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We present a detailed investigation of photometric, spectroscopic, and polarimetric observations of the Type II SN 2023ixf. Earlier studies have provided compelling evidence for a delayed shock breakout from a confined dense circumstellar matter (CSM) enveloping the progenitor star. The temporal evolution of polarization in SN~2023ixf revealed three distinct peaks in polarization evolution at 1.4 d, 6.4 d, and 79.2 d, indicating an asymmetric dense CSM, an aspherical shock front and clumpiness in the low-density extended CSM, and an aspherical inner ejecta/He-core. SN 2023ixf displayed two dominant axes, one along the CSM-outer ejecta and the other along the inner ejecta/He-core, showcasing the independent origin of asymmetry in the early and late evolution. The argument for an aspherical shock front is further strengthened by the presence of a high-velocity broad absorption feature in the blue wing of the Balmer features in addition to the P-Cygni absorption post 16 d. Hydrodynamical light curve modeling indicated a progenitor of 10 solar mass with a radius of 470 solar radii and explosion energy of 2e51 erg, along with 0.06 solar mass of 56-Ni, though these properties are not unique due to modeling degeneracies. The modeling also indicated a two-zone CSM: a confined dense CSM extending up to 5e14 cm, with a mass-loss rate of 1e-2 solar mass per year, and an extended CSM spanning from 5e14 cm to at least 1e16cm with a mass-loss rate of 1e-4 solar mass per year, both assuming a wind-velocity of 10 km/s. The early nebular phase observations display an axisymmetric line profile of [OI], red-ward attenuation of the emission of Halpha post 125 days, and flattening in the Ks-band, marking the onset of dust formation., Comment: 32 pages, 15 figures, 1 Table, Accepted in the Astrophysical Journal

Galactic transient black hole candidate (BHC) MAXI J1535-571 was discovered on 2017 September 02 simultaneously by {\it MAXI}/GSC and {\it Swift}/BAT instruments. It has also been observed by India's first multi-wavelength astronomy-mission satellite {\it AstroSat}, during the rising phase of its 2017-18 outburst. We make both the spectral and the temporal analysis of the source during 2017 September 12-17 using data of {\it AstroSat}'s Large Area X-ray Proportional Counter (LAXPC) in the energy range of $3-40$~keV to infer the accretion flow properties of the source. Spectral analysis is done with the physical two-component advective flow (TCAF) solution-based {\it fits} file. From the nature of the variation of the TCAF model fitted physical flow parameters, we conclude and confirm that the source was in the intermediate spectral state during our analysis period. We observe sharp type-C quasi-periodic oscillations (QPOs) in the frequency range of $\sim 1.75-2.81$~Hz. For a better understanding of the nature and evolution of these type-C QPOs, a dynamic study of the power density spectra is done. We also investigate the origin of these QPOs from the shock oscillation model. We find that non-satisfaction of Rankine-Hugoniot conditions for non-dissipative shocks and not their resonance oscillations is the cause of the observed type-C QPOs., Comment: 15 pages, 6 figures, 3 tables (accepted for publication in Ap&SS)

We carry out a detailed study of the spectral and the timing properties of the stellar-mass black hole candidate XTE J1752-223 during its 2009-10 outburst using RXTE PCA data in the $2.5-25$ keV energy range. Low frequency quasi-periodic oscillations (LFQPOs) are seen in the power density spectrum (PDS). The spectral analysis is done using two types of models: one is the combined disk black body plus power-law model and the other is Transonic flow solution based Two Component Advective Flow (TCAF) model. RXTE PCA was non-operational during 2009 Nov. 16 to 2010 Jan. 18 and thus we study light curve profiles and evolution of hardness ratios using MAXI GSC and Swift BAT data. Based on the evolution of the temporal and the spectral properties of the source during its 2009-10 outburst, we find that the object evolved through the following spectral states: hard, hard-intermediate and soft-intermediate/soft. From the TCAF model fitted spectral analysis, we also estimate the probable mass of the black hole to be in the range of $8.1-11.9$ $M_\odot$, and more precisely, the mass appears to be is $10\pm{1.9}~M_\odot$., Comment: 12 pages, 7 Figures, 3 Tables (Accepted for publication in MNRAS)

