arXiv subjects

Amit Pathak

Publications and source records attributed to Amit Pathak.

At least 19 recordsLinked to original sources

arXiv2026

The Cosmic Ultraviolet Background at the Galactic Poles

We have used archival GALEX data to separate the cosmic ultraviolet background at the Galactic Poles into two components: the dust scattered light and an offset. We have modeled the dust-scattered light using a single- scattering model finding 1 sigma limits of 0.54 -- 0.71 for the albedo (a) and 0.74 -- 0.83 for the phase function asymmetry factor (g) at 1530 {\AA} and 0.66 -- 0.73 for a and 0.71 -- 0.77 for g at 2360 {\AA}, that is, the grains are moderately reflective and highly forward-scattering. The offsets are 277 -- 284 photon units at 1530 {\AA} and 513 -- 520 photon units at 2360 {\AA}. We have estimated other Galactic and extragalactic contributors to the offset finding that 161 +- 18 photon units is unaccounted for at 1530 {\AA} and 335 +- 38 at 2360 {\AA}. The offsets are constant over these regions with variances of 20 -- 30 photon units.

astro-ph.GA↗

arXiv2026

Relativistic Scattering in the Funnel of Cygnus X-3

Cygnus X-3 presents significant challenges to standard accretion models. Recent polarimetric observations by IXPE reveal high polarization degrees (PD) in the hard state ($\sim 23\%$) and unexpectedly significant polarization in the soft state ($\sim 12\%$), which are difficult to reconcile with static scattering models at low inclination ($i \approx 30^\circ$). We present a relativistic scattering model within a funnel-shaped geometry that resolves this discrepancy. We show that a single funnel-outflow configuration with variable bulk velocity $\beta$ can reproduce both polarization states, with lower velocities ($\beta \approx 0$) yielding $\sim 12\%$ polarization (soft state) and mildly relativistic velocities ($\beta \lesssim 0.4$) producing $\sim 23\%$ polarization (hard state) at $i \approx 30^\circ$ for half funnel opening angles of $\sim 13^\circ$-$16^\circ$. Relativistic aberration modifies the effective scattering angle in the comoving frame, enhancing polarization in the hard state while recovering the static limit in the soft state. The model also yields a consistent estimate of the intrinsic luminosity, of order $\sim 10^{40}$ erg s$^{-1}$, supporting a super-Eddington interpretation. This framework provides a unified explanation of the observed polarization properties of Cygnus X-3.

astro-ph.HE↗

arXiv2026

Search for new physics with baryons at BESIII

The BESIII experiment at BEPCII provides a clean environment to test baryon-dark sector connections and baryon-number violation. Using $10^{10}$ $J/\psi$ events, searches were performed for $\Sigma^{+}\to p+\text{inv}$, $\Xi^{-}\to\pi^{-}+\text{inv}$, and $\bar{\Lambda}-\Lambda$ oscillations. No excess was observed, yielding $\mathcal{B}(\Sigma^{+}\to p+\text{inv})<3.2\times10^{-5}$, $\mathcal{B}(\Xi^{-}\to\pi^{-}+\text{inv})<(4-7)\times10^{-5}$, and $P(\Lambda)<4.4\times10^{-6}$ (90\% C.L.). These are the most stringent limits on invisible baryon decays and $\Delta B=2$ transitions in strange baryons, probing new physics beyond the Standard Model.

hep-ex↗

arXiv2025

PDRs4All XIX. The 6 to 9 $\mu$m region as a probe of PAH charge and size in the Orion Bar

Infrared emission from polycyclic aromatic hydrocarbons (PAHs) play a major role in determining the charge balance of their host environments that include photo-dissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds. We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR. We employ JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program ''PDRs4All'' and synthetic images in the JWST MIRI filters. We investigate the spatial morphology of the AIBs at 6.2, 7.7, 8.6, and 11.0 $\mu$m that commonly trace PAH cations, and the neutral PAH-tracing 11.2 $\mu$m AIB, their (relative) correlations, and the relationship with existing empirical prescriptions for AIBs. The 6.2. 7.7, 8.6, 11.0, and 11.2 $\mu$m AIBs are similar in spatial morphology, on larger scales. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 $\mu$m vs. the 6.2 and 7.7 $\mu$m AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 $\mu$m AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 $\mu$m AIB is carried by large, compact, cationic PAHs, and the 11.0 $\mu$m AIB's correlation to it implies, so is this band. The 6.2/8.6 and 7.7/8.6 PAH band ratios thus probe PAH size. We conclude that the 6.2/11.2 AIB ratio is the most reliable proxy for charged PAHs, within the cohort. We outline JWST MIRI imaging prescriptions that serve as effective tracers of the PAH ionization fraction as traced. This study showcases the efficacy of the 6-9 $\mu$m AIBs to probe the charge state and size distribution of the emitting PAHs, offering insights into the physical conditions of their host environments. JWST MIRI photometry offers a viable alternative to IFU spectroscopy for characterizing this emission in extended objects.

