arXiv subjects

S. Bhattacharyya

Publications and source records attributed to S. Bhattacharyya.

At least 19 recordsLinked to original sources

arXiv2026

Single Particle Excitations, Band Structures and Octupole Correlation in $^{65}$Zn

The excitation scheme of the $^{65}$Zn ($Z = 30, N = 35$) nucleus has been probed following its population in the $^{63}$Cu($\alpha$,pn) reaction at E$_{beam}$ = 30 MeV and using an array of Compton suppressed HPGe clovers as the detection system. This work has identified several new transitions of the nucleus and have modified the placements of some of the previously known ones. The multipolarities and the electric/ magnetic nature of the observed $\gamma$-ray rays have been measured, using the conventional methodologies. The spin-parity assignments for the levels have consequently been made; some of the spin-parities are new while others are either validation of the existing values or are modified results based on the present analysis. The experimental level scheme exhibits collective as well as single particle structures. The measured level energies have been compared with those calculated in the framework of the large basis shell model using a model space of $p_{3/2}, f_{5/2}, p_{1/2}, g_{9/2}$ orbitals and two different interactions. The collective excitations of the nucleus were probed through the properties of its band structures and through the calculations of the Total Routhian Surface (TRS) for the associated deformations/ shapes. The results of this study brings out the essential features of evolving structural characteristics and developing collectivity with increasing number of nucleons outside a doubly-magic core and with their occupancy of deformation driving high-$j$ orbitals.

nucl-ex↗

arXiv2026

Chasing Gamma-Ray Signals from Binary Neutron Star Coalescences with the Cherenkov Telescope Array: Prospects and Observing Strategies

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB~170817A, heralded the birth of multi-messenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV--TeV GW follow-up strategies for CTAO using a multi-step simulation pipeline and to estimate the expected rate of joint GW-GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV--TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star-black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

astro-ph.HE↗

arXiv2026

First observation of multi-phonon $γ$-vibrations in an odd-odd nuclear system

The identification of the first multi-phonon $γ$-vibrational bands in an odd-odd neutron-rich nucleus of the nuclear chart is presented. These high spin structures of hard to access $^{104}_{41}$Nb$_{63}$, produced in fission, were studied by combining a spectrometer with isotopic resolution coupled to a $γ$-ray tracking array and independently high-fold $γ$ coincidence measurements. Triaxial Projected Shell Model calculations for the high-spin states are in good agreement with the measured observables for the yrast, one-phonon and two-phonon $γ$ bands. The possibility of an oblate shape of an isomeric state and coexistence of triaxial and oblate configurations are investigated from the decay of the 141 keV isomer. The present work illustrates the robustness of vibration excitations in the presence of odd valence proton and neutron as well as the possibly coexisting shapes beyond the $N=60$ transitional region.

nucl-ex↗

arXiv2026

Level structure of light neutron-rich La isotopes beyond the N=82 shell closure

The high spin excited states of Lanthanum isotopes $^{140-143}$La, above the $N=82$ closed shell, have been populated in fission reactions. The prompt $γ$-ray transitions were measured using two complementary methods; a) in coincidence with the isotopically identified fragments produced in the fission of the $^{238}$U+$^{9}$Be system using the VAMOS++ and the AGATA spectrometers and b) high statistics three-fold $γ-γ-γ$ and four-fold $γ-γ-γ-γ$ coincidence data from the spontaneous fission of $^{252}$Cf using the Gammasphere. This work reports the first identification of a pair of parity doublet structures in $^{143}$La and the new high spin level structure in $^{140-142}$La from prompt $γ$-ray spectroscopy. The level structures are interpreted in terms of the systematics of neighbouring odd-$Z$ nuclei above $Z=50$ shell closure and large-scale shell model calculations. The present results indicate the presence of stable octupole deformation, in $^{143}$La. The excitation energy pattern and their comparison with neighbouring isotones, moving away from the N=82 closed shell, point towards a transition from single particle structures to an alternating parity rotational band structure in the La isotopic chain.

nucl-ex↗

arXiv2026

Spectral connection between far UV and soft X-ray emission from active galactic nuclei using AstroSat