We present spectral and timing properties of the newly discovered X-ray transient source, MAXI J1535-571, which is believed to be a Galactic X-ray binary containing a black hole candidate (BHC) as the primary object. After its discovery on 2017 Sep. 2, it has been monitored regularly in multi-wavelength bands by several satellites. We use archival data of Swift (XRT and BAT) and MAXI (GSC) satellite instruments to study accretion flow dynamics of the source during the outburst. During its outburst, the source became very bright in the sky with a maximum observed flux of $5$~Crab in the $2-10$~keV GSC band. Similar to other transient BHCs, it also shows signatures of low frequency quasi-periodic oscillations (QPOs) during the outburst. Spectral data of different instruments are fitted with the transonic flow solution based two-component advective flow (TCAF) model fits file to find the direct accretion flow parameters. Evolution of spectral states and their transitions are understood from the model fitted physical flow parameters and nature of QPOs. We also estimate probable mass of the black hole from our spectral analysis as $7.9-9.9~M_\odot$ or $8.9\pm1.0~M_\odot$., Comment: 14 pages, 6 figures, 2 tables

We determine mass of MAXI J1659-152 using two different methods, viz: constant Normalization parameter point of view while fitting with TCAF, $M_{BH}$-$\chi^2_{red}$ variation from TCAF fits. We find a mass range of $4.06-7.93~M_\odot$ with a most probable value at $6 \pm 2 M_\odot$.

The theoretical concept of Chakrabarti-Titarchuk two Component Advective Flow (TCAF) model was introduced around two decade ago in mid-90s. Recently after the inclusion of TCAF model into XSPEC as an additive table model, we find that it is quite capable to fit spectra from different phases of few transient black hole candidates (TBHCs) during their outbursts. This quite agrees with our theoretical understanding. Here, a brief summary of our recent studies of spectral and temporal properties of few TBHCs during their X-ray outbursts with TCAF will be discussed.

We study black hole candidate (BHC) MAXI~J1836-194 during its 2011 outburst with Two Component Advective Flow (TCAF) model using RXTE/PCU2 data in $2.5-25$~keV band. From spectral fit, accretion flow parameters such as Keplerian disk rate ($\dot{m_d}$), sub-Keplerian halo rate ($\dot{m_h}$), shock location ($X_{s}$) and compression ratio (R) are extracted directly. During the entire phase of the outburst, quasi-periodic oscillations (QPOs) are observed sporadically. From the nature of the variation of accretion rate ratio (ARR=$\dot{m_h}$ / $\dot{m_d}$) and QPOs, entire period of the outburst is classified in two spectral states, such as, hard (HS), hard-intermediate (HIMS). Unlike other transient BHCs, no signature of soft (SS) and soft-intermediate (SIMS) spectral states are observed during entire phase of the outburst, Comment: 2 pages, 1 Figure (ASI Conf. Ser. for RETCO II Conference Proceeding, Edited by I. Chattopadhyay, A. Nandi, S. Das, and S. Mandal)

Galactic black hole candidates GRS 1915+105 and IGR J17091-3624 have many similarities in their light curves and spectral properties. However, very little is known about the orbital elements of their companions. In case the orbits are eccentric, tidal forces by the black hole on the companion can cause modulations of accretion rates in orbital time scale. We look for these modulations in the light curves of these two objects and find that their periodicities are around 28.3 d (29.0 d) and 32.2 d respectively. Eccentricities are at the most 0.071 and 0.46 respectively. We conclude that both these objects have long orbital periods and are eccentric. This could be a reason why light curves have several similar variability class transitions as reported in the literature., Comment: This paper has been withdrawn by the author due to an important error in the adopted methods of data analysis

Indian Centre for Space Physics is engaged in pioneering balloon borne experiments with typical payloads less than ~ 3.5kg. Low cost rubber balloons are used to fly them to a height of about 40km. In a double balloon system, the booster balloon lifts the orbiter balloon to its cruising altitude where data is taken for a longer period of time. In this Paper, we present our first scientific report on the variation of Cosmic Rays and muons with altitude and detection of several solar flares in X-rays between 20keV and 100keV. We found the altitude of the Pfotzer maximum at Tropic of Cancer for cosmic rays and muons and catch several solar flares in hard X-rays. We find that the hard X-ray (> 40keV) sky becomes very transparent above Pfotzer maximum. We find the flare spectrum to have a power-law distribution. From these studies, we infer that valuable scientific research could be carried out in near space using low cost balloon borne experiments. Published in Online version of Indian Journal of Physics., Comment: 19 pages, 13 Figures and 1 Table

