arXiv subjects

Shantanu Desai

Publications and source records attributed to Shantanu Desai.

At least 19 recordsLinked to original sources

arXiv2026

Probing cosmic anisotropy with galaxy clusters and supernovae

Using $Λ$CDM and Padé-(2,1) cosmography, we study directional variations in the Hubble constant, $H_0$, using galaxy clusters (GC) and Type Ia Supernovae (SNe), from the PantheonPlus compilation, employing the hemisphere decomposition method. Since there is a degeneracy between $H_0$ and absolute magnitude $M_B$ for SNe, Cepheid-host calibration is usually required to constrain $H_0$. To complement the Cepheid-host calibration in the SNe analysis, we use GC X-ray scaling relations as a complementary probe of anisotropy. We consider five redshift bins on the full SNe sample $[z_\mathrm{min}, 0.5]$ where $z_\mathrm{min}= \{0.015, 0.03, 0.05, 0.1, 0.2\}$. Our results show that the combined GC sample is consistent with statistical isotropy and that there is a $\sim 2.5 σ$ anisotropy preference in the lowest-redshift bin in the SNe-only case. This preference weakens when low-redshift SNe are progressively removed. In the SNe$+$GC analysis, we find that any anisotropy preference is driven by how the $H_0-M_B$ and the $H_0-k$ (where $k$ is the normalization parameter in the GC scaling relation) degeneracies are treated. The low-redshift bin anisotropy preference in the SNe-only analysis persists in the SNe$+$GC case when Cepheid-host calibration is used while the results are consistent with statistical isotropy when $k$ is fixed. Overall, our results do not provide compelling evidence for a statistically significant anisotropy in the local expansion rate. Our findings highlight the importance of combining complementary cosmological probes and examining the dependence of anisotropy constraints on calibrations and parameter degeneracies when assessing claims of cosmic anisotropy.

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arXiv2026

Bayesian and frequentist constraints on dark energy parameterizations using non-CMB data

Data from the Dark Energy Spectroscopic Instrument (DESI) BAO combined with the cosmic microwave background (CMB) have provided evidence for dynamical dark energy (DDE) with a phantom crossing ($w(z) < -1$) when constraining $w_0-w_a$ parameterizations. Given the tension between BAO and CMB data within $Λ$CDM, we use only non-CMB data: Type Ia Supernovae (SNe) from the DES-Dovekie compilation, DESI BAO DR2, Gamma-Ray Bursts (GRBs) and Redshift-Space-Distortion (RSD) measurements. We use the dataset combinations GRB$+$BAO, BAO$+$SNe and GRB$+$BAO$+$SNe, each with RSD. We perform both Bayesian and frequentist analyses of two quintessence-inspired thawing dark energy parameterizations: the thawing quintessence parameterization (TQD) and the Algebraic Thawing parameterization (AT), together with the Chevallier-Polarski-Linder (CPL) and the Generalized Emergent Dark Energy parameterizations (GEDE). The GRB sample, used in this work, contributes only marginally to the constraints, while SNe tighten them considerably. The CPL equation of state shows a phantom crossing at $z\sim 0.5$, while TQD and AT improve the best-fit $χ^2$ over $Λ$CDM, without showing phantom crossing. When SNe are included, TQD is preferred by the Bayes factor (moderately), AIC and DIC (positively). For CPL and AT, AIC and DIC prefer the extended model positively, while the Bayes factor weakly prefers $Λ$CDM over CPL and is inconclusive for AT. This difference is consistent with the Occam penalty from the prior volume in the Bayes factor. GEDE, with the DE density restricted to be suppressed at high redshifts is disfavored by all three metrics. Hence, the preference for DDE depends on the parameterization, the dataset and the comparison metric used.

