arXiv subjects

Pooja Bhattacharjee

Publications and source records attributed to Pooja Bhattacharjee.

16 recordsLinked to original sources

arXiv2025

Galaxy Mergers Collectively Illuminate the $\gamma$-Ray Sky

The origin and acceleration mechanism of cosmic rays (CRs) remain fundamental open questions. Galaxy mergers are proposed as very high-energy CR accelerators, which are expected to produce high-energy (HE) $\gamma$ rays and neutrinos through interactions with the ambient gas and low-energy background radiation fields. For the first time, we systematically study the HE $\gamma$-ray emission from galaxy mergers utilising 16.7 years of Fermi Large Area Telescope (Fermi-LAT) data with the sample list compiled from eight survey catalogs. Our analysis finds 8 galaxy mergers that exhibit $\gamma$-ray emission with significance $\gtrsim5\sigma$ in the 1-500 GeV energy range. A stacking analysis of the remaining faint galaxy mergers yields a combined $\gamma$-ray emission detected at $\sim 35\sigma$ significance, a best-fit spectral index of $\Gamma \approx 2.07$, and an energy flux of $\sim \rm 2\times10^{-14}~erg~cm^{-2}~s^{-1}$. We compare the stacked spectral energy distributions of the galaxy mergers with the projected sensitivity of the upcoming $\gamma$-ray telescope Cherenkov Telescope Array (CTA). Furthermore, we find that 18 previously unassociated Fermi-LAT sources are spatially coincident with galaxy mergers. Our findings establish galaxy mergers as a new class of HE $\gamma$-ray sources. Future neutrino and $\gamma$-ray observatories will be crucial to discover the particle acceleration mechanism in these newly identified CR sources.

astro-ph.HE↗

arXiv2025

Probing the Parameter Space of Axion-Like Particles Using Simulation-Based Inference

Axion-like particles (ALPs), hypothetical pseudoscalar particles that couple to photons, are among the most actively investigated candidates for new physics beyond the Standard Model. Their interaction with gamma rays in the presence of astrophysical magnetic fields can leave characteristic, energy-dependent modulations in observed spectra. Capturing such subtle features requires precise statistical inference, but standard likelihood-based methods often fall short when faced with complex models, large number of nuisance parameters and limited analytical tractability. In this work, we investigate the application of simulation-based inference (SBI), specifically Truncated Marginal Neural Ratio Estimation (TMNRE), to constrain ALP parameters using simulated observations from the upcoming Cherenkov Telescope Array Observatory (CTAO). We model the gamma-ray emission from the active galactic nucleus NGC 1275, accounting for photon-ALP mixing, extragalactic background light (EBL) absorption, and the full CTAO instrument response. Leveraging the Swyft framework, we infer posteriors for the ALP mass and coupling strength and demonstrate its potential to extract meaningful constraints on ALPs from future real gamma-ray data with CTAO.

astro-ph.HE↗

arXiv2025

New constraints on $Z^\prime$ from captured dark matter annihilation in astrophysical objects

Considering dark matter capture in astrophysical objects such as neutron stars and brown dwarfs, followed by their annihilation into two neutrino and four neutrino final states, we derive new constraints on the mass and coupling of a novel abelian gauge boson $Z^\prime$ arising from an anomaly-free $U(1)$ extension of the Standard Model. We further confront these astrophysical limits with complementary bounds from the Planck-observed relic abundance via the freeze-in mechanism, big bang nucleosynthesis (BBN), gravitational wave signatures from cosmic strings, and searches at energy and intensity frontier experiments.

hep-ph↗

arXiv2025

Can local White Dwarfs constrain Dark Matter interactions?

