arXiv subjects

Aman Upadhyay

Publications and source records attributed to Aman Upadhyay.

4 recordsLinked to original sources

arXiv2026

Discovery of a Candidate 2 keV Cyclotron Resonance Scattering Feature in the HLX NGC 3583 X-1

We present a broadband X-ray study of the transient hyperluminous X-ray source (HLX), 2SXPS J111416.1+481833, in the galaxy NGC 3583, using archival XMM-Newton, NuSTAR, Chandra data, and long-term Swift/XRT monitoring. The source episodically enters the hyperluminous regime with X-ray luminosities $L_X > 10^{41}$ erg s$^{-1}$ and drops by a factor of $>45$ from its peak into a deep low state. We detect a clear spectral cutoff at $\sim$5-6 keV in the broadband spectra, which are well modeled by a soft thermal component combined with optically thick thermal Comptonization or an inner advection-dominated disk. In the XMM-Newton spectra, we detect a statistically significant ($\gtrsim 3.9 σ$) absorption line centered at $E_{\rm line} \approx 1.97 \pm 0.04$ keV with a width of $σ_{\rm line} \approx 74 \pm 40$ eV. We primarily interpret the line as a candidate proton Cyclotron Resonance Scattering Feature (CRSF), implying a local magnetic field strength of $B \sim 4 \times 10^{14}$ G. Alternative interpretations, such as an origin in an ionized outflow, were explored and found to be less likely. We do not detect coherent X-ray pulsations, placing 90% confidence upper limits on the pulsed fraction of 19.3% in the 0.3-10 keV band and 36.3% in the 3-15 keV band. The combination of extreme luminosity, a hard spectral state, and the detection of a candidate cyclotron line provides strong evidence for a highly magnetized neutron star accretor.

astro-ph.HE↗

arXiv2026

Probing spectral variability in NGC 4490 ULX-8 over 24 years of XMM-Newton, Chandra and Swift-XRT observations

We present a spectral variability study of the ultraluminous X-ray source NGC 4490 ULX-8 based on 14 Chandra, 6 XMM-Newton and 19 Swift-XRT observations obtained between 2000 and 2024. The X-ray spectra are modelled using absorbed power-law and absorbed multicolour disc blackbody models. The best-fit photon indices span 0.9-2.7, while the inferred inner disc temperatures lie in the range 1.0-1.6 keV. We detect pronounced long-term variability in the unabsorbed X-ray luminosity on multi-year timescales, while variability within individual observations is comparatively modest. A Hardness-Intensity Diagram of the source shows no clear transition between hard and soft states; however, two recent observations taken on 2022 December 1 and 2024 May 4 show a sharp increase in brightness. The spectra across all observations are dominated by smooth, single-component curvature in the 0.3-10 keV band, consistent with the broadened-disc regime of ultraluminous X-ray sources. A correlation analysis reveals a weak positive X-ray luminosity-photon index trend that remains statistically supported after controlling for related degeneracies, indicating that it is not driven solely by fitting covariance. The luminosity-inner disc temperature relation is only weakly constrained, but remains compatible, within uncertainties, with both thin-disc and slim-disc scalings. Using disc parameters derived from higher-quality XMM-Newton spectra, we obtain model-dependent estimates of the characteristic inner disc radius and compact-object mass as functions of inclination and spin. The reported results are consistent with a stellar-mass black hole accretor operating at or near the Eddington limit.

astro-ph.HE↗

arXiv2025

A Study of Spectral Variability between flaring and non-flaring state in M74 X-1

We conducted an extensive long-term spectral and timing study of the ultraluminous X-ray source (ULX) M74 X-1, using data taken between 2001 and 2021 by Chandra and XMM-Newton X-ray observatories. Our analysis shows that flares are present in some observations, whereas they are absent in others. Flaring state exhibits two-component spectra at a lower average flux level, whereas the non-flaring state displays single-component spectra at a higher average flux level. The M74 X-1 spectra are best described by the combination of accretion disk and Comptonization components, a dual thermal disk blackbody model, and a modified multi-temperature disk blackbody model. Using the dual thermal disk blackbody model, we obtain cool and hot temperatures of $T_{in}$ (cool) = $0.38^{+0.08}_{-0.06}$ keV and $T_{in}$ (hot) = $1.67^{+0.18}_{-0.13}$ keV, respectively, suggesting two temperature emitting regions and indicating possible presence of outflowing wind along with the accretion disk. We found a Gaussian feature at $E_{line}$ = $0.96^{+0.05}_{-0.11}$ keV with $σ$ = $0.11^{+0.13}_{-0.06}$ keV in the spectra of the flaring state which can be interpreted as the unresolved wind feature in the system when compared to similar feature seen in other ULXs. Plotting the hardness luminosity diagram, we get a trend of increasing hardness with luminosity, suggesting the presence of geometrical beaming in a low-inclination system. Additionally, using the hot disk blackbody component from the dual thermal disk blackbody model, we estimate the mass of the compact object to be M = $7.1^{+1.4}_{-1.3}$ M$_\odot$, classifying it as a stellar-mass black hole and confirming super-Eddington accretion in the system.

astro-ph.HE↗

arXiv2024

Complete Optimal Non-Resonant Anomaly Detection

We propose the first-ever complete, model-agnostic search strategy based on the optimal anomaly score, for new physics on the tails of distributions. Signal sensitivity is achieved via a classifier trained on auxiliary features in a weakly-supervised fashion, and backgrounds are predicted using the ABCD method in the classifier output and the primary tail feature. The independence between the classifier output and the tail feature required for ABCD is achieved by first training a conditional normalizing flow that yields a decorrelated version of the auxiliary features; the classifier is then trained on these features. Both the signal sensitivity and background prediction require a sample of events accurately approximating the SM background; we assume this can be furnished by closely related control processes in the data or by accurate simulations, as is the case in countless conventional analyses. The viability of our approach is demonstrated for signatures consisting of (mono)jets and missing transverse energy, where the main SM background is $Z(νν) +\text{jets}$, and the data-driven control process is $γ+\text{jets}$.

hep-ph↗