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arXiv2026

Search for high-energy neutrinos from Einstein Probe detected Fast X-ray Transients using 14 years of IceCube muon track data

We perform a time-integrated search for high energy neutrinos from 27 fast X-ray transients (FXTs) detected by Einstein Probe, using 14 years of publicly available IceCube muon-track data. We employ the unbinned maximum likelihood method which is used for IceCube point source searches. We do not find any statistically significant excess from any of the FXTs. The largest TS value is obtained for EP240801a (TS=4.77), corresponding to a local $p$-value of 0.04, which is not significant. We therefore calculate 90\% confidence level upper limit on the differential muon neutrino flux. The most stringent upper limit is obtained for EP241021a, with $E^2 Φ(E)=2.1\times 10^{-10}$ (GeV cm$^{-2}$ s$^{-1}$), for a $E^{-2}$ spectrum at a pivot energy of 100 TeV.

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arXiv2026

Study of pulse jitter characteristics of PSR J1744-1134: Towards high precision pulsar timing with InPTA

Variations in the shapes of individual pulses introduce a broadband noise component in pulsar times of arrival beyond what is expected from radiometer noise, referred to as the jitter noise. This noise limits the timing accuracy and, consequently, the sensitivity required for the detection of nanohertz gravitational waves using pulsar timing arrays. Accurate characterization of jitter noise, including its amplitude and frequency dependence, is therefore essential for each pulsar. In this work, we present the first detailed study of jitter noise in PSR J1744-1134 over the frequency range 300-500 MHz, using observations from the Indian Pulsar Timing Array experiment conducted with the upgraded Giant Metrewave Radio Telescope. We detect clear signatures of jitter in this pulsar through pulse-level analysis and Bayesian inference. Furthermore, we investigated the frequency dependence of the jitter noise and compared our results with earlier measurements at higher frequencies. The observed differences in jitter amplitude suggest a possible frequency evolution, highlighting the importance of low-frequency studies for improving pulsar timing precision and accuracy. We also investigated the effect of jitter on the high precision dispersion measure derived from our observations and found that jitter is not the limiting factor for DM precision achieved for this pulsar in the given observation timescale.

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arXiv2026

Search for Gamma-ray emission from Abell 119 galaxy cluster using INTEGRAL/ISGRI, COMPTEL, and DAMPE data

We present a comprehensive search for non-thermal high-energy $γ$-ray emission from the nearby merging galaxy cluster Abell~119 ($z=0.044$) using archival observations spanning over seven decades in photon energy. Our analysis combines hard X-ray data from INTEGRAL/ISGRI (30--100~keV), MeV $γ$-ray observations from COMPTEL (0.75--30~MeV), and GeV--TeV data from the DArk Matter Particle Explorer (DAMPE; 3~GeV--1~TeV). No statistically significant emission is detected at the cluster position in any energy band. In the hard X-ray regime, we derive a $3σ$ upper limit of $F_{30-100\,\mathrm{keV}} \lesssim 8.5 \times 10^{-11}$~erg~cm$^{-2}$~s$^{-1}$ from ISGRI mosaic imaging. Reanalysis of archival COMPTEL data yields 95\% confidence-level upper limits ranging from $\sim 9 \times 10^{-11}$ to $\sim 8 \times 10^{-10}$~erg~cm$^{-2}$~s$^{-1}$ across 0.75--30~MeV. In the GeV--TeV range, DAMPE \rthis{places upper limits on the} differential energy flux to $\sim 10^{-12}$--$10^{-10}$~erg~cm$^{-2}$~s$^{-1}$ (95\% confidence level). These results provide independent multi-band constraints on the reported GeV excess from recent Fermi-LAT studies. While our DAMPE limits do not exclude the flux levels claimed in those analyses, the absence of confirmation across keV--MeV--GeV bands indicates that any non-thermal emission from Abell~119 remains tentative.

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arXiv2026

Limit on high energy neutrino emission from Abell 119 using IceCube 10-year muon track data

We carry out a search for high energy muon neutrino emission from the galaxy cluster Abell 119, motivated by a recent tentative detection of GeV gamma-ray emission from this cluster using the Fermi-LAT telescope, which hinted at a hadronic origin. For this purpose, we used the 10-year muon track data from 2008-2018, provided by the IceCube Collaboration and implement the unbinned maximum likelihood method. We do not find any statistically significant excess and the observed value of the test statistic is consistent with a null result. We then obtain upper limits (at 90\% confidence level) on the differential muon neutrino energy flux from this cluster, whose value is equal to $1.2 \times 10^{-9}~\mathrm{GeV}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$ at a pivot energy of 100 TeV, after assuming a neutrino spectral index of 2.0. The neutrino flux expected under the hadronic interpretation is, however, approximately an order of magnitude smaller than our flux limit, and hence the hadronic model cannot be ruled out based on our upper limit. Among the next generation neutrino detectors, IceCube-Gen2 and TRIDENT neutrino detector should be able to confirm or rule out a hadronic origin for the gamma-ray emission in Abell 119 with 10 years of exposure.