We investigate whether nearby white dwarfs (WDs) can constrain dark matter (DM) interactions with ordinary matter. As experimental sensitivity improves, driven by the Gaia mission, the sample volume of nearby WDs has been increasing over recent years. We carefully select a sample of ten cold, isolated, non-magnetic WDs within 13~pc of the Sun. We model their carbon-oxygen interior using a finite temperature relativistic equation of state and model atmospheres to infer their core temperatures. This enables us to perform a thorough estimation of the DM capture rate and evaporation mass using actual astrophysical observations. Given the low local DM density, we focus on DM that annihilates into long-lived mediators, which escape the WD and later decay into photons. While \textit{Fermi}-LAT data shows no significant gamma-ray excess, future telescopes, CTA North \& South, LHAASO, SWGO, could probe DM-nucleon cross sections down to $\sim 10^{-41}~\text{cm}^2$ for DM masses above the TeV scale. Our results are competitive with current direct detection bounds (e.g., LZ) in the multi-TeV regime. This work underscores the importance of systematic WD studies in the broader landscape of DM detection and demonstrates the synergy between astrophysical and terrestrial searches in exploring DM interactions.

hep-ph↗

arXiv2025

Searching for dark matter annihilating into light long-lived mediators from stars inside dwarf spheroidal galaxies

Several astrophysical and cosmological observations suggest the existence of dark matter (DM) through its gravitational effects, yet its nature remains elusive. Despite the lack of DM signals from direct detection experiments, efforts continue to focus on the indirect detection of DM from DM-rich astrophysical objects. Dwarf spheroidal galaxies (dSphs) are among the most promising targets for such searches. In this work, we aim to investigate the expected DM capture rate from the stellar component of ten nearby DM-rich dSphs, assuming that the accumulated DM eventually annihilates into light, long-lived mediators (LLLMs) which decay into gamma rays outside the dSphs. We analyze nearly 16 years of {\it Fermi}-LAT data to search for DM annihilation through LLLMs, and, from the observed stacked flux upper limits, set limits on the DM-nucleon scattering cross section for the case of a generic DM scenario. Additionally, we incorporate the Sommerfeld enhancement (SE) effect into the DM annihilation process assuming scalar DM model, and obtain bounds on the DM-nucleon scattering cross section of $\sim~10^{-36} {\rm cm}^2$ for DM masses around 100 GeV. This allows us to explore an alternative avenue for exploring DM phenomena from dSphs and compare our results with the bounds reported by direct DM detection experiments and other celestial bodies.

astro-ph.HE↗

arXiv2024

Gamma-ray flux limits from brown dwarfs: Implications for dark matter annihilating into long-lived mediators

Brown dwarfs (BDs) are celestial objects representing the link between the least massive main-sequence stars and giant gas planets. In the first part of this article, we perform a model-independent search of a gamma-ray signal from the direction of nine nearby BDs in 13 years of \Fermi-LAT data. We find no significant excess of gamma rays, and we, therefore, set 95\% confidence level upper limits on the gamma-ray flux with a binned-likelihood approach. In the second part of the paper, we interpret these bounds within an exotic mechanism proposed for gamma-ray production in BDs: If the dark matter (DM) of the universe is constituted of particles with non-negligible couplings to the standard model, BDs may efficiently accumulate them through scatterings. DM particles eventually thermalise, and can annihilate into light, long-lived, mediators which later decay into photons outside the BD. Within this framework, the current Fermi-LAT stacked upper limits on the gamma-ray flux of the selected BDs do not enable setting a bound on the DM-nucleon cross-section. An improvement of approximately a factor of 9 in the gamma-ray limits is required to obtain bounds on the scattering cross-section $σ_{χn}$ $\sim 10^{-36}$ cm$^{2}$ for DM masses below 10 GeV. This improvement would cover a larger portion of the parameter space in mediator decay length and DM mass. The code and data to reproduce the results of this study are available on GitLab at \github{https://gitlab.in2p3.fr/francesca.calore/brown-dwarfs-gamma}.

astro-ph.HE↗

arXiv2024

Probing the Dark Matter Capture Rate in a Local Population of Brown Dwarfs with IceCube Gen 2

This study explores the potential for dark matter annihilation within brown dwarfs, investigating an unconventional mechanism for neutrino production. Motivated by the efficient accumulation of dark matter particles in brown dwarfs through scattering interactions, we focus on a mass range above 10 GeV, considering dark matter annihilation channels $χχ\rightarrow ν\barν ν\barν$ through long-lived mediators. Using the projected sensitivity of IceCube Generation 2, we assess the detection capability of the local population of brown dwarfs within 20 pc and exclude dark matter-nucleon scattering with cross-sections as low as a few multiples of $10^{-36}~\rm cm^{2}$.

