arXiv subjects

Pulkit Ojha

Publications and source records attributed to Pulkit Ojha.

3 recordsLinked to original sources

arXiv2025

Radiation pressure instability: from heart-beat states in black hole binary systems to Quasars and Changing-Look AGN

Radiation-pressure instability was identified soon after the seminal classical accretion disk models of Shakura-Sunyaev and Novikov-Thorne, yet its full implications remain an active area of investigation. These models form the backbone of our understanding of accretion onto compact objects and successfully describe the phenomenology of black hole and neutron star X-ray binaries, as well as luminous active galactic nuclei (AGN), in the regime of high mass accretion rates. At luminosities approaching a significant fraction of the Eddington limit (L/LEdd > 0.1), standard thin disks are predicted to become thermally unstable due to the dominance of radiation pressure. This prediction has found empirical support in several Galactic stellar-mass black hole systems, where the instability manifests as quasi-periodic, large-amplitude luminosity oscillations, so-called "heartbeat states", and has been proposed as a driver of observed signatures of deterministic chaos in accretion-driven light curves. The scope of radiation-pressure-induced variability extends beyond stellar-mass black holes: both black holes across mass scales and accreting neutron stars can exhibit related behavior, though the presence of a boundary layer in neutron stars adds complexity and offers a unique laboratory for testing the interplay between accretion dynamics and the central object. On extragalactic scales, the instability has been invoked to explain the duty cycles and apparent short lifetimes of radio-loud AGN, as well as the dramatic spectral-state transitions seen in Changing-Look AGN. (...)

astro-ph.HE↗

arXiv2025

Long-term evolution of Sco X-1: implications for the current spin frequency and ellipticity of the neutron star

Sco X-1 is the brightest observed extra-solar X-ray source, which is a neutron star (NS) low-mass X-ray binary (LMXB), and is thought to have a strong potential for continuous gravitational waves (CW) detection due to its high accretion rate and relative proximity. Here, we compute the long-term evolution of its parameters, particularly the NS spin frequency ($\nu$) and the surface magnetic field ($B$), to probe its nature and its potential for CW detection. We find that Sco X-1 is an unusually young ($\sim7\times10^6$ yr) LMXB and constrain the current NS mass to $\sim 1.4-1.6~{\rm M}_\odot$. Our computations reveal a rapid $B$ decay, with the maximum current value of $\sim 1.8\times10^8$ G, which can be useful to constrain the decay models. Note that the maximum current $\nu$ value is $\sim 550$ Hz, implying that, unlike what is generally believed, a CW emission is not required to explain the current source properties. However, $\nu$ will exceed an observed cut-off frequency of $\sim 730$ Hz, and perhaps even the NS break-up frequency, in the future, without a CW emission. The minimum NS mass quadrupole moment ($Q$) to avoid this is $\sim (2-3)\times10^{37}$ g cm$^2$, corresponding to a CW strain of $\sim 10^{-26}$. Our estimation of current $\nu$ values can improve the CW search sensitivity.

astro-ph.HE↗

arXiv2024

Long-term evolution of spin and other properties of neutron star low-mass X-ray binaries: implications for millisecond X-ray pulsars

A neutron star (NS) accreting matter from a companion star in a low-mass X-ray binary (LMXB) system can spin up to become a millisecond pulsar (MSP). Properties of many such MSP systems are known, which is excellent for probing fundamental aspects of NS physics when modelled using the theoretical computation of NS LMXB evolution. Here, we systematically compute the long-term evolution of NS, binary and companion parameters for NS LMXBs using the stellar evolution code MESA. We consider the baryonic to gravitational mass conversion to calculate the NS mass evolution and show its cruciality for the realistic computation of some parameters. With computations using many combinations of parameter values, we find the general nature of the complex NS spin frequency ($\nu$) evolution, which depends on various parameters, including accretion rate, fractional mass loss from the system, and companion star magnetic braking. Further, we utilize our results to precisely match some main observed parameters, such as $\nu$, orbital period ($P_{\rm orb}$), etc., of four accreting millisecond X-ray pulsars (AMXPs). By providing the $\nu$, $P_{\rm orb}$ and the companion mass spaces for NS LMXB evolution, we indicate the distribution and plausible evolution of a few other AMXPs. We also discuss the current challenges in explaining the parameters of AMXP sources with brown dwarf companions and indicate the importance of modelling the transient accretion in LMXBs as a possible solution.

astro-ph.HE↗