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Solar flare intensity is strongly dependent on the phase in the solar cycle, the structure and dynamics of the magnetic field near sunspots, and also on occasional solar coronal mass ejections. In this paper we study some of the solar flares detected by the stratospheric balloon borne experiments of Indian Centre for Space Physics. We also observe a gamma-ray burst which is believed to be originated from sudden energy release in gamma rays. In the hard X-ray region of 10 − 100 keV, we present and analyze data from various classes of solar flares and a gamma-ray burst. Because of natural constraints present in balloon borne experiments we receive data up to about a height of ∼ 42 km. The Earth’s residual atmosphere at this height absorbs the lower energy part of the spectrum. Moreover, the background radiation (mainly secondary cosmic rays) introduces noise. We show how we circumvent these limitations and create the accurate light curves and the spectra of a few solar flares and a gamma-ray burst.

We present a comprehensive analysis of pre-seismic anomalies as computed from the ground and space-based techniques during the recent Samos earthquake in Greece on 30 October 2020, with a magnitude M = 6.9. We proceed with a multi-parametric approach where pre-seismic irregularities are investigated in the stratosphere, ionosphere, and magnetosphere. We use the convenient methods of acoustics and electromagnetic channels of the Lithosphere–Atmosphere–Ionosphere-Coupling (LAIC) mechanism by investigating the Atmospheric Gravity Wave (AGW), magnetic field, electron density, Total Electron Content (TEC), and the energetic particle precipitation in the inner radiation belt. We incorporate two ground-based IGS GPS stations DYNG (Greece) and IZMI (Turkey) for computing the TEC and observed a significant enhancement in daily TEC variation around one week before the earthquake. For the space-based observation, we use multiple parameters as recorded from Low Earth Orbit (LEO) satellites. For the AGW, we use the SABER/TIMED satellite data and compute the potential energy of stratospheric AGW by using the atmospheric temperature profile. It is found that the maximum potential energy of such AGW is observed around six days before the earthquake. Similar AGW is also observed by the method of wavelet analysis in the fluctuation in TEC values. We observe significant energetic particle precipitation in the inner radiation belt over the earthquake epicenter due to the conventional concept of an ionospheric-magnetospheric coupling mechanism by using an NOAA satellite. We first eliminate the particle count rate (CR) due to possible geomagnetic storms and South Atlantic Anomaly (SAA) by the proper choice of magnetic field B values. After the removal of the statistical background CRs, we observe a significant enhancement of CR four and ten days before the mainshock. We use Swarm satellite outcomes to check the magnetic field and electron density profile over a region of earthquake preparation. We observe a significant enhancement in electron density one day before the earthquake. The parameters studied here show an overall pre-seismic anomaly from a duration of ten days to one day before the earthquake.

We focus on the possible thermal channel of the well-known Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) mechanism to identify the behavior of thermal anomalies during and prior to strong seismic events. For this, we investigate the variation of Surface Latent Heat Flux (SLHF) as resulting from satellite observables. We demonstrate a spatio-temporal variation in the SLHF before and after a set of strong seismic events occurred in Kathmandu, Nepal, and Kumamoto, Japan, having magnitudes of 7.8, 7.3, and 7.0, respectively. Before the studied earthquake cases, significant enhancements in the SLHF were identified near the epicenters. Additionally, in order to check whether critical dynamics, as the signature of a complex phenomenon such as earthquake preparation, are reflected in the SLHF data, we performed a criticality analysis using the natural time analysis method. The approach to criticality was detected within one week before each mainshock.

Atmospheric disturbances caused by seismic activity are a complex phenomenon. The Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) mechanism gives a detailed idea to understand these processes to study the possible impacts of a forthcoming earthquake. The atmospheric gravity wave (AGW) is one of the most accurate parameters for explaining such LAIC process, where seismogenic disturbances can be explained in terms of atmospheric waves caused by temperature changes. The key goal of this work is to study the perturbation in the potential energy associated with stratospheric AGW prior to many large earthquakes. We select seven large earthquakes having Richter scale magnitudes greater than seven (M>7.0) in Japan (Tohoku and Kumamoto), Mexico (Chiapas), Nepal, and the Indian Ocean region, to study the intensification of AGW using the atmospheric temperature profile as recorded from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite. We observe a significant enhancement in the potential energy of the AGW ranging from 2 to 22 days prior to different earthquakes. We examine the conditions of geomagnetic disturbances, typhoons, and thunderstorms during our study and eliminate the possible contamination due to these events.

We present perturbations in very low frequency (VLF) signals received at Ionospheric & Earthquake Research Centre (IERC) (Lat. 22.50 N, Long. 87.48 E) during and prior to two earthquakes, one on 11 March 2011 at 11:16:24 LT (M = 9) in Honshu, Japan and another on 12 May 2015 at 12:35:19 LT (M = 7.3) in Kodari, Nepal. The VLF signal transmitted from JJI (22.2 kHz) in Japan (Lat. 32.05 N, Long. 131.51 E) showed strong shift in VLF-sunrise terminator times towards night-time few days prior to both the earthquakes. These two earthquakes took place near the transmitter JJI and receiver IERC respectively. We simulated the VLF sunrise terminator time shifts using the long wavelength propagation capability (LWPC) code. To effectively represent the D-region ionospheric variabilities, we assumed a mean dynamic perturbation over the path and presented them with a set of effective Wait's parameters (β eff , ). We have successfully reproduced the temporal trend of the normalized VLF signal amplitude at VLF sunrise terminators for a few days around both the earthquakes. We used Wait's exponential model for estimating the altitude profile of D-region electron density ( ) at VLF sunrise terminator times around both the earthquakes and quantified the changes of those profiles.