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arXiv2024

Confronting the dark matter capture rate with a continuous gravitational wave probe of local neutron stars

Continuous gravitational waves (CGWs) from various astrophysical sources are one of the many future probes of upcoming gravitational wave (GW) search missions. Neutron stars (NSs) with deformity are one of the leading sources of CGW emissions. In this work, for the first time, a novel attempt to estimate the dark matter (DM) capture rate is performed using CGW as the probe to the local NS population. Competitive bounds on DM capture from the local NS population are reported when compared with DM direct search experiments and other astrophysical observations.

astro-ph.HE↗

arXiv2024

Chasing Gravitational Waves with the Cherenkov Telescope Array

The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA.

astro-ph.HE↗

arXiv2023

Gammapy: A Python package for gamma-ray astronomy

In this article, we present Gammapy, an open-source Python package for the analysis of astronomical $γ$-ray data, and illustrate the functionalities of its first long-term-support release, version 1.0. Built on the modern Python scientific ecosystem, Gammapy provides a uniform platform for reducing and modeling data from different $γ$-ray instruments for many analysis scenarios. Gammapy complies with several well-established data conventions in high-energy astrophysics, providing serialized data products that are interoperable with other software packages. Starting from event lists and instrument response functions, Gammapy provides functionalities to reduce these data by binning them in energy and sky coordinates. Several techniques for background estimation are implemented in the package to handle the residual hadronic background affecting $γ$-ray instruments. After the data are binned, the flux and morphology of one or more $γ$-ray sources can be estimated using Poisson maximum likelihood fitting and assuming a variety of spectral, temporal, and spatial models. Estimation of flux points, likelihood profiles, and light curves is also supported. After describing the structure of the package, we show, using publicly available $γ$-ray data, the capabilities of Gammapy in multiple traditional and novel $γ$-ray analysis scenarios, such as spectral and spectro-morphological modeling and estimations of a spectral energy distribution and a light curve. Its flexibility and power are displayed in a final multi-instrument example, where datasets from different instruments, at different stages of data reduction, are simultaneously fitted with an astrophysical flux model.

astro-ph.IM↗

arXiv2022

Study of potential self-annihilation signal from dark matter particles in some prospective astrophysical dark matter sources

With the growing interest in indirect detection for dark matter signature, the thesis aims to investigate the signal originating from the self-annihilation of dark matter candidates. The methods for targeting the dark matter signal are two-fold, on one hand, we explore the gamma rays resulting from dark matter particles. On the other hand, we focus on complementary radio properties.

astro-ph.HE↗

arXiv2021

Gamma-ray and Synchrotron Radiation from Dark Matter annihilations in Ultra-faint Dwarf Galaxies

The very large (100-1000) mass-to-light ratio applicable to the ultra-faint dwarf galaxies (UFDs) implies a high concentration of dark matter, thus rendering them ideal theatres for indirect signatures of dark matter. In this paper, we consider 14 recently discovered UFDs and study the electromagnetic radiation emanating from them over a wide range, from gamma ray down to radio frequencies. We analyze the Fermi-LAT data on high energy gamma rays and radio fluxes at the GMRT and VLA to obtain upper limits on annihilation cross section $\langleσv\rangle$ in a model independent way. We further discuss the sensitivity of the Square Kilometer Array radio telescope in probing the synchrotron radiation from the aforementioned UFDs. We also investigate the dependences of the said upper limits on the uncertainties in the determination of various astrophysical parameters.

hep-ph↗

arXiv2020

Multiwavelength analysis of low surface brightness galaxies to study possible dark matter signature

