arXiv subjects

Shantanu Desai

Publications and source records attributed to Shantanu Desai.

At least 55 records · Page 3Linked to original sources

arXiv2025

How stable are the impact factors of major astrophysics journals

We examine the long-term trends in impact factor over the past decade (2015-2014) for seven flagship journals widely used in Astrophysics: ApJ, AJ, ApJL, ApJS,A &A, JCAP, and MNRAS. We check the variation of both the traditional impact factor as well as the median-based impact factor, which we had studied in a previous work. We find that ApJS exhibits the largest drop in impact factor from 14.9 to 9.0, while ApJ and MNRAS show a steady decline from 6.7 to 5.2 and 5.6 to 5.0, respectively. However, the impact factors for AJ and ApJL have shown a steady increase from 4.55 to 5.35 and 6.4 to 9.7, respectively. The median-based impact factors are much more stable showing variations of at most $\pm 1$ within the last decade.

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arXiv2025

Constraints on Dark Energy Models Using Late Universe Probes

We use late Universe probes - Type Ia Supernovae from the PantheonPlus compilation, Quasars, and Dark Energy Spectroscopic Instrument (DESI) Data Release 1 (DR1) BAO data - along with Cosmic Chronometers or Megamasers to constrain various dark energy parameterizations. These include the standard $Λ$CDM model ($w_0=-1,w_a=0$), as well as the Chevallier-Polarski-Linder (CPL), Barboza-Alcaniz (BA), Jassal-Bagla-Padmanabhan (JBP), Exponential (EXP), and Transitional Dark Energy (TDE) parameterizations. We find that across all parameterizations, the constrained values of $w_0$ and $w_a$ remain within $(1-2)σ$ of the standard $Λ$CDM model, irrespective of spatial curvature, dataset combinations, or prior choices. We find from Bayesian model comparison that $Λ$CDM remains the most favored model for both flat and non-flat cases, with results remaining robust under different priors. Across all dataset combinations, we reaffirm the fact that LRG1 and LRG2 data points from the DESI BAO dataset are responsible for driving the preference for dynamical dark energy.

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arXiv2025

Comparison of $R_h=ct$ and $Λ$CDM using DESI DR1 measurements

We use DESI DR1 BAO measurements of the ratio of tranvserse comoving distance to Hubble distance in order to test the compatibility of $R_h=ct$ over flat $Λ$CDM. For this purpose, we used Bayesian model selection to evaluate the efficacy of these models given the observed data. When we consider the BAO measurements up to redshift of 1.3, both models are equally favored. However, when we consider the Lyman-$α$ QSO measurement at redshift of 2.33, we find Bayes factors of greater than 100 for flat $Λ$CDM over $R_h=ct$ using two different priors for $Ω_m$, indicating that $Λ$CDM is decisively favored over $R_h=ct$. The same is the case when we combine all the measurements. Therefore, the DESI DRI measurements rule out $R_h=ct$ cosmology, albeit this is driven by the Lyman-$α$ QSO measurement at $z=2.33$.

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arXiv2025

The Indian Pulsar Timing Array Data Release 2: I. Dataset and Timing Analysis

The Indian Pulsar Timing Array (InPTA) employs unique features of the upgraded Giant Metrewave Radio Telescope (uGMRT) to monitor dozens of the International Pulsar Timing Array (IPTA) millisecond pulsars (MSPs), simultaneously in the 300-500 MHz and the 1260-1460 MHz bands. This dual-band approach ensures that any frequency-dependent delays are accurately characterized, significantly improving the timing precision for pulsar observations, which is crucial for pulsar timing arrays. We present details of InPTA's second data release that involves 7 yrs of data on 27 IPTA MSPs. This includes sub-banded Times of Arrival (ToAs), Dispersion Measures (DM), and initial timing ephemerides for our MSPs. A part of this dataset, originally released in InPTA's first data release, is being incorporated into IPTA's third data release which is expected to detect and characterize nanohertz gravitational waves in the coming years. The entire dataset is reprocessed in this second data release providing some of the highest precision DM estimates so far and interesting solar wind related DM variations in some pulsars. This is likely to characterize the noise introduced by the dynamic inter-stellar ionised medium much better than the previous release thereby increasing sensitivity to any future gravitational wave search.