We present the UV/X-ray joint spectral analyses of four Seyfert~1 galaxies (PG 0804+761, NGC 7469, SWIFT J1921.1-5842, and SWIFT J1835.0+3240) using the data acquired with the Ultraviolet Imaging Telescope and Soft X-ray Telescope onboard AstroSat. We model the intrinsic UV/X-ray continuum with the accretion disk, warm and hot Comptonization using the OPTXAGNF and FAGNSED models, where the disk seed photons are Comptonized in the warm and hot corona. The Eddington ratio of the four Seyferts ranges from 0.01 to 1. In the case of SWIFT J1835.0$+$3240, we infer a compact warm corona ($ R_{warm} - R_{hot} \lesssim 18 r_{g}$) while, PG 0804+761, NGC 7469, and SWIFT J1921.1-5842 may exhibit a larger warm Comptonizing region ($\gtrsim 32r_{g}$). We could constrain the spin parameter in PG 0804+761, $a^{\star} = 0.76_{-0.20}^{+0.08}$ (1$σ$ error), with the FAGNSED model. In SWIFT J1835.0+3240 and SWIFT J1921.1-5842, the UV/X-ray spectral variability may be driven by the thermal Comptonization of the disk seed photons in the hot corona. Furthermore, the observed spectral hardening with the decrease in disk temperature and accretion rate compared to earlier observations may indicate a state transition in SWIFT J1835.0+3240 from a high/soft to a low/hard state.

astro-ph.HE↗

arXiv2025

Multi-Frequency Study of FRB20201124A with the uGMRT

We present results from multi-epoch observations of the repeating fast radio burst FRB 20201124A with the upgraded Giant Metrewave Radio Telescope (uGMRT) during its active phase between 8 May and 28 May 2021. The bursts exhibit significant morphological diversity, including multiple sub-bursts, downward frequency drifts, and intrinsic widths ranging from 1.032 - 32.159 ms. Bursts were detected in both Band 4 (550 - 950 MHz) and Band 5 (1060 - 1460 MHz), with the last Band 5 burst occurring on 24 May, while Band 4 activity persisted until 28 May, indicating a frequency-dependent decline. Consecutive bursts were observed with separations of 16.7 - 291.5 ms, revealing short repetition intervals or potential sub-second quasi-periodicity. The waiting-time and energy distributions are bimodal, suggesting at least two distinct emission timescales and energy modes. Burst fluence ranges from 1.72 - 78.47 Jy ms, and the cumulative fluence distribution follows a broken power law. Multi-frequency analysis further shows closely spaced burst pairs across Band 4 and Band 5, with sub-second offsets of 1.08 - 1.15 s, and no strict simultaneity with contemporaneous FAST detections. These findings demonstrate that FRB 20201124A exhibits closely spaced, patchy, multi-frequency emission with frequency-dependent activity, highlighting the complex and dynamic nature of repeating FRBs.

astro-ph.HE↗

arXiv2025

Shape evolution in neutron-rich odd-even $^{105-109}$Nb isotopes

Neutron-rich nuclei around $Z\sim40$ exhibit multiple shape transitions. This region shows one of the sharpest transitions in the nuclear chart, from a spherical vibrator at $N=58$ to a strongly deformed prolate shape at $N=60$, with largest deformations seen for $_{38}$Sr and $_{40}$Zr. Below $Z=36$, a spherical-to-oblate transition is predicted, while above $Z=42$ and $N\ge60$, the shape evolves from axial to triaxial. Even-$Z$ nuclei have been well studied, but odd-$Z$ isotopes such as Nb offer additional insights into these mechanisms. The Nb isotopes lie at the boundary between axially deformed Zr and triaxially deformed Mo nuclei. This work explores the structure of neutron-rich Nb nuclei up to $N=68$, aiming to understand shape evolution with isospin and the onset of triaxiality. Two complementary fission experiments were used: (i) $^{238}$U+$^9$Be at GANIL in inverse kinematics with AGATA, EXOGAM, and VAMOS++, allowing prompt and delayed $γ$-ray spectroscopy with isotopic identification; (ii) spontaneous fission of $^{252}$Cf with the Gammasphere array providing high-fold $γ$-coincidence data. The level scheme of $^{105}$Nb was significantly extended with two new negative-parity bands. A revised scheme is proposed for $^{107}$Nb, differing from previous results, and new structures are reported in $^{109}$Nb. The signature splitting analysis indicates triaxial deformation for positive-parity bands, while negative-parity bands show axial symmetry, similar to Zr. This reveals a shape coexistence in neutron-rich Nb nuclei.

