arXiv subjects

Sohan Ghodla

Publications and source records attributed to Sohan Ghodla.

10 recordsLinked to original sources

arXiv2025

Early Formation of Supermassive Black Holes via Dark Star Gravitational Instability

We show that dark stars, which are dark-matter-powered stars in the early universe, can grow by accretion to masses in the range $\mathscr{O}\left ({10}^4\right )-\mathscr{O}\left ({10}^7\right)\,{M_\odot}$ before the general-relativistic Feynman-Chandrasekhar instability causes their dynamical collapse to black holes. These accreting dark star configurations avoid standard stellar nuclear- and weak-interaction evolution that would lead to their demise long before they reached this supermassive size. Remarkably, this mechanism for supermassive black hole (SMBH) genesis is relatively robust to initial dark star mass, formation epoch, accretion rate and its history. The SMBHs produced this way can serve as seeds for even larger SMBHs $({\gtrsim}10^9\,M_\odot)$ that have been discovered at high redshift.

astro-ph.CO↗

arXiv2025

Reconstructing PTA measurements via early seeding of supermassive black holes

Motivated by recent findings that PTA's nHz signal may be dominated by supermassive black hole (SMBH) binaries ($M \gtrsim 10^9 M_\odot$), and high redshift quasar observations revealing unexpectedly massive SMBHs, we calculate the implications of early seeded SMBHs for the PTA signal. As an application, we explore two prominent scenarios of high-$z$ SMBHs seeding mechanisms: direct collapse black holes (DCBHs) and collapse of Dark Stars. We show that Dark Star seeded SMBHs, with comoving seed number density of $\mathcal{O}(10^{-3}) \, {\rm Mpc}^{-3}$ can be the dominant contributor to the PTA signal while the DCBH channel may contribute sub-dominantly. We also suggest ways to place an upper bound on the seed number density.

astro-ph.CO↗

arXiv2024

The evolution of accretor stars in binary systems due to accretion of increasingly helium-rich material

The recent discovery of examples of intermediate-mass helium stars have offered new insights into interacting binaries. These observations will allow significant improvements in our understanding of helium stars. However, in the creation of these stars their companions may accrete a significant amount of helium-rich stellar material. These creates stars with unusual composition profiles -- stars with helium-rich cores, hydrogen-rich lower envelopes and a helium-rich outer envelope. Thus the mean molecular weight reaches a minimum in the the middle of the star rather than continuously decreasing outwards in mass. To demonstrate this structure we present Cambridge STARS model calculations of an example interacting binary systems where the helium-rich material is transferred, and compare it to one where the composition of the accreted mass is fixed to the companion's surface composition. We show that the helium-rich material leads to the accretor being 0.2 dex hotter and 0.15 dex more luminous than models where the composition is not helium rich. We use a simple BPASS v2.2 population model to estimate that helium-rich mass transfer occurs in 23 per cent of massive binaries that undergo mass transfer. This suggests this is a common process. This binary process has implications for the discrepancy between spectroscopic and gravitational masses of stars, the production of ionizing photons and possibly the modelling of high redshift galaxies.

astro-ph.SR↗

arXiv2024

Galactic and extragalactic probe of dark matter with LISA's binary black holes

The upcoming LISA mission will be able to detect gravitational waves from galactic and extragalactic compact binaries. Here, we report on LISA's capability to probe dark matter around these binaries if the latter constitute black holes. By analyzing the variation in the chirp mass of the binary, we show that depending on the black hole masses, LISA should be able to probe their surrounding dark matter to a luminosity distance of $\approx 1$ Gpc if such binaries are observed within the inner $\approx 10$ pc of their galactic center for particle-like dark matter or near the galactic solitonic core for wave-like dark matter. In the case a null result is recorded during the course of observation of \emph{well-localized} binaries, one can still rule out certain parameter spaces of dark matter as being the dominant contributor to the matter budget of the Universe.

astro-ph.CO↗

arXiv2024

The effect of interstellar medium on LVK's black holes

Gravitational radiation alone is not efficient in hardening the orbit of a wide binary black hole (BBH). By employing a toy model for the interstellar medium (ISM) surrounding BBHs, here we discuss the effect of this baryonic medium on BBH dynamics. Depending on the BBH's mass, we show that a binary surrounded by an isotropic cold neutral medium (i.e., an asymptotic temperature $T_{\infty} \approx 100$ K) with a time-averaged particle density of $\langle n_H \rangle = \mathcal{O}(1)$ cm$^{-3}$ can play a significant role in hardening the binary orbit over a $\mathcal{O}(10^9)$ yr time scale. Additionally, this causes the black hole's mass to grow at a rate $\propto m^2$. We thus discuss the impact of the ISM on the LIGO-Virgo-KAGRA (LVK) observables and quantify the properties of the ISM under which the latter could act as an additional important pathway for driving a subset of LVK's BBH mergers.