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arXiv2025

Search for transient gamma-ray emission from magnetar flares using Fermi-LAT

We search for transient gamma-ray emission in the energy range from 0.1-10 GeV using data from the Fermi-LAT telescope in coincidence with magnetar flares. For our analysis, we look for coincidence with 15 distinct flares from 11 magnetars using three time windows of $\pm$ 1 day, $\pm$ 7 days, and $\pm$ 15 days. For 14 of these flares from 10 magnetars, we do not see any statistically significant gamma-ray emission. However, we see one possible gamma-ray flare from one magnetar, namely 1E 1048.1-5937, with combined significance of $4.4σ$, (after considering the look-elsewhere effect) observed after about 10 days from the start of the X-ray flare. However, this magnetar is located close to the galactic plane (with galactic latitude of -0.52\degree), and this signal could be caused by contamination due to diffuse flux from gamma-ray sources in the galactic plane.

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arXiv2025

Determination of Hubble constant from Megamaser Cosmology Project using Profile Likelihood

The Megamaser Cosmology Project inferred a value for the Hubble constant given by $H_0=73.9 \pm 3.0 $ km/sec/Mpc. This value was obtained using Bayesian inference by marginalizing over six nuisance parameters, corresponding to the velocities of the megamaser galaxy systems. We obtain an independent estimate of the Hubble constant with the same data using frequentist inference. For this purpose, we use profile likelihood to dispense with the aforementioned nuisance parameters. The frequentist estimate of the Hubble constant is given by $H_0=73.5^{+3.0}_{-2.9}$ km/sec/Mpc and agrees with the Bayesian estimate to within $0.2σ$, and both approaches also produce consistent confidence/credible intervals. Therefore, this analysis provides a proof-of-principle application of profile likelihood in dealing with nuisance parameters in cosmology, which is complementary to Bayesian analysis.

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arXiv2025

Constraints on Lorentz Invariance Violation from Gamma-ray Burst rest-frame spectral lags using Profile Likelihood

We reanalyze the spectral lag data for 56 Gamma-Ray Bursts (GRBs) in the cosmological rest frame to search for Lorentz Invariance Violation (LIV) using frequentist inference. For this purpose, we use the technique of profile likelihood to deal with the nuisance parameters, corresponding to a constant time lag in the GRB rest frame and an unknown intrinsic scatter, while the parameter of interest is the energy scale for LIV ($E_{QG}$). With this method, we do not obtain a global minimum for $χ^2$ as a function of $E_{QG}$ up to the Planck scale. Thus, we can obtain one-sided lower limits on $E_{QG}$ in a seamless manner. Therefore, the 95\% c.l. lower limits which we thus obtain on $E_{QG}$ are then given by: $E_{QG}\geq 2.07 \times 10^{14}$ GeV and $E_{QG}\geq 3.71\times 10^{5}$ GeV, for linear and quadratic LIV, respectively.

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arXiv2025

A Stacked Analysis of GeV Gamma-Ray Emission from SPT-SZ Galaxy Clusters with 16 Years of Fermi-LAT Data

We report a statistically significant detection of cumulative $γ$-ray emission from a stacked sample of SPT-SZ selected galaxy clusters using 16.4 years of data from the Fermi Gamma-Ray Space Telescope's Large Area Telescope (LAT). By analyzing a population of clusters with individual Test Statistic (TS) values $<$ 9.0, we identify a robust cumulative signal with a TS of 75.2, which corresponds to approximately $8.4σ$ significance. In contrast, clusters with TS $<$ 4.0 yield a weaker cumulative signal of TS equal to 9.65 (2.65$σ$), consistent with background fluctuations. The derived $γ$-ray spectrum is well characterized by a power law model with a best-fit spectral index of $-2.59 \pm 0.20$ and an integrated flux of $1.67^{+1.35}_{-1.07} \times 10^{-11}$ ph cm$^{-2}$ s$^{-1}$. The high TS values in the full sample are likely driven by AGN-dominated clusters. However, when clusters with TS between 4 and 9 are excluded, the remaining low-TS population shows a stacked signal consistent with hadronic emission.

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arXiv2025

A meta-analysis of impact factors of astrophysics journals

We calculate the 2024 impact factors for the 38 most widely used journals in Astrophysics, using the citations collated by NASA/ADS (Astrophysics Data System) and compare them to the official impact factors. This includes journals which publish papers outside of astrophysics such as PRD, EPJC, Nature, etc. We also propose a new metric to gauge the impact factor based on the median number of citations in a journal and calculate the same for all the journals. We find that the ADS-based impact factors are mostly in agreement, albeit higher than the official impact factors for most journals. The journals with the maximum fractional difference in median-based and old impact factors are JHEAP and PTEP. We find the maximum difference between the ADS and official impact factor for Nature.