astro-ph.GA↗

arXiv2025

Information Extraction from Visually Rich Documents using LLM-based Organization of Documents into Independent Textual Segments

Information extraction (IE) from Visually Rich Documents (VRDs) containing layout features along with text is a critical and well-studied task. Specialized non-LLM NLP-based solutions typically involve training models using both textual and geometric information to label sequences/tokens as named entities or answers to specific questions. However, these approaches lack reasoning, are not able to infer values not explicitly present in documents, and do not generalize well to new formats. Generative LLM-based approaches proposed recently are capable of reasoning, but struggle to comprehend clues from document layout especially in previously unseen document formats, and do not show competitive performance in heterogeneous VRD benchmark datasets. In this paper, we propose BLOCKIE, a novel LLM-based approach that organizes VRDs into localized, reusable semantic textual segments called $\textit{semantic blocks}$, which are processed independently. Through focused and more generalizable reasoning,our approach outperforms the state-of-the-art on public VRD benchmarks by 1-3% in F1 scores, is resilient to document formats previously not encountered and shows abilities to correctly extract information not explicitly present in documents.

cs.IR↗

arXiv2025

Spectral Analysis of Cyg X-3 using simultaneous AstroSat and Insight-HXMT Observations

We present the results from the spectral analysis of Cygnus X-3 using simultaneous data from AstroSat and Insight-HXMT during its soft state. A pure reflection spectrum, including emission lines of iron, silica, and sulfur, provides a good fit to the spectra. Orbital phase-resolved analysis shows no significant spectral parameter variations, except for the normalization. Leveraging IXPE polarization results, we model the funnel-shaped geometry and estimate scattered flux and observed polarization for various funnel parameters and observer inclinations. We consider two scenarios: reflection from the funnel walls and scattering by gas within the funnel. Our results reconfirm previous findings, showing that reflection can produce a high polarization degree (PD) of 23$\%$, but not a low PD of 12$\%$. Conversely, scattering can produce a PD of 10-12$\%$, but not as high as 23$\%$ for a fixed observer inclination of $30^\circ$. Scattering results align with previous findings without absorption, but with absorption, PD drops significantly with increasing funnel opening angle. Thus, we can identify common funnel parameters that can produce the different observed PDs in the soft and hard states. The intrinsic luminosity of the source was estimated by comparing the results from a plane disk and the funnel model, to be $\sim7$ $\times$ 10$^{40}$ erg/s for 12$\%$ PD (scattering) and $\sim5$ $\times$ 10$^{41}$ erg/s for 23$\%$ PD (reflection). However, for the reflection model, the luminosity may decrease to $\sim$ 10$^{40}$ erg/s when the 23$\%$ PD observed is taken as a lower limit.

astro-ph.HE↗

arXiv2025

From Public Square to Echo Chamber: The Fragmentation of Online Discourse

This paper examines how social media algorithms and filter bubbles contribute to the fragmentation of online discourse, fostering ideological divides and undermining shared understanding. Drawing on Michael Sandels philosophical emphasis on community and shared values, the study explores how digital platforms amplify discrimination discourse including sexism, racism, xenophobia, ableism, homophobia, and religious intolerance during periods of heightened societal tension. By analyzing the dynamics of digital communities, the research highlights mechanisms driving the emergence and evolution of discourse fragments in response to real world events. The findings reveal how social media structures exacerbate polarization, restrict cross group dialogue, and erode the collective reasoning essential for a just society. This study situates philosophical perspectives within a computational analysis of social media interactions, offering a nuanced understanding of the challenges posed by fragmented discourse in the digital age.

cs.CY↗

arXiv2025

Research on the Interstellar Medium and Star Formation in the Galaxy: An Indian Perspective

Although the star formation process has been studied for decades, many important aspects of the physics involved remain unsolved. Recent advancement of instrumentation in the infrared, far-infrared and sub-millimetre wavelength regimes have contributed to a significantly improved understanding of processes in the interstellar medium (ISM) leading to star formation. The future of research on the ISM and star formation looks exciting with instruments like the JWST, ALMA, etc., already contributing to the topic by gathering high-resolution high-sensitivity data and with several larger ground- and space-bound facilities either being planned or constructed. India has a sizable number of astronomers engaged in research on topics related to the ISM and star formation. In this white paper invited by the Astronomical Society of India to prepare a vision document for Indian astronomy, we review the Indian contributions to the global understanding of the star formation process and suggest areas that require focused efforts both in creating observing facilities and in theoretical front in India, in order to improve the impact of our research in the coming decades.