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arXiv2026

A comprehensive assessment of weak-lensing inferred circular velocity profiles of isolated galaxies

We fit the circular velocity data, derived from weak lensing observations of isolated galaxies selected from the KiDS survey in four baryonic mass bins, using three different dark matter profiles. These include NFW, Burkert, and pseudo-isothermal profile. We find that the NFW and Burkert profiles cannot adequately fit the circular velocity data for all the four baryonic mass bins, whereas only the pseudo-isothermal profile can fit the data for all the four bins.

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arXiv2026

Determination of neutron star radius from pulse profile modeling using profile likelihood

In recent years, NICER data have been extensively used to determine neutron star masses and radii using pulse profile modeling. Pulse profile modeling is implemented with the {\tt X-PSI} package and the best-fit parameters are typically obtained using Bayesian inference. Using simulated data, we demonstrate the first ever application of frequentist inference to determine the neutron star radius, where the nuisance parameters are treated using profile likelihood. We find that the profile likelihood technique can recover the true radius to $< 1σ$. The uncertainty in the estimated radius is also comparable to that obtained from Bayesian analysis while being computationally much faster. Therefore, this work serves as a proof-of-principle application of frequentist inference to estimate the neutron star radius using pulse profile modeling and complements the Bayesian inference technique currently used. We have also made our analysis codes for frequentist inference using {\tt X-PSI} publicly available.

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arXiv2026

Galaxy Morphology Classification: Uncertainty Modeling and Out of Distribution Detection

We present a comprehensive framework for galaxy morphology classification that combines enhanced ``out-of-distribution (OOD)'' detection with improved uncertainty quantification. Using ``Galaxy Zoo DECaLS'', we trained a ResNet-34 architecture under three configurations: standard cross-entropy loss as a baseline, IsoMaxPlus loss function for OOD detection, and hybrid IsoMaxPlus+MonteCarlo Dropout for enhanced uncertainty quantification. IsoMaxPlus replaces the conventional SoftMax logits with distance-based class representations, preserving inter-class separability and enabling reliable OOD detection, without requiring additional architectural modifications or hyperparameter tuning. Coalescing IsoMaxPlus with MC Dropout provides a fast Bayesian approximation by performing multiple stochastic forward passes during inference. Our results show that IsoMaxPlus substantially improves OOD detection, increasing TNR@TPR95 by nearly $90\%$ relative to the cross-entropy baseline, while maintaining a competitive accuracy of 97\% across nine morphological classes. Additionally, with MC Dropout, the model yields more stable predictions and a reduction in calibration error, providing more reliable uncertainty estimates. The Expected Calibration Error (ECE) is reduced from $0.0095$ to $0.0026$ ($\approx73\%$) for IsoMaxPlus and $0.0033$ ($\approx65\%$) when combined with MC Dropout, compared to the baseline. Misclassified or underconfident predictions exhibit higher predictive entropy and lower minimum distance scores, providing an interpretable metric for identifying unreliable predictions. These methods are well-suited for current and upcoming large-scale surveys, where reliable automated morphological classification and awareness of uncertainty are essential for identifying rare or previously unseen galaxy morphologies.

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arXiv2026

Which is the best day of the week to submit to arXiv:astro-ph?