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arXiv2025

Effect of Peak Absolute Magnitude of Type Ia Supernovae and Sound Horizon Values on Hubble Tension using DESI results

We apply data-motivated priors on the peak absolute magnitude of Type Ia supernovae ($M$) and the sound horizon at the drag epoch ($r_d$), to study how the $M-r_d$ degeneracy affects low redshift measurements of the Hubble constant, and then compare these estimates to the Planck estimated value of the Hubble constant. We use the data from Pantheon$+$, Cosmic Chronometers, and the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1) Baryon Acoustic Oscillations (BAO) results for this purpose. We reaffirm the fact that there is a degeneracy between $M$ and $r_d$, and modifying the $r_d$ values to reconcile the discrepancy in Hubble constant values also requires a change in the peak absolute magnitude $M$. For certain $M$ and $r_d$ priors, the discrepancy is found to reduce to be as low as (1.2-2) $σ$ when considering the spatially flat $Λ$CDM model. We also notice that for our datasets considered, the Gaussian prior combination of $M \in \mathcal{N} (-19.253,0.027)$ (obtained from SH0ES) and $r_d \in \mathcal{N} (147.05,0.3)$ Mpc (determined from Planck CMB measurements) is least favored as compared to other prior combinations for the $Λ$CDM model.

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arXiv2025

Search for spatial coincidence between IceCube neutrinos and gamma-ray bright red dwarfs

We search for a spatial coincidence between high energy neutrinos detected by the IceCube neutrino detector and ten red dwarfs which have been observed in gamma-rays. For our analysis, we use the unbinned maximum likelihood method to look for a statistically significant excess. We do not find any such spatial association between any of the red dwarfs and Icecube-detected neutrinos. Therefore, we conclude that none of the gamma-ray bright red dwarfs contribute to the diffuse neutrino flux measured by IceCube.

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arXiv2025

Constraint on Lorentz Invariance Violation for spectral lag transition in GRB 160625B using profile likelihood

We reanalyze the spectral lag data for GRB 160625B using frequentist inference in order to constrain the energy scale ($E_{QG}$) of Lorentz Invariance Violation (LIV). For this purpose, we use profile likelihood to deal with the astrophysical nuisance parameters. This is in contrast to Bayesian inference implemented in previous works, where marginalization was carried out over the nuisance parameters. We show that with profile likelihood, we do not find a global minimum for $χ^2$ as a function of $E_{QG}$ below the Planck scale for both linear and quadratic models of LIV, whereas bounded credible intervals were previously obtained using Bayesian inference. Therefore, we can set one-sided lower limits in a straightforward manner. We find that $E_{QG} \geq 2.55 \times 10^{16}$ GeV and $E_{QG} \geq 1.85 \times 10^7$ GeV at 95\% c.l., for linear and quadratic LIV, respectively. Therefore, this is the first proof-of-principles application of profile likelihood method to the analysis of GRB spectral lag data to constrain LIV.

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arXiv2025

Gravitational physics in the context of Indian astronomy: A vision document

Contributions from the Indian gravity community have played a significant role in shaping several branches of astronomy and astrophysics. This document reviews some of the most important contributions and presents a vision for gravity research in the context of astronomy \& astrophysics in India. This is an expanded version of one of the chapters in the recently released Vision Document of the Astronomical Society of India.

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arXiv2024

Improving DM estimates using low-frequency scattering-broadening estimates

A pulsar's pulse profile gets broadened at low frequencies due to dispersion along the line of sight or due to multi-path propagation. The dynamic nature of the interstellar medium makes both of these effects time-dependent and introduces slowly varying time delays in the measured times-of-arrival similar to those introduced by passing gravitational waves. In this article, we present an improved method to correct for such delays by obtaining unbiased dispersion measure (DM) measurements by using low-frequency estimates of the scattering parameters. We evaluate this method by comparing the obtained DM estimates with those, where scatter-broadening is ignored using simulated data. A bias is seen in the estimated DMs for simulated data with pulse-broadening with a larger variability for a data set with a variable frequency scaling index, $α$, as compared to that assuming a Kolmogorov turbulence. Application of the proposed method removes this bias robustly for data with band averaged signal-to-noise ratio larger than 100. We report the measurements of the scatter-broadening time and $α$ from analysis of PSR J1643$-$1224, observed with upgraded Giant Metrewave Radio Telescope as part of the Indian Pulsar Timing Array experiment. These scattering parameters were found to vary with epoch and $α$ was different from that expected for Kolmogorov turbulence. Finally, we present the DM time-series after application of this technique to PSR J1643$-$1224.

