arXiv subjects

Karan Akbari

Publications and source records attributed to Karan Akbari.

7 recordsLinked to original sources

arXiv2026

A Cross-Band (X-ray $\times$ Optical) Periodicity Search for Supermassive Black Hole Binaries: A Null Result and the First Completeness-Corrected Constraint

We present the first sample-level search for supermassive black hole binaries (SMBHBs) requiring coherent quasi-periodicity at a common period in the X-ray and optical bands, over 1194 Swift-BAT hard X-ray AGN (Stage 1) and 175 4XMM-DR14 AGN (Stage 2). No source is a co-periodic candidate. Each light curve is modelled as a damped random walk (DRW) and searched with a Lomb-Scargle periodogram and a look-elsewhere-corrected Monte-Carlo significance. Because DRW red noise is largely independent between corona and disc, we require both bands individually significant with periods coincident within 5%, and gate the survivors with the model-independent null-signal-template test of Robnik et al. (2024). Over $P=100$-$900$ d the completeness-corrected 95% upper limit on the co-periodic fraction is amplitude-dependent: $\lesssim 3\%$ for hard-X-ray fractional modulation $\gtrsim 0.3$, $\approx 15\%$ (precision-limited) at 0.2, and uninformative below $\sim 0.15$ (the $\epsilon=1$ floor is 0.25%). The sensitivity is set by the hard X-ray monitoring, not the optical photometry or the statistics, the opposite of the usual assumption. Daily MAXI and RXTE/ASM monitoring of the brightest AGN raises the X-ray completeness 5-8-fold, and the search remains null. Integrated over the BAT black-hole mass function, the expected all-amplitude co-periodic fraction is $\sim 3\times10^{-2}\, f_{\rm bin}\, \delta_{\rm mod}$ (modulo a factor $g<1$), with $f_{\rm bin}$ the sub-pc binary fraction, $\delta_{\rm mod}$ the modulating duty cycle, and $g$ the fraction reaching recoverable hard-X-ray amplitude, so a null is expected. We deliver a validated cross-band framework and the first completeness-corrected constraint on the co-periodic fraction, ready for the denser X-ray monitoring of Einstein Probe and eROSITA.

astro-ph.HE↗

arXiv2026

Importance and Science Outcomes from the first XSPECT/XPoSat Workshop

This paper summarizes the science outcomes of the first Workshop on Data Analysis using observations from the XSPECT payload onboard the XPoSat, which brought together early-career researchers and experts to explore the instrument's scientific capabilities through lectures and hands-on analyses. Participants performed end-to-end data analysis, including calibration, spectral modeling, and timing studies, on seven sources comprising Neutron Star Low-Mass X-ray Binaries, pulsars, and Black Hole X-ray Binaries, demonstrating the instrument's scientific potential. The observations, obtained during the first year of XSPECT operations, together with in-house developed software, were provided to the participants, making them the first users outside the instrument team to analyze XSPECT data. For NS-LMXBs, Aql X-1 exhibited a classical Type-I X-ray burst, enabling constraints on the stellar radius through spectral fitting. Sco X-1, observed across its complete Z-track, revealed systematic spectral evolution driven by accretion-rate fluctuations and disk-corona coupling, while Cir X-1 displayed orbital phase-dependent transitions between hard and soft states, reflecting changes in accretion geometry. Among accretion-powered pulsars, GX 301-2 showed a double-peaked, energy-dependent pulse profile and strong iron fluorescence lines due to stellar wind reprocessing, whereas Vela X-1 exhibited orbital phase-dependent absorption and steady coronal temperatures. Among BH-XRBs, Cyg X-1 transitioned from a hard to soft-intermediate state with increasing disk contribution and spectral softening, while Cyg X-3 remained in the intermediate state with multiple emission lines originating from a clumpy stellar wind. The workshop outcomes highlight the scientific promise of XSPECT and the importance of collaborative training in maximizing the science from XSPECT and future Indian space astronomy missions.