We examine whether the citations received by refereed astronomy papers vary systematically with the day of the week of their arXiv preprint release. We use NASA ADS citation data for all refereed articles published in 2020--2023 in six journals (MNRAS, ApJ, ApJL, ApJS, A\&A and JCAP; 37,173 papers with astro-ph primary preprints), combined with version-1 submission timestamps from the arXiv API. We group papers both by submission weekday and by the day their announcement batch appears on arXiv, accounting for arXiv's weekday deadline and lack of weekend announcements. Given the heavy skew of citation distributions, our primary statistic is the median citations per group, with bootstrap confidence intervals. Papers submitted on weekends, all of which appear in the Monday listing, have median citations significantly lower than weekday submissions in five of the six journals, by 12--26 \%; the sixth (JCAP) shows the same direction, but not significantly, consistent with its small weekend sample. Median citations are indistinguishable across the five weekdays. These results extend the recently reported weekend citation disadvantage in particle and nuclear physics to astronomy. The link is correlational, consistent with either compositional variations in weekend submissions or reduced visibility in the enlarged Monday listings.

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arXiv2026

Can the Efron-Petrosian Method Recover the Inverse-Square Distance Law for Simulated Radio Pulsar Fluxes?

We test whether the Efron-Petrosian (E-P) method can recover the inverse-square law dependence of the radio pulsar flux, using a synthetic catalog generated according to the specifications of the Parkes Multi-beam survey using the {\tt PsrPopPy} software. We find that the E-P method cannot automatically reproduce the inverse-square scaling law for the radio pulsar flux, even though this scaling is built into the synthetic data by construction, except over a narrow range of flux thresholds, and even here we don't get pristine agreement. The main reason for the deviation is that the synthetic radio pulsar catalog is truncated based on a cut on the pulsar signal to noise ratio (SNR), which has a non-linear dependence on the flux along with considerable scatter. We show that the disagreement is exacerbated as we raise the SNR threshold. We then demonstrate that if we create a synthetic catalog based on a flux cut (instead of an SNR-based threshold), we can recover the true distance exponent, with an accuracy ranging from pristine agreement to within $\pm 1 σ$ depending on the chosen flux threshold. Therefore, the E-P method cannot be used to determine the correct scaling of radio pulsar flux with distance. Our results also demonstrate that the E-P method breaks down when the detection threshold scales non-linearly with flux along with scatter.

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arXiv2026

Profile Reconstruction from Temporally Stable Emission Components for Timing PSR J1713+0747

The assumption of long-term pulse-profile stability underpins high-precision pulsar timing and forms the basis of pulsar timing array experiments. However, several millisecond pulsars exhibit temporal profile variability that can introduce systematic biases in pulse time of arrival measurements and compromise timing precision. We present a profile-domain analysis of PSR J1713+0747 at low radio frequencies, in the 300-500 MHz band, using upgraded GMRT observations for the Indian Pulsar Timing Array experiment. We model frequency-resolved pulse profiles using a Bayesian Gaussian decomposition framework in which individual Gaussian components are associated with persistent emission regions through informative phase priors that permit modest temporal variations. By tracking the evolution of the decomposed components across observing epochs and frequency sub-bands, we identify central Gaussian components that remain precisely localized despite changes in the integrated pulse morphology. We then reconstruct pulse profiles with realistic noise using these central components and perform timing analysis. Our approach provides a physically motivated framework for mitigating pulse-profile variability and offers a generic methodology for recovering robust timing information from pulsars exhibiting profile evolution.

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arXiv2026

The Indian Pulsar Timing Array Data Release 2: III. Search for a Stochastic Gravitational Wave Background

We present the first independent search for an isotropic stochastic gravitational wave background in the second data release of the Indian Pulsar Timing Array, comprising of 27 millisecond pulsars monitored simultaneously in two frequency bands with the upgraded Giant Metrewave Radio Telescope over a maximum 7.2 year baseline. Building on a comprehensive single pulsar noise analysis, we search for a common uncorrelated red noise process within a Bayesian inference framework and with the noise-marginalized optimal statistics, and we test the robustness of the result through per-pulsar dropout analyses and solar-wind exclusion cuts. Leaving the spectral index free, we recover a broad amplitude posterior, $\log_{10} A_{\rm CURN} = -13.71^{+1.06}_{-3.28}$, with an unconstrained spectral index $γ_{\rm CURN} = 2.98^{+3.62}_{-2.70}$ and a Savage-Dickey Bayes factor of $2.5$ for a common red process over the no signal model. The optimal-statistic signal to noise ratios for the monopole, dipole, and Hellings-Downs correlations are all consistent with zero. Fixing the spectral index to $γ= 13/3$, the value predicted by an idealized toy model in which the background is sourced by a population of supermassive black hole binaries in circular orbits evolving purely under leading-order gravitational radiation reaction, we place a $95\%$ upper limit on the common-process amplitude of $A_{\rm GWB} < 3.4\times10^{-14}$, stable across solar elongation cuts of $10^\circ$, $20^\circ$, and $30^\circ$. This limit lies approximately an order of magnitude above the amplitudes reported by other, longer-running pulsar timing array experiments. We also demonstrate through simulated datasets with the addition of simple chromatic and achromatic noise components that it will take at least a 10 year baseline to start recovering the common red noise signal.

