the kinematic model for the jets. We find that the line with a higher energy can be best identified with a FeXXVI Ly-$α$ transition while the line with lower energy can be identified with a FeXXV (1s2p - 1s$^2$) transition. We observe that the X-ray flux on March 13th, 2004 (when the base of the jet is exposed) is more than twice compared to that on Oct. 2nd, 2003 (when the base is covered by the companion). We find the flux to continue to remain high at least till another orbital period. We believe that this is because SS 433 was undergoing a weak flaring activity during the recent observation.

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arXiv2005

SS 433: Results of a Recent Multi-wavelength Campaign

We conducted a multi-wavelength campaign in September-October, 2002, to observe SS 433. We used 45 meter sized 30 dishes of Giant Meter Radio Telescope (GMRT) for radio observation, 1.2 meter Physical Research Laboratory Infra-red telescope at Mt Abu for IR, 1 meter Telescope at the State Observatory, Nainital for Optical photometry, 2.3 meter optical telescope at the Vainu Bappu observatory for spectrum and Rossi X-ray Timing Explorer (RXTE) Target of Opportunity (TOO) observation for X-ray observations. We find sharp variations in intensity in time-scales of a few minutes in X-rays, IR and radio wavelengths. Differential photometry at the IR observation clearly indicated significant intrinsic variations in short time scales of minutes throughout the campaign. Combining results of these wavelengths, we find a signature of delay of about two days between IR and Radio. The X-ray spectrum yielded double Fe line profiles which corresponded to red and blue components of the relativistic jet. We also present the broadband spectrum averaged over the campaign duration.

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arXiv2005

Spectral Signatures of Winds from Accretion Disks Around Black Holes

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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arXiv2004

Computation of outflow rates from accretion disks around black holes

We self-consistently estimate the outflow rate from the accretion rates of an accretion disk around a black hole in which both the Keplerian and the sub-Keplerian matter flows simultaneously. While Keplerian matter supplies soft-photons, hot sub-Keplerian matter supplies thermal electrons. The temperature of the hot electrons is decided by the degree of inverse Comptonization of the soft photons. If we consider only thermally-driven flows from the centrifugal pressure-supported boundary layer around a black hole, we find that when the thermal electrons are cooled down, either because of the absence of the boundary layer (low compression ratio), or when the surface of the boundary layer is formed very far away, the outflow rate is negligible. For an intermediate size of this boundary layer the outflow rate is maximal. Since the temperature of the thermal electrons also decides the spectral state of a black hole, we predict that the outflow rate should be directly related to the spectral state.

astro-ph↗
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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arXiv2004

Possible Photometric Evidence of Ejection of Bullet Like Features in the Relativistic Jet source SS433

SS433 is well-known for its precessing twin jets having optical bullets inferred through {\it spectroscopic} observation of $H_α$ lines. Recently, Chakrabarti et al. (2002) described processes which may be operating in accretion disk of SS433 to produce these bullets. In a recent multi-wavelength campaign, we find sharp rise in intensity in time-scales of few minutes in X-rays, IR and radio waves through {\it photometric} studies. We interpret them to be possible evidence of ejection of bullet-like features from accretion disks.

astro-ph↗
arXiv2002

On the Ejection Mechanism of Bullets in SS 433

We discuss plausible mechanisms to produce bullet-like ejecta from the precessing disk in the SS 433 system. We show that non-steady shocks in the sub-Keplerian accretion flow can provide the basic timescale of the ejection interval while the magnetic rubber-band effect of the toroidal flux tubes in this disk can yield flaring events.

astro-ph↗
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.

astro-ph↗
arXiv2000

Understanding Galactic Black Hole Candidate GRS 1915+105

We interpret observed features of the black hole candidate GRS1915+105 in the light of the advective flow paradigm. The light curve of this black hole is seen to be of four classes, Soft, Hard, Semi-Soft and Intermediate. Each class can display more than one type of light curve making altogether twelve or more types of light curves. In each type, the mode of accretion may be one or more of three types: which we identify as HW (hard state with winds), CC (Compton cooling) and EA (enhanced accretion) respectively. Apart from these three states there are two more fundamental states in a black hole accretion which are soft and hard states. We also find that winds and outflows could soften spectra in hard states and the enhanced accretion could harden the soft states. These are observed in the spectra of GRS 1915+105. Generally, we find that advective flow paradigm can satisfactorily explain all the features of the enigmatic black hole GRS 1915+105.

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