astro-ph.GA↗

arXiv2024

Investigating C-D out-of-plane vibrational modes in PAHs as a tool to study interstellar deuterium-containing PAHs

Previous as well as recent observations by ISO, Spitzer, AKARI, SOFIA, JWST etc. have revealed various characteristics of mid-infrared emission bands between 3-20 micron. Subsequently, several forms of organics including Polycylic Aromatic Hydrocarbons (PAHs)/PAH-like molecules are proposed as carriers for these bands. Deuterated PAH (PAD) is one such substituted PAH, which is proposed as a potential candidate carrier for weak emission bands at 4.4 and 4.65 micron, detected towards few astronomical targets and are characteristics of aromatic and aliphatic C-D stretching modes in a PAD molecule, respectively. However, the 4.4 micron band is not widely detected. In order to validate PADs as carriers for mid-infrared emission bands, an additional alternative tool is crucial. If PAHs are deuterated, they should also possess an inherent signature from the C-D out-of-plane (C-Doop) vibrations, which are at the longer wavelength side. In this report, features due to C-Doop modes in PAHs bearing a single to multiple deuterium atoms are reported by performing quantum-chemical calculations. This paper reports that some of the C-Doop vibrations appear at the 14-19 micron range. Also, the strength of C-Doop modes is not proportional to the D/H ratio in PAHs. In addition, a moderate change in the spectra of deuterated PAHs is observed from that of the undeuterated counterparts, as deuteration would alternate the adjacency class of the C-H bonds and the symmetry of the molecule. We discuss the efficiency and usefulness of these bands to constrain the form of PAHs emitting mid-infrared emission bands.

astro-ph.GA↗

arXiv2024

Theoretical Investigation of Interstellar 3-Pyrroline: Formation, Rotational and Vibrational Spectroscopy

The recent detection of CN-functionalized aromatics partly addresses the long-standing mystery of the apparent absence of five- and six-membered rings in interstellar environments. N-heterocycles, which are crucial as the fundamental structures of nucleobases, have been a focus of these aromatic searches due to their biological significance. Although N-heterocycles have not been conclusively detected in astrophysical environments, their presence in chondrites and meteorites signifies their interstellar and circumstellar connection. Precise spectral data identifies the unique signatures of molecules, confirming their presence in space. In this light, the present work reports an extensive computational investigation on interstellar 3-pyrroline; a five-membered ring N-heterocycle. This includes an alternative formation route in cold interstellar environments and highly accurate rotational and vibrational spectroscopy. The results indicate that 3-pyrroline can form on dust grain surfaces from vinyl cyanide, as its formation from pyrrole through double hydrogenation may lead to the formation of pyrrole itself via a H$_{2}$ abstraction process. 3-pyrroline's rotational transition at 52.3 GHz offers a potential tool for its detection in cold interstellar regions. Additionally, the strongest infrared features of 3-pyrroline at 16.09 $\mu$m and $\sim$3.50 $\mu$m are observable with JWST. The provided data is crucial for laboratory identification and future interstellar observations of 3-pyrroline at both radio and IR wavelengths.

astro-ph.GA↗

arXiv2022

A comprehensive rotational study of astronomical iso-pentane within 84 to 111 GHz

The rotational line survey by ALMA (Atacama Large Millimeter/submillimeter Array) recently revealed the presence of i-C3H7CN (i-PrCN) and n-C3H7CN (n-PrCN) in 3-mm atmospheric window between 84 to 111 GHz towards the hot core region Sagittarius B2(N) (Sgr B2(N)). This was the first interstellar detection of a linear straight chain molecule. In this light, we report the rotational spectra of C5H12 isomeric group in the same frequency range. We performed quantum chemical calculations for spectroscopic parameters. The pure rotational spectrum of the species has been simulated using the PGOPHER program. The rotational spectrum of this molecule makes it a good candidate for future astronomical detections since the radio lines can be calculated to very high accuracy in mm/sub-mm wave region.