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arXiv2026

Impact of CMB low-$\ell$ EE polarization data on dark energy parameterizations

Measurement of the optical depth to reionization ($τ_\mathrm{reio}$) is largely driven by the large-scale EE polarization data of CMB ($\ell<30$). Removing the low-$\ell$ EE data potentially alleviates various cosmological tensions. In this work, we study the effect of the low-$\ell$ EE polarization measurements on the CPL, JBP and BA dark energy parameterizations using CMB data from Planck and ACT DR6, combined with DESI BAO and PantheonPlus compilation of Type Ia supernovae. We find that excluding low-$\ell$ EE data shifts $τ_\mathrm{reio}$ and $A_s$ to higher values through the unbroken $A_s-τ_\mathrm{reio}$ degeneracy with a $\sim(1.4-1.8)σ$ shift in $A_s$ for $Λ$CDM and JBP, and a milder $\sim 1σ$ shift for CPL and BA. The equation of state (EOS) for all three parameterizations moves towards the quintessence regime ($w(z) > -1$) upon exclusion of low-$\ell$ EE data, driven primarily by the strengthening of the $w_a-A_s$ and the $w_a-τ_\mathrm{reio}$ correlations, with a small effect from the correlations of $w_0$ with $A_s$ and $τ_\mathrm{reio}$. The most prominent effect occurs in JBP, where the EOS lies entirely within the quintessence regime at $1σ$ when excluding low-$\ell$ EE data. Model comparison through AIC shows positive evidence in favor of CPL and BA and weak evidence in favor of JBP, robust to the inclusion of low-$\ell$ EE data and CMB data used. DIC model comparison shows strong evidence in favor of CPL and BA when low-$\ell$ EE data is included, and positive evidence when it is excluded. For JBP, we get positive (weak) evidence when including (excluding) low-$\ell$ EE data over $Λ$CDM.

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arXiv2026

A MINOT-based Study of Gamma-ray emission from SPT-CL J2012-5649/Abell 3667

We present an analysis of the non-thermal properties of the merging galaxy cluster SPT-CL J2012-5649/Abell~3667 ($z = 0.0556$, $M_{500} = 7.16 \times 10^{14}\ M_\odot$) using the MINOT non-thermal emission modeling framework. The predicted hadronic gamma-ray flux from $pp$ interactions in the $1$--$300\ \mathrm{GeV}$ band is $2.82 \times 10^{-11}\ \mathrm{cm^{-2}\ s^{-1}}$ within $R_{500}$, rising to $1.15 \times 10^{-10}\ \mathrm{cm^{-2}\ s^{-1}}$ at the truncation radius ($3.7\,R_{500}$), in broad agreement with the Fermi-LAT reported flux of $1.3 \times 10^{-10}\ \mathrm{cm^{-2}\ s^{-1}}$. Approximately $76\%$ of the predicted hadronic flux originates from beyond $R_{500}$. The Inverse Compton contribution from cosmic-ray electrons is subdominant relative to the hadronic $π^0$-decay gamma-ray component by a factor of ${\sim}20$ in the $1$--$300\ \mathrm{GeV}$ energy band, and therefore does not contribute significantly to the observable signal. The best fit of the observed Fermi-LAT data is achieved with a hadronic emission assuming a proton spectrum $\propto E^{-3.5}$ and a proton energy density almost in equipartition with the thermal energy density. Even though those parameters may look extreme, they could well be explained by the particle acceleration by low Mach number accretion shocks.