nucl-ex↗

arXiv2025

Prospects for dark matter observations in dwarf spheroidal galaxies with the Cherenkov Telescope Array Observatory

The dwarf spheroidal galaxies (dSphs) orbiting the Milky Way are widely regarded as systems supported by velocity dispersion against self-gravity, and as prime targets for the search for indirect dark matter (DM) signatures in the GeV-to-TeV $γ$-ray range owing to their lack of astrophysical $γ$-ray background. We present forecasts of the sensitivity of the forthcoming Cherenkov Telescope Array Observatory (CTAO) to annihilating or decaying DM signals in these targets. An original selection of candidates is performed from the current catalogue of known objects, including both classical and ultra-faint dSphs. For each, the expected DM content is derived using the most comprehensive photometric and spectroscopic data available, within a consistent framework of analysis. This approach enables the derivation of novel astrophysical factor profiles for indirect DM searches, which are compared with results from the literature. From an initial sample of 64 dSphs, eight promising targets are identified -- Draco I, Coma Berenices, Ursa Major II, Ursa Minor and Willman 1 in the North, Reticulum II, Sculptor and Sagittarius II in the South -- for which different DM density models yield consistent expectations, leading to robust predictions. CTAO is expected to provide the strongest limits above $\sim$10 TeV, reaching velocity-averaged annihilation cross sections of $\sim$5$\times$10$^{-25}$ cm$^3$ s$^{-1}$ and decay lifetimes up to $\sim$10$^{26}$ s for combined limits. The dominant uncertainties arise from the imprecise determination of the DM content, particularly for ultra-faint dSphs. Observation strategies are proposed that optimise either deep exposures of the best candidates or diversified target selections.

astro-ph.HE↗

arXiv2025

Competition between the neutron-proton pair break-ups delineating the level structure of 202Po

High-spin spectroscopic study of $^{202}$Po ($Z$ = 84, $N$ = 118) has been carried out using the $^{195}$Pt($^{12}$C, 5n)$^{202}$Po fusion-evaporation reaction. An extended level scheme has been proposed up to an excitation energy of $E_x\approx$ 8 MeV and angular momentum of 27$\hbar$, with the addition of 57 newly observed $γ$-ray transitions, along with the revisions in the placement of 8 already known transitions and the multipolarities of 4 of these transitions. The energy of the unobserved 8$^+ \rightarrow 6^+$ transition has been proposed to be 9.0(5) keV, which resolves the uncertainty in the excitation energy of the levels above the 6$^{+}$ state. Three new sequences of $M1$ transitions have also been identified in the high excitation energy regime and included in the proposed level scheme. The large-scale shell model calculations for $Z>82$ and $N<126$ valence space have been carried out using PBPOP interaction which explained the overall level scheme for both the positive and negative parity states. The calculations successfully reproduced the purity of the proton $πh_{9/2}$ dominated $8^+$ isomeric state, and also explained the missing $E2$ decay of the ${12}^+$ isomeric state in terms of changing nucleonic configurations.

nucl-ex↗

arXiv2025

Imaging the Photochemistry of Cyclobutanone using Ultrafast Electron Diffraction: Experimental Results

We investigated the ultrafast structural dynamics of cyclobutanone following photoexcitation at $λ=200$ nm using gas-phase megaelectronvolt ultrafast electron diffraction. Our investigation complements the simulation studies of the same process within this special issue. It provides information about both electronic state population and structural dynamics through well-separable inelastic and elastic electron scattering signatures. We observe the depopulation of the photoexcited S$_2$ state of cyclobutanone with n3s Rydberg character through its inelastic electron scattering signature with a time constant of $(0.29 \pm 0.2)$ ps towards the S$_1$ state. The S$_1$ state population undergoes ring-opening via a Norrish Type-I reaction, likely while passing through a conical intersection with S$_0$. The corresponding structural changes can be tracked by elastic electron scattering signatures. These changes appear with a delay of $(0.14 \pm 0.05)$ ps with respect the initial photoexcitation, which is less than the S$_2$ depopulation time constant. This behavior provides evidence for the ballistic nature of the ring-opening once the S$_1$ state is reached. The resulting biradical species react further within $(1.2 \pm 0.2)$ ps via two rival fragmentation channels yielding ketene and ethylene, or propene and carbon monoxide. Our study showcases both the value of gas-phase ultrafast diffraction studies as an experimental benchmark for nonadiabatic dynamics simulation methods and the limits in the interpretation of such experimental data without comparison to such simulations.