Low Surface Brightness (LSB) galaxies have very diffuse, low surface density stellar disks which appear faint in optical images. They are very rich in neutral hydrogen (HI) gas, which extends well beyond the stellar disks. Their extended HI rotation curves and stellar disks indicate that they have very massive dark matter (DM) halos compared to normal bright galaxies. Hence, LSB galaxies may represent valuable laboratories for the indirect detection of DM. In this paper, we search for WIMP annihilation signatures in four LSB galaxies and present an analysis of nearly nine years of data from the Fermi Large Area Telescope (LAT). Above 500 MeV, no excess emission was detected from the LSB galaxies. We obtain constraints on the DM cross-section for different annihilation channels, for both individual and stacked targets. In addition to this, we use radio data from the Very Large Array (VLA) radio telescope in order to derive DM constraints, following a multiwavelength approach. The constraints obtained from the four considered LSB galaxies are nearly 3 orders of magnitude weaker than the predicted limits for the thermal relic abundances and the combined limits achieved from Fermi-LAT observations of dwarf spheroidal galaxies. Finally, we discuss the possibility of detecting emission from LSB galaxies using the upcoming ground-based $γ$-ray and radio observatories, namely the Cherenkov Telescope Array (CTA) and the Square Kilometre Array (SKA).

astro-ph.HE↗

arXiv2019

Analysis of Fermi-LAT data from Tucana-II: Possible constraints on the Dark Matter models with an intriguing hint of a signal

Tucana-II (Tuc-II), a recently discovered and confirmed Ultra Faint Dwarf Spheroidal galaxy, has a high mass to light ratio as well as a large line-of-sight stellar velocity dispersion, thus making it an ideal candidate for an indirect dark matter (DM) search. In this paper, we have analyzed nine years of $γ$-ray data obtained from the \textit{Fermi}-LAT instrument from the direction of Tuc-II. The fact that a very weak significant $γ$-ray excess ($2.2σ$) over the background of Tuc-II have been detected from the location of this galaxy. We have observed that this excess of $γ$-ray emission from the of location Tuc-II rises with longer periods of data. If WIMP pair annihilation is assumed for this faint emission, for $b\bar{b}$ annihilation channel the test statistics (TS) value peaks at DM mass $\sim$ 14 GeV and for $τ^{+}τ^{-}$ annihilation channel it peaks at DM mass 4 GeV. It is then called for an estimation of the $95\%$ confidence level upper limit of the possible velocity weighted self-annihilation cross-section of the DM particles (WIMPs) within Tuc-II by fitting the observed $γ$-ray flux with spectra expected for DM annihilation. The estimated upper limits of the cross-sections from Tuc-II are then compared with two other dwarf galaxies that are considered to be good DM candidates in several studies. We have also compared our results with the cross-sections obtained in various popular theoretical models of the WIMPs to find that our results impose reasonable tight constraints on the parameter spaces of those DM models. In the concluding section, we compared our results with the similar results obtained from a combined dSph analysis by the \textit{Fermi}-LAT collaboration as well as the results obtained from the studies of DM in the dwarf galaxies by the major ground-based Cherenkov experiments.

astro-ph.HE↗

arXiv2018

Investigating the region of 3C 397 in High Energy Gamma rays

We investigate the supernova remnant (SNR) 3C 397 and its neighboring pulsar PSR J1906+0722 in high energy gamma rays by using nearly six years of archival data of {\it Large Area Telescope} on board {\it Fermi Gamma Ray Space Telescope} (Fermi-LAT). The off-pulse analysis of gamma-ray flux from the location of PSR J1906+0722 reveals an excess emission which is found to be very close to the radio location of 3C 397. Here, we present the preliminary results of this gamma-ray analysis of 3C 397 and PSR J1906+0722.

astro-ph.HE↗

arXiv2018

Constraints on dark matter models from the observation of Triangulum-II with the Fermi Large Area Telescope

Triangulum-II, a newly discovered dwarf spheroidal galaxy, is a strong candidate for the indirect search of dark matter through the detection of $γ$-ray emission that could originate from the pair- annihilation of weakly interacting massive particles (WIMPs). We here report on the analysis of almost seven years Fermi Gamma-Ray Space Telescope data of Triangulum-II which was taken during its all sky survey operation mode. No excess $γ$-ray emission has been detected above 100 MeV from Triangulum-II. We derive the upper limits on $γ$-ray flux assuming both the power-law spectra and the spectra related to WIMP annihilation. In this work, we have considered several theoretical WIMP (neutralinos here) models envisioning both thermal and non-thermal production of WIMPs, and put limits on pair-annihilation cross-section of WIMPs to constrain the parameter space related to those theoretical models.

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