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arXiv2024

Calibration of luminosity correlations of gamma-ray bursts using quasars

In order to test the efficacy of Gamma-ray Bursts (GRB) as cosmological probes, we characterize the scatter in the correlations between six pairs of GRB observables, which have previously also been studied in arXiv:2011.14040. However, some of these observables depend on the luminosity distance, for which one needs to assume an underlying cosmological model. In order to circumvent this circularity problem, we use X-ray and UV fluxes of quasars as distance anchors to calculate the luminosity distance in a model-independent manner, which in turn is used to calculate the GRB-related quantities. We find that all six pairs of regression relations show high intrinsic scatter for both the low and high redshift sample. This implies that these GRB observables cannot be used as model-independent high precision cosmological probes.

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arXiv2024

The ORT and the uGMRT Pulsar Monitoring Program : Pulsar Timing Irregularities & the Gaussian Process Realization

The spin-down law of pulsars is generally perturbed by two types of timing irregularities: glitches and timing noise. Glitches are sudden changes in the rotational frequency of pulsars, while timing noise is a discernible stochastic wandering in the phase, period, or spin-down rate of a pulsar. We present the timing results of a sample of glitching pulsars observed using the Ooty Radio Telescope (ORT) and the upgraded Giant Metrewave Radio Telescope (uGMRT). Our findings include timing noise analysis for 17 pulsars, with seven being reported for the first time. We detected five glitches in four pulsars and a glitch-like event in PSR J1825-0935. The frequency evolution of glitch in pulsars, J0742-2822 and J1740-3015, is presented for the first time. Additionally, we report timing noise results for three glitching pulsars. The timing noise was analyzed separately in the pre-glitch region and post-glitch regions. We observed an increase in the red noise parameters in the post-glitch regions, where exponential recovery was considered in the noise analysis. Timing noise can introduce ambiguities in the correct evaluation of glitch observations. Hence, it is important to consider timing noise in glitch analysis. We propose an innovative glitch verification approach designed to discern between a glitch and strong timing noise. The novel glitch analysis technique is also demonstrated using the observed data.

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arXiv2024

Search for GeV gamma-ray emission from SPT-CL J2012-5649 with six years of DAMPE data

We search for gamma-ray emission from the galaxy cluster SPT-CL J2012-5649 in the energy range from 3 GeV to 1 TeV using the DArk Matter Particle Explorer (DAMPE) telescope. For our analysis, we use three different templates: point source, radial disk, and radial Gaussian. We do not detect a signal with significance $>3σ$ for any of these templates at any location within $R_{200}$ of the cluster center. We obtain 95\% C.L. upper limit on the energy flux ranging between $\sim 10^{-6}$ and $10^{-4} \rm{MeV~cm^{-2}~s^{-1}}$ depending on the energy range. These upper limits are consistent with a previously reported non-zero flux detected by Fermi-LAT at $6σ$ significance. This work represents the first proof of principle search for gamma-ray emission from a single galaxy cluster using DAMPE data.

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arXiv2024

A test of MOND and Emergent Gravity with SMACS J0723.3-7327 using eROSITA observations

We implement a test of MOND and Verlinde's Emergent Gravity using the galaxy cluster SMACS J0723-7327, which has been recently imaged using the eROSITA X-ray telescope as well as with JWST. We test MOND using two independent methods. The first method involves comparing the dynamical MOND mass and baryonic mass, while the second method entails a comparison of the MOND-estimated temperature with the observed temperature. We then compare the unseen mass predicted by Emergent Gravity with the estimated dark matter mass. We find that MOND shows a mass discrepancy in the central regions at high significance levels. The observed temperature profile is in marginal disagreement with that in the MOND paradigm. However, the Emergent Gravity Theory agrees in accurately accounting for the dynamical mass in the inner regions within $1σ$. Our results are qualitatively consistent with the earlier tests on other clusters.

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