physics.chem-ph↗

arXiv2025

The birth of Be star disks I. From localized ejection to circularization

Classical Be stars are well known to eject mass, but the details governing the initial distribution and evolution of this matter into a disk are poorly constrained by observations. By combining high-cadence spectroscopy with contemporaneous space photometry from TESS, we have sampled about 30 mass ejection events in 13 Be stars. Our goal is to constrain the geometrical and kinematic properties of the ejecta, facilitating the investigation into the initial conditions and evolution, and understanding its interactions with preexisting material. The photometric variability is analyzed together with measurements of the rapidly changing emission features to identify the onset of outburst events and obtain information about the geometry of the ejecta and its evolution. All Be stars observed with sufficiently high cadence exhibit rapid oscillations of line asymmetry with a single frequency in the days following the start of the event. The emission asymmetry cycles break down after roughly 5 - 10 cycles, with the emission line profile converging toward approximate symmetry. In photometry, several frequencies typically emerge at relatively high amplitude at some point during the mass ejection process. In all observed cases, freshly ejected material was initially within a narrow azimuthal range, indicating it was launched from a localized region on the star. The material orbits the star with a frequency consistent with the near-surface Keplerian orbital frequency. This material circularizes into a disk configuration after several orbital timescales. This is true whether or not there was a preexisting disk. We find no evidence for precursor phases prior to the ejection of mass in our sample. The several photometric frequencies that emerge during outburst are at least partially stellar in origin. (Abstract abridged)

astro-ph.SR↗

arXiv2024

Measurement of enhanced electric dipole transition strengths at high spin in $^{100}$Ru: Possible observation of octupole deformation

The majority of atomic nuclei have deformed shapes and nearly all these shapes are symmetric with respect to reflection. There are only a few reflection asymmetric pear-shaped nuclei that have been found in actinide and lanthanide regions, which have static octupole deformation. These nuclei possess an intrinsic electric dipole moment due to the shift between the center of charge and the center of mass. This manifests in the enhancement of the electric dipole transition rates. In this article, we report on the measurement of the lifetimes of the high spin levels of the two alternate parity bands in $^{100}$Ru through the Doppler Shift Attenuation Method. The estimated electric dipole transition rates have been compared with the calculated transition rates using the triaxial projected shell model without octupole deformation, and are found to be an order of magnitude enhanced. Thus, the observation of seven inter-leaved electric dipole transitions with enhanced rates establish $^{100}$Ru as possibly the first octupole deformed nucleus reported in the A $\approx$ 100 mass region.

nucl-ex↗

arXiv2024

Very High-energy Gamma-Ray Episodic Activity of Radio Galaxy NGC 1275 in 2022-2023 Measured with MACE

The radio galaxy NGC 1275, located at the central region of Perseus cluster, is a well-known very high-energy (VHE) gamma-ray emitter. The Major Atmospheric Cherenkov Experiment Telescope has detected two distinct episodes of VHE (E > 80 GeV) gamma-ray emission from NGC 1275 during 2022 December and 2023 January. The second outburst, observed on 2023 January 10, was the more intense of the two, with flux reaching 58$\%$ of the Crab Nebula flux above 80 GeV. The differential energy spectrum measured between 80 GeV and 1.5 TeV can be described by a power law with a spectral index of $Γ= - 2.90 \pm 0.16_{stat}$ for both flaring events. The broadband spectral energy distribution derived from these flares, along with quasisimultaneous low-energy counterparts, suggests that the observed gamma-ray emission can be explained using a homogeneous single-zone synchrotron self-Compton model. The physical parameters derived from this model for both flaring states are similar. The intermediate state observed between two flaring episodes is explained by a lower Doppler factor or magnetic field, which subsequently returned to its previous value during the high-activity state observed on 2023 January 10.