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arXiv2026

Search for GeV gamma-ray emission from PSZ G181.06+48.47 galaxy cluster using Fermi-LAT data

We present a search for high energy gamma rays in the energy range from 1--300~GeV from the galaxy cluster PSZ2~G181.06+48.47 using 17.9~years of Fermi-LAT data. A binned likelihood analysis employing the 16-year source catalog reveals a significant $γ$-ray excess at the cluster position. Modelling the emission as a point source yields a detection with $\mathrm{TS}=19.0 (4.3 σ)$, a photon index of $Γ=3.20 \pm 0.62$, and an integrated photon flux of $(9.75\pm2.74)\times10^{-11}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, but leaves statistically significant residual emission at the cluster center. Replacing the point-source hypothesis with a RadialGaussian spatial template significantly improves the fit, with a preferred width of $σ=0.4^{\circ}$ and an extension significance of $\mathrm{TS}_{\rm ext}=36.8 (6.0σ)$. The adopted Gaussian model yields a source detection significance of $\mathrm{TS}=55.0 (7.4σ)$, a photon index of $Γ=2.62\pm0.26$, and an integrated photon flux of $(3.22\pm0.50)\times10^{-10}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, while reducing the residual emission at the cluster position to a level consistent with zero. The cluster spectral energy distribution shows significant emission only in the lowest energy interval (1.0--3.13~GeV), while all higher-energy bins are consistent with upper limits, indicating a soft $γ$-ray spectrum with no evidence for emission above $\sim5$~GeV. These results provide strong evidence that the $γ$-ray emission associated with PSZ2~G181.06+48.47 is spatially extended on a scale of $σ\approx0.4^{\circ}$ and is more consistent with diffuse intracluster emission than with a single unresolved point source.

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arXiv2026

A Search for Hard X-ray/Soft $γ$-ray Emission from SPT-CL J2012-5649 (Abell 3667) Using INTEGRAL/ISGRI

We present a search for hard X-ray/soft $γ$-ray emission from the merging galaxy cluster SPT-CL J2012-5649 (Abell~3667) using archival INTEGRAL/ISGRI observations. This cluster located at $z=0.0556$ hosts prominent radio relics associated with merger-driven shocks, suggesting the presence of relativistic electrons capable of producing inverse-Compton (IC) emission in the hard X-ray to soft $γ$-ray regime. We searched for emission in the 30--300~keV energy range using the INTEGRAL Off-line scientific analysis software with a total effective exposure of 2817~s. No significant emission was detected at the cluster position in the aforementioned energy interval. The extracted ISGRI spectrum is consistent with pure background, and no physically meaningful model parameters can be constrained. From the mosaic variance maps, we derive a $3σ$ upper limit of $F_{30-300\,\mathrm{keV}} < 3.63 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. This limit rules out bright IC scenarios and constrains the efficiency of merger-driven particle acceleration in SPT-CL J2012-5649. Our results provide the most stringent soft $γ$-ray constraint on this system to date and highlight the need for next-generation hard X-ray missions, such as HEX-P or eXTP, to probe IC emission at theoretically predicted levels in merging clusters.

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arXiv2026

Probing dipole and quadrupole anisotropy in Gamma-ray bursts from Swift dataset

Testing the validity of the cosmological principle's assumption of large-scale isotropy remains crucial for modern cosmology. We investigate the angular distributions of gamma-ray bursts using the GRB catalog from Neil Gehrels Swift Observatory (Swift) for an independent probe of isotropy. Using the HEALPix spherical harmonic decomposition, we estimate the dipole and quadrupole amplitudes and compare them against the null hypothesis obtained from 500 isotropic Monte Carlo realizations. Our results show 2.9$σ$ dipole and 7.2$σ$ quadrupole amplitude when applied to the raw data. To account for observational biases, we then create an exposure map using the pointing history, roll angle, and the partial coding fraction of the Swift Telescope. Reevaluating the null hypothesis using this map reduces the significance of these anisotropies to less than $1σ$. Therefore, our findings confirm statistical isotropy of the GRB sky using the Swift data, consistent with previous studies. We have also made the Swift exposure map publicly available.

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