We fit spectra of galactic transient source GX 339-4 during its 2013 outburst using Two Component Advective Flow (TCAF) solution. For the first time, we are fitting combined NuSTAR and Swift observation with TCAF. We use TCAF to fit 0.8–9.0 keV Swift and 4–79 keV NuSTAR spectra along with the LAOR model. To fit the data we use disk accretion rate, halo accretion rate, size of the Compton cloud and the density jump of advective flows at this cloud boundary as model parameters. From TCAF fitted flow parameters, and energy spectral index we conclude that the source was in the hard state throughout this particular outburst. The present analysis also gives some idea about the broadening of Fe K α with the accretion rate. Since TCAF does not include Fe line yet, we make use of the ‘LAOR model’ as a phenomenological model and find an estimate of the Kerr parameter to be ∼ 0.99 for this candidate.

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from Nuclear Spectroscopic Telescope Array (NuSTAR). We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely, in the soft–intermediate) spectral state during this short period of X-ray activity. We notice a faint but broad Fe Kα emission line at around 6.5 keV. We also estimate the probable mass of the black hole to be 6.5−0.7+0.5M⊙, with 90% confidence.

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after 26 years of inactivity of the black hole. We study the accretion flow properties of the source during its initial phase of the outburst using Swift/XRT and Swift/BAT data in the energy range of 0.5–150 keV. We have done spectral analysis with the two component advective flow (TCAF) model fits file. Several flow parameters such as two types of accretion rates (Keplerian disk and sub-Keplerian halo), shock parameters (location and compression ratio) are extracted to understand the accretion flow dynamics. We calculated equipartition magnetic field Beq for the outburst and found that the highest Beq∼900 Gauss. Power density spectra (PDS) showed no break, which indicates no or very less contribution of the Keplerian disk component, which is also seen from the result of the spectral analysis. No signature of prominent quasi-periodic oscillations (QPOs) is observed in the PDS. This is due to the non-satisfaction of the condition for the resonance shock oscillation as we observed mismatch between the cooling timescale and infall timescale of the post-shock matter.

The galactic black hole candidate (BHC) XTE J1118+480 during its 2000 outburst has been studied in a broad energy range using the archival data of PCA and HEXTE payloads of Rossi X-ray Timing Explorer. Detailed spectral and temporal properties of the source are studied. Low and very low frequency quasi-periodic oscillations (QPOs), with a general trend of increasing frequency are observed during the outburst. Spectral analysis is done using the combined data of the PCA and HEXTE instruments with two types of models: the well-known phenomenological power-law model and the current version of the fits file of two-component advective flow (TCAF) solution as an additive table model in XSPEC. During the entire period of the outburst, a non-thermal power-law component and the TCAF model fitted to the sub-Keplerian halo rate were found to be highly dominant. We suggest that this so-called outburst is due to enhanced jet activity. Indeed, the ‘outburst’ subsides when this activity disappears. We estimated the X-ray fluxes coming from the base of the jet and found that the radio flux is correlated with this X-ray flux. Though the object was in the hard state in the entire episode, the spectrum becomes slightly softer with the rise in the Keplerian disk rate in the late declining phase. We also estimated the probable mass of the source from our spectral analysis with the TCAF solution. Our estimated mass of XTE J1118+480 is 6.99−0.74+0.50M⊙\\(6.99^{+0.50}_{-0.74}~M_{\\odot }\\) i.e., in the range of 6.25–7.49M⊙\\(7.49~M _{\\odot }\\).

Work on the chemical evolution of pre-biotic molecules remains incomplete since the major obstacle is the lack of adequate knowledge of rate coefficients of various reactions which take place in interstellar conditions. In this work, we study the possibility of forming three pyrimidine bases, namely, cytosine, uracil and thymine in interstellar regions. Our study reveals that the synthesis of uracil from cytosine and water is quite impossible under interstellar circumstances. For the synthesis of thymine, reaction between uracil and : CH 2 is investigated. Since no other relevant pathways for the formation of uracil and thymine were available in the literature, we consider a large gas-grain chemical network to study the chemical evolution of cytosine in gas and ice phases. Our modeling result shows that cytosine would be produced in cold, dense interstellar conditions. However, presence of cytosine is yet to be established. We propose that a new molecule, namely, C 4 N 3 OH 5 could be observable in the interstellar region. C 4 N 3 OH 5 is a precursor (Z isomer of cytosine) of cytosine and far more abundant than cytosine. We hope that observation of this precursor molecule would enable us to estimate the abundance of cytosine in interstellar regions. We also carry out quantum chemical calculations to find out the vibrational as well as rotational transitions of this precursor molecule along with three pyrimidine bases.