astro-ph.HE↗

arXiv2024

Multi-epoch UV $-$ X-ray spectral study of NGC 4151 with AstroSat

We present a multi-wavelength spectral study of NGC 4151 based on five epochs of simultaneous AstroSat observations in the near ultra-violet (NUV) to hard X-ray band ($\sim 0.005-80$ keV) during $2017 - 2018$. We derived the intrinsic accretion disk continuum after correcting for internal and Galactic extinction, contributions from broad and narrow line regions, and emission from the host galaxy. We found a bluer continuum at brighter UV flux possibly due to variations in the accretion disk continuum or the UV reddening. We estimated the intrinsic reddening, $E(B-V) \sim 0.4$, using high-resolution HST/STIS spectrum acquired in March 2000. We used thermal Comptonization, neutral and ionized absorption, and X-ray reflection to model the X-ray spectra. We obtained the X-ray absorbing neutral column varying between $N_H \sim 1.2-3.4 \times 10^{23} cm^{-2}$, which are $\sim 100$ times larger than that estimated from UV extinction, assuming the Galactic dust-to-gas ratio. To reconcile this discrepancy, we propose two plausible configurations of the obscurer: (a) a two-zone obscurer consisting of dust-free and dusty regions, divided by the sublimation radius, or (b) a two-phase obscurer consisting of clumpy, dense clouds embedded in a low-density medium, resulting in a scenario where a few dense clouds obscure the compact X-ray source substantially, while the bulk of UV emission arising from the extended accretion disk passes through the low-density medium. Furthermore, we find a positive correlation between X-ray absorption column and $NUV-FUV$ color and UV flux, indicative of enhanced winds possibly driven by the 'bluer-when-brighter' UV continuum.

astro-ph.HE↗

arXiv2024

Dark Matter Line Searches with the Cherenkov Telescope Array

Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g.~box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

hep-ph↗

arXiv2024

Existance of octupole correlation in 116Sn

The negative parity states in 116Sn have been investigated in terms of octupole correlation. The same is probed by using the Indian National Gamma Array (INGA) facility at Variable Energy Cyclotron Centre, Kolkata using the reaction, 114Cd(α,2n) 116Sn at 34 MeV energy. Three new γ-transitions relevant to the present investigation are reported and the spin-parities of the associated levels are assigned based on the DCO-ratios and polarisation measurements. The interband transitions between the positive parity ground band and the negative parity band are newly observed and the corresponding extracted ratio of transition probability, B(E1)/B(E2) indicated the existence of octupole correlation in these nuclei. The enhanced transition rates for E1 and E3 transitions between these opposite parity bands and corroboration of soft octupole deformation ultimately aid the onset of octupole excitation in this isotope.

nucl-ex↗

arXiv2023

Search for the origin of wobbling motion in the $ A \approx 130 $ region: The case of $^{131}$Xe

In-beam $ γ$-ray spectroscopy of $^{131}$Xe has been carried out to study the structure of the intruder $ νh_{11/2} $ band. Excited states were populated via an $ α$-induced fusion-evaporation reaction at E$ _α = 38 $ MeV. Inspection of $ γγ$-coincidence data resulted in the identification of a new rotational sequence. Based on the systematics of excitation energy, assigned spin-parity, decay pattern, and the electromagnetic character of the inter-band $ ΔI = 1 $ $ γ$-transitions, this sequence is proposed as the unfavoured signature partner of the $ νh_{11/2} $ band. The structure of this band is further illuminated in the light of the triaxial particle rotor model (TPRM). The possibility of wobbling excitation in $ N = 77 $ Xe-Ba-Ce isotones has been explored in a systematic manner.

nucl-ex↗

arXiv2023

Particle Identification at VAMOS++ with Machine Learning Techniques

Multi-nucleon transfer reaction between 136Xe beam and 198Pt target was performed using the VAMOS++ spectrometer at GANIL to study the structure of n-rich nuclei around N=126. Unambiguous charge state identification was obtained by combining two supervised machine learning methods, deep neural network (DNN) and positional correction using a gradient-boosting decision tree (GBDT). The new method reduced the complexity of the kinetic energy calibration and outperformed the conventional method, improving the charge state resolution by 8%

physics.ins-det↗