The RT-2 Experiment, a low energy gamma-ray telescope, onboard CORONAS-PHOTON satellite is designed to study the temporal, spectral and spatial properties of the hard X-ray solar flares mainly in the energy range of 15 - 100 keV, which is extendable upto 1000 keV. During the operational period of ~ 9 months, it has been able to detect a few solar flares and at least four Gamma-Ray Bursts (GRBs). In this paper, we discuss the properties of the GRBs as observed by RT-2 in the energy band of 15 to ~ 1000 keV. We will present the results of spectral and timing properties of the GRBs (specially for the GRB 090618) using RT-2 detectors. Temporal analysis suggests that all four GRBs belong to the category of long duration bursts., Comment: 13 pages, 6 figures, ASI Conf. Series, BASI, Vol. 5, p. 91-102

The joint X and gamma-ray observations of GRB 090618 by a large number of satellites offer an unprecedented possibility of testing crucial aspects of theoretical models. In particular, it allows us to test (a) the formation of an optically thick e+e- baryon plasma self-accelerating to Lorentz factors in the range 200 < g < 3000; (b) its transparency condition with the emission of a component of 10^{53-54} baryons in the TeV region and (c) the collision of these baryons with the circumburst medium clouds, characterized by dimensions of 10^{15-16} cm. In addition, these observations offer the possibility of testing a new understanding of the thermal and power-law components in the early phase of this GRB. We test the fireshell model of GRBs in one of the closest (z = 0.54) and most energetic (Eiso = 2.90 x 10^{53} ergs) GRBs, namely GRB 090618. We analyze its emission using several spectral models, with special attention to the thermal and power-law components. We determine the fundamental parameters of a canonical GRB within the context of the fireshell model. We find evidences of the existence of two different episodes in GRB 090618. The first episode lasts 50 s and is characterized by a spectrum consisting of thermal component, which evolves between kT = 54 keV and kT = 12 keV. The second episode, which lasts for \sim 100s, behaves as a canonical long GRB with a Lorentz gamma factor at transparency of g = 495, a temperature at transparency of 29.22 keV and with characteristic masses of the surrounding clouds of \sim 10^{22-24} g. We support the recently proposed two-component nature of GRB 090618 by using specific theoretical analysis and illustrate that the 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., Comment: 19 pages, 18 figures. The abstract was reduced from the original one

GRB 090618 offered an unprecedented opportunity to have coordinated data, by the best of the X and Gamma Ray observatories, of the nearest (z = 0.54) energetic source (10$^{54}$ erg). Using the Fermi-GBM observations of this GRB, we have analyzed this source to explore the possibility of having components yet to be observed in other sources. We show that it is not possible to interpret GRB 090618 within the framework of the traditional single component GRB model. We argue that the observation of the first episode of duration of around 50s could not be a part of a canonical GRB, while the residual emission could be modeled easily with the models existing in literature. In this work we have considered the case of the fireshell scenario., Comment: 2011 Fermi Symposium proceedings - eConf C110509

We present the results of an analysis of the prompt gamma-ray emission from GRB 090618 using the RT-2 Experiment onboard the Coronas-Photon satellite. GRB 090618 shows multiple peaks and a detailed study of the temporal structure as a function of energy is carried out. As the GRB was incident at an angle of 77 degree to the detector axis, we have generated appropriate response functions of the detectors to derive the spectrum of this GRB. We have augmented these results using the publicly available data from the Swift BAT detector and show that a combined spectral analysis can measure the spectral parameters quite accurately. We also attempt a spectral and timing analysis of individual peaks and find evidence for a systematic change in the pulse emission characteristics for the successive pulses. In particular, we find that the peak energy of the spectrum, E_p, is found to monotonically decrease with time, for the successive pulses of this GRB., Comment: 12 pages, 6 figures, 3 tables, Accepted for publication in The Astrophysical Journal

In this review we shall comment on a few recent findings which strengthen the view that the black hole accretion has substantial amount of sub-Keplerian component. The manifestation of this component is many fold. We discuss some of them. A general outline of the complex structure that emerges from the multitude of observations is presented. A detailed outline of what might be going on in outburst sources is also discussed. The relationship amount the spectral and timing properties can be best understood by this picture. We claim that the sub-Keplerian advective disk paradigm is a complete package. Since signatures of sub-Keplerian motion is already increasing in the literature, the whole package must be correct., Comment: 7 Pages, 2 Figures, Proceeding of the 2nd Kolkata Conference on "Observational Evidence for the Black Holes in the Universe", Published in AIP, 2008

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., Comment: 5 Pages, 2 Figures, Proceeding of the 2nd Kolkata Conference on "Observational Evidence for the Black Holes in the Universe", Published in AIP, 2008