astro-ph.GA↗

arXiv2022

Rotational spectra of interstellar N- and CN-PAHs: pyrene and coronene

The detection of benzonitrile (C6H5CN), 1- and 2-cyano-naphthalene (C10H7CN) in the cold, dark molecular cloud TMC-1 at centimetre (cm) wavelengths has opened up prospects for the detection of other N- and CN-containing polycyclic aromatic hydrocarbons (PAHs). In this light, the pure rotational spectra of N-pyrene (C15H9N), CN-pyrene (C15H9CN), N-coronene (C23H11N) and CN-coronene (C23H11CN) are reported here for the first time. The B3LYP/6-311+G(d,p) level of theory, in the Density Functional Theory (DFT) calculations, achieves the best performance for calculating the spectroscopic parameters and simulating the rotational spectra. The large permanent dipole moment of CN-PAHs makes them the most suitable PAH species for detection in the interstellar medium. Additionally, pyrene's smaller partition function makes CN-pyrene a prime candidate to be discovered in cold, dark molecular clouds such as the TMC-1. The present work sets a benchmark for theoretical rotational spectra of N- and CN-containing PAHs and may act as a guide for laboratory experiments and observational searches.

astro-ph.GA↗

arXiv2022

Astrochemical model to study the abundances of branched carbon-chain molecules in a hot molecular core with realistic binding energies

Straight-chain (normal-propyl cyanide, n - C3H7CN) and branched-chain (iso-propyl cyanide, i - C3H7CN) alkyl cyanides are recently identified in the massive star-forming regions (Sgr B2(N) and Orion). These branched-chain molecules indicate that the key amino acids (side-chain structures) may also be present in a similar region. The process by which this branching could propagate towards the higher-order (butyl cyanide, C4H9CN) is an active field of research. Since the grain catalysis process could have formed a major portion of these species, considering a realistic set of binding energies are indeed essential. We employ quantum chemical calculations to estimate the binding energy of these species considering water as a substrate because water is the principal constituent of this interstellar ice. We find significantly lower binding energy values for these species than were previously used. It is noticed that the use of realistic binding energy values can significantly change the abundance of these species. The branching is more favorable for the higher-order alkyl cyanides with the new binding energies. With the inclusion of our new binding energy values and one essential destruction reaction (i - C3H7CN + H -> CH3C(CH3)CN + H2, having an activation barrier of 947 K), abundances of t - C4H9CN dramatically increased.

astro-ph.GA↗

arXiv2022

Laboratory measurements of stretching band strengths of deuterated Quenched Carbonaceous Composites (D-QCC)

The observed large variation in the abundance of deuterium (D) in the interstellar medium (ISM) suggests that a significant fraction of D may be depleted into polycyclic aromatic hydrocarbons (PAHs). Signatures of deuteration of PAHs are expected to appear most clearly through C-D stretching modes at 4.4--4.7 micron, whose strengths in emission spectra relative to those of C-H stretching modes at 3.3--3.5 micron provide the relative abundance of D to hydrogen (H) in PAHs once we have accurate relative band strengths of both stretching modes. We report experimental results of the band strength of C-D stretching modes relative to C-H. We employ a laboratory analogue of interstellar carbonaceous dust, Quenched Carbonaceous Composite (QCC), and synthesize deuterated QCC (D-QCC) by replacing the starting gas of CH$_4$ of QCC by mixtures of CH$_4$ and CD$_4$ with various ratios. Infrared spectra of D-QCC are taken to estimate the relative band strengths of the stretching modes, while the D/H ratios in the D-QCC samples are measured with a nano-scale secondary ion mass spectrometer (NanoSIMS). We obtain that the relative strength of aromatic and aliphatic C-D to C-H stretches is 0.56 +/- 0.04 and 0.38 +/- 0.01 per D/H, respectively. The ratio for the aromatic stretches is in good agreement with the results of theoretical calculations, while that of aliphatic stretches is smaller than that of aromatic. The present results do not significantly change the D/H ratios in the interstellar PAHs previously estimated from observed spectra.

astro-ph.GA↗

arXiv2021

Theoretical study of infrared spectra of interstellar PAH molecules with N, NH & NH$_2$ incorporation

This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with incorporation of N, NH and NH$_2$ using density functional theory (DFT). The properties of their vibrational modes in 2--15 $\mu \rm m$ are investigated in relation to the Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated with NH$_2$ and N (at inner positions), produce intense infrared bands at 6.2, 7.7 and 8.6 $\mu \rm m$ that have been normally attributed to ionized PAHs so far. The present results suggest that strong bands at 6.2 and 11.2 $\mu \rm m$ can arise from the same charge state of some N-containing PAHs, arguing that there might be some N-abundant astronomical regions where the 6.2 to 11.2 $\mu \rm m$ band ratio is not a direct indicator of PAHs' ionization. PAHs with NH$_2$ and N inside the carbon structure show the UIR band features characteristic to star-forming regions as well as reflection nebulae (Class A), whereas PAHs with N at the periphery have similar spectra to the UIR bands seen in planetary nebulae and post-AGB stars (Class B). The presence of N atom at the periphery of a PAH may attract H or H$^{+}$ to form N-H and N-H$_2$ bonds, exhibiting features near 2.9--3.0 $\mu \rm m$, which are not yet observationally detected. The absence of such features in the observations constrains the contribution of NH and NH$_2$ substituted PAHs that could be better tested with concentrated observations in this range. However, PAHs with N without H either at the periphery or inside the carbon structure do not have the abundance constraint due to the absence of 2.9--3.0 $\mu \rm m$ features and are relevant in terms of positions of the UIR bands. Extensive theoretical and experimental studies are required to obtain deeper insight.