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arXiv2026

A Calibration Audit of a Gaia XP White-Dwarf Main-Sequence Binary Catalog: How Much the BP-Band Residual Contaminates at Its Bound and Above

A Gaussian-process classifier on Gaia DR3 XP spectra produced $\sim$30,000 white-dwarf main-sequence binary candidates (Li et al. 2025), with probabilities but no likelihood. The corrected spectra agree with external libraries to better than 2% below 400 nm and 1% redward (Huang et al. 2024), and that residual sits where a hot white dwarf also adds flux. We asked whether it contaminates the selection at its bound. It does, weakly. The spurious rate runs about 1.7 times the baseline there. Put into the blue band, 2% of total flux is a median 55% local excess across our MS templates, 27 times that number read locally. We inject a blue taper clipped to that bound, under the templates' published per-pixel errors, with a $\Delta\chi^2$ threshold standing in for the classifier. The spurious rate is $0.09\pm0.02$ across 20 seeds on a 0.053 baseline, 19 of 20 above it. The unclipped 2% taper, above the redward bound, gives $0.11\pm0.03$. At that taper the amortized posterior's 90% coverage falls to 0.85 from a clean 0.88 on the flat-model leg, with PIT uniformity rejected at $p<10^{-5}$ in all four seeds. The selection fails in bulk only above a 10-15% local excess, and the rate reaches 0.96 near 50%. A model-comparison gate certifies binarity only above a white-dwarf flux share near 0.05. At share 0.01, near the catalog's median of 0.0096, the $\Delta\chi^2$ preference inverts at a 20% local excess. Outside Huang's color-magnitude box, 40% of the catalog, the post-correction residual is not measured.

astro-ph.IM↗

arXiv2026

Circumstellar Disc and X-ray Variability in the Be/X-ray Binary SXP 5.05 During its 2024 Outburst

Be/X-ray binaries provide a unique opportunity to study the interaction between neutron stars and circumstellar discs. SXP 5.05 is a particularly rare system, exhibiting eclipse-like X-ray variability attributed to obscuration by the Be star disc rather than a simple stellar eclipse. Motivated by its unusual geometry and the well-studied 2013 outburst, we present a multiwavelength analysis of its 2024 outburst using NICER X-ray observations and long-term optical monitoring from OGLE. The X-ray light curve shows a declining outburst with lower peak intensity and shorter duration compared to 2013, indicating a reduced accretion episode. The spectral evolution, characterized through hardness ratios, reveals a transition from a soft, high-intensity state to a harder, low-intensity state. Coherent pulsations near 5.05 s are detected throughout the observations, with properties consistent with previous measurements. The optical light curves show a reduced variability amplitude relative to 2013, possibly from a less extended or less dense circumstellar disc. Orbital-phase-folded optical profiles reveal a persistent, phase-locked dip structure, indicating a stable non-axisymmetric disc component that evolves across outburst phases. Together, these results support a picture in which the observed variability is driven by changes in disc structure and viewing geometry. SXP 5.05 thus remains a key system for probing the time-dependent properties of Be star discs through combined X-ray and optical observations.

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arXiv2026

Misspecification in amortized X-ray spectral inference: a detection benchmark, a gain shift three schemes cannot see, and marginalizing it out

Amortized neural posterior estimation fits X-ray spectra in milliseconds, and pipeline use will lean on post-hoc checks to catch detector systematics the simulator misses. We measure what those checks see, on simulated spectra from real XMM-Newton EPIC-pn and NICER responses. Of four misspecification families, an unmodelled 6.4 keV line and partial covering are catchable per spectrum (AUC 0.97 and 0.84). A wrong continuum family reaches 0.66. One flow passed every recovery check yet under-covered (coverage deviation 0.114), and a converged NICER flow (0.011) fails simulation-based calibration on all five parameters. Recovery metrics do not certify calibration. Three schemes catch the line: the posterior-predictive check, importance sampling against the exact Poisson likelihood (effective sample size, 0.68-0.82), and the nested-sampling evidence (count-controlled, $-67$ to $-892$ nats). None catches the 3 per cent gain shift, 2.4 to 14.4 times the EPIC-pn energy-scale accuracy (AUC 0.44-0.54; 36-cell means 0.50 and 0.49 on both responses; paired evidence $+0.33 \pm 1.37$ nats). The effective sample size stays null-consistent from 0.1 to 10 per cent, realistic amplitudes included. The shift still biases the photon index by $+0.02$ (0.03 to 0.11$\sigma$) at both count levels. Retraining with a per-simulation nuisance gain adds no inference-time cost, and the gain posterior returns the prior. The bias survives, and exact gain-marginalized nested sampling shows the flow capturing at most 0.163 of its gain constraint (continuous-prior convention) at high counts. Marginalization over posited calibration nuisances is the working defence for X-ray SBI at catalog scale; no check we ran supplies a warning.