astro-ph.GA↗

arXiv2021

Evidence for coronal temperature variation in Seyfert 2 ESO 103--035 using NuSTAR observations

We report flux-resolved spectroscopic analysis of the active galactic nucleus (AGN) ESO 103--035 using \textit{NuSTAR} observations. Following an earlier work, we fit the spectra using a thermal Comptonization model with a relativistic reflection component to obtain estimates of the coronal temperature for two flux levels. The coronal temperature was found to increase from 24.0$^{+6.8}_{-3.4}$ to 55.3$^{+54.6}_{-7.2}$ keV (errors at 1-$\sigma$ confidence level) as the flux increased from $9.8$ to $11.9 \times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$ in the 3--78 keV band. A marginal variation in the high energy photon index allows for both, a non-varying optical depth and for the optical depth to have varied by a factor of $\sim$2. This is in contrast to a previous work on \textit{NuSTAR} flux resolved spectroscopy of the AGN, Ark 564, where the temperature was found to decrease with flux along with a $10$\% variation in the optical depth. The results maybe understood in a framework where AGN variability is either dominated by coronal heating variation leading to correlated increase of temperature with flux and the opposite effect being seen when the variability is dominated by changes in the seed photon flux.

astro-ph.HE↗

arXiv2021

Chemical complexity of phosphorous bearing species in various regions of the Interstellar medium

Phosphorus related species are not known to be as omnipresent in space as hydrogen, carbon, nitrogen, oxygen, and sulfur-bearing species. Astronomers spotted very few P-bearing molecules in the interstellar medium and circumstellar envelopes. Limited discovery of the P-bearing species imposes severe constraints in modeling the P-chemistry. In this paper, we carry out extensive chemical models to follow the fate of P-bearing species in diffuse clouds, photon-dominated or photodissociation regions (PDRs), and hot cores/corinos. We notice a curious correlation between the abundances of PO and PN and atomic nitrogen. Since N atoms are comparatively abundant in diffuse clouds and PDRs than in the hot core/corino region, PO/PN reflects < 1 in diffuse clouds, << 1 in PDRs, and > 1 in the late warm-up evolutionary phase of the hot core/corino regions. During the end of the post-warm-up phase, we obtain PO/PN > 1 for hot core and < 1 for its low mass analog. We employ a radiative transfer model to investigate the transitions of some of the P-bearing species in diffuse cloud and hot core regions and estimate the line profiles. Our study estimates the required integration time to observe these transitions with ground-based and space-based telescopes. We also carry out quantum chemical computation of the infrared features of PH3 along with various impurities. We notice that SO2 overlaps with the PH3 bending-scissoring modes around ~ (1000 - 1100) cm-1. We also find that the presence of CO2 can strongly influence the intensity of the stretching modes around ~ 2400 cm-1 of PH3 .

astro-ph.GA↗

arXiv2020

DFT study on interstellar PAH molecules with aliphatic side groups

Polycyclic Aromatic Hydrocarbon (PAH) molecules have been long adjudged to contribute to the frequently detected distinct emission features at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7 {\mu}m with weaker and blended features distributed in the 3-20 {\mu}m region. The comparatively weaker 3.4 {\mu}m emission feature has been attributed to have an aliphatic origin as carrier. PAH with aliphatic functional group attached to it is one of the proposed potential candidate carriers for the 3.4 {\mu}m emission band, however, the assignment of carrier is still enigmatic. In this work, we employ Density Functional Theory (DFT) calculation on a symmetric and compact PAH molecule; coronene (C24H12) with aliphatic side group to investigate any spectral similarities with observed features at 3-4 {\mu}m. The side groups considered in this study are -H (hydrogenated), -CH3 (methyl), -CH2-CH3 (ethyl) and -CH=CH2 (vinyl) functional groups. Considering the possible presence of deuterium (D) in PAHs, we also include D in the aliphatic side group to study the spectral behavior. We present a detailed analysis of the IR spectra of these molecules and discuss possible astrophysical implications.

astro-ph.GA↗