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arXiv2026

Probing heartbeat oscillations from the black hole X-ray binary GRS 1915+105 using spectral-timing analysis

GRS 1915+105 is a black hole X-ray binary whose $\rho$-class ("heartbeat") oscillations ($\sim$50--100 s) are attributed to radiation-pressure instabilities in the inner accretion disk at near-Eddington luminosities. We present a phase-resolved spectral and timing analysis of 24 Swift XRT observations (1--10 keV) and broadband AstroSat SXT+LAXPC data (0.8--30 keV), dividing each cycle into five phases. The narrow-band XRT fits show an apparent anti-correlation between the inner disk temperature ($T_{\rm in} \sim 1.7$--$1.5$ keV) and apparent radius ($R_{\rm in} \sim 22$--$38$ km) across the cycle. The broadband AstroSat fits, however, are statistically consistent with a constant disk temperature: a joint fit with $T_{\rm in}$ tied across all five phases gives $T_{\rm in} = 1.275 \pm 0.020$ keV ($\chi^2_\nu = 1.003$; $\Delta\chi^2 = +4.3$ for 4 added constraints), whereas tying the disk normalization as well is rejected ($\Delta\chi^2 = +175.9$), leaving a $\sim$20% variation in apparent $R_{\rm in}$ ($18.1 \pm 0.7$ to $21.9 \pm 0.7$ km). The coronal electron temperature rises from $\sim$6 to $\sim$14.5 keV approaching the burst, with the photon index tracking it. We attribute the larger XRT disk swings to its limited bandpass, where coronal Comptonization is unconstrained and the disk parameters absorb coronal variability; the dominant variability is therefore coronal, consistent with Vadawale et al. (2001), and the residual $R_{\rm in}$ change is plausibly a color-correction effect (Zoghbi et al. 2016). Hardness--intensity and color--color diagrams show clear spectral hysteresis. Our broadband coverage provides a phase-resolved test of disk constancy and favors coronal evolution as the driver of the spectral variability across the $\rho$ cycle.

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arXiv2025

A continuous transition from Type-C Quasi Periodic Oscillations to the Heartbeat state in the Black hole X-ray binary 4U 1630-47

We present a timing analysis of the black hole X-ray binary (BHXRB) 4U 1630-47 using AstroSat observations from 10-19 March 2023, for the first time capturing a rare and rapid transition in variability properties. Within less than a day, the source evolved from a type-C quasi-periodic oscillation (QPO) state, with centroid frequencies between 3-5 Hz, to the Heartbeat state, characterized by a broad peak in the power density spectrum at ~25 mHz, corresponding to a ~40 s modulation period. As the source evolved, it passed through a transition track where the QPO features weakened and ultimately disappeared in the Heartbeat state. In the hardness-intensity Diagram, the QPOs occur at higher hardness and lower intensity, followed by a brightening phase as the source moved towards the soft intermediate state, and finally reached the Heartbeat state through a transition towards lower hardness. In the power-color diagram, this transition is marked by a clear shift to a distinct region of power color space, separate from the range occupied by other observed states. This work establishes 4U 1630-47 as another system, apart from GRS 1915+105, where a continuous transition from QPO to Heartbeat state has been observed. Notably, 4U 1630-47 is the only system where the QPO is absent during the heartbeat state. This provides us with another probe to understand the physical mechanism governing this transition and the overall accretion mechanism in BHXRBs.

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