astro-ph.HE↗

arXiv2023

The effect of stellar rotation on black hole mass and spin

The gravitational wave signature of a binary black hole (BBH) merger is dependent on its component mass and spin. If such black holes originate from rapidly rotating progenitors, the large angular momentum reserve in the star could drive a collapsar-like supernova explosion, hence substantially impacting these characteristics of the black holes in the binary. To examine the effect of stellar rotation on the resulting black hole mass and spin, we conduct a 1D general relativistic study of the end phase of the collapse. We find that the resulting black hole mass at times differs significantly from the previously assumed values. We quantify the dependence of the black hole spin magnitude on the hydrodynamics of the accretion flow, providing analytical relations for calculating the mass and spin based on the progenitor's pre-collapse properties. Depending on the nature of the accretion flow, our findings have implications for the black hole upper mass gap resulting from pair-instability supernovae, the maximum mass of a maximally rotating stellar black hole, and the maximum effective spin of a BBH formed in tidally locked helium star - black hole binary.

astro-ph.HE↗

arXiv2023

Observational implications of cosmologically coupled black holes

It was recently suggested that "cosmologically coupled" black holes with masses that increase in proportion to the volume of the Universe might constitute the physical basis of dark energy. We take this claim at face value and discuss its potential astrophysical implications. We show that the gravitational wave emission in binary systems would be significantly enhanced so that the number of black hole mergers would exceed the observed rate by orders of magnitude, with typical masses much larger than those seen by the LIGO-Virgo-KAGRA network. Separately, if the mass growth happens at fixed angular momentum, the supermassive black holes in matter-deficient elliptical galaxies should be slowly rotating. Finally, cosmological coupling would stabilize small black holes against Hawking radiation-induced evaporation.

astro-ph.CO↗

arXiv2023

Sustained super-Eddington accretion around neutron stars & black holes

Recently, it was shown that the formation of a photon-trapping surface might not be sufficient to ensure unimpeded super-Eddington (SE) accretion. In light of this finding, here we derive a condition such that sustained and unimpeded SE accretion could be achieved in optically thick slim accretion disks surrounding neutron stars (NSs) and black holes (BHs). For this, we calculate a semi-analytic approximation of the self-similar global radial velocity expression for an advection-dominated flow. Neglecting the influence of relativistic jets on the accretion flow, we find that for Eddington fraction $\dot{m} \gtrsim 1.5 (\epsilon/0.1)^{3/5}$ ($\epsilon$ being the accretion efficiency) sustained SE accretion might be possible in slim disks around BHs irrespective of their spin. The same condition holds for NSs when $\epsilon > 0.03$. The presence of a surface magnetic field might truncate the disk at the magnetosphere of the NS, resulting in lower efficiencies and consequently changing the condition to $\dot{m} > 0.013 \epsilon^{-19/31}$. Our approach suggests that sustained SE accretion might almost always be possible around NSs and BHs hosting accretion disks.

astro-ph.HE↗

arXiv2022

Evaluating chemically homogeneous evolution in stellar binaries: Electromagnetic implications -- Ionizing photons, SLSN-I, GRB, Ic-BL

We investigate the occurrence of rapid-rotation induced chemically homogeneous evolution (CHE) due to strong tides and mass accretion in binaries. To this end, we generalize the relation in Packet (1981) to calculate the minimum angular momentum (AM) accretion required by a secondary star to experience accretion-induced CHE. Contrary to traditionally assumed 5-10 percent accretion of initial mass ($Z \lesssim 0.004$, $M \gtrsim$ 20 M$_{\odot}$) for spinning up the accretor (resulting in CHE) this value can drop to $\sim$ 2 percent for efficient AM accretion while for certain systems it could be substantially larger. We conduct a population study using \textsc{bpass} by evolving stars under the influence of strong tides in short-period binaries and also account for the updated effect of accretion-induced spin-up. We find accretion CHE (compared to tidal CHE) to be the dominant means of producing homogeneous stars even at 10 percent AM accretion efficiency during mass transfer. Unlike tidal CHE, it is seen that CH stars arising due to accretion can retain a larger fraction of their AM till core collapse. Thus we show that accretion CHE could be an important formation channel for energetic electromagnetic transients like GRBs, Ic-BL (SLSN-I, Ic-BL) under the collapsar (magnetar) formalism and a single CH star could lead to both the transients under their respective formation scenario. Lastly, we show that under the current treatment of CHE, the emission rate of ionizing photons by such stars decreases more rapidly at higher metallicities than previously predicted.

astro-ph.SR↗

arXiv2022

Forward Modelling the O3(a+b) GW transient mass distributions with BPASS by varying compact remnant mass and SNe kick prescriptions

We present forward modeling from the BPASS code suite of the population of observed gravitational wave (GW) transients reported by the LIGO/VIRGO consortium (LVC) during their third observing run, O3(a+b). Specifically, we predict the expected chirp mass and mass ratio distributions for GW transients, taking account of detector sensitivity to determine how many events should have been detected by the current detector network in O3(a+b). We investigate how these predictions change by alternating between four different remnant mass estimation schemes and two supernovae (SNe) kick prescriptions. We find that none of the model populations resulting from these variations accurately match the whole O3(a+b) GW transient catalog. However, agreement from some models to part of the catalog suggests ways to achieve a more complete fit. These include reducing the number of low mass black holes (BHs) close to the mass gap, while also increasing the number of higher mass BHs below the pair-instability SN limit. Finally, we find that the interaction between the value of the remnant mass from a stellar model and the choice of SN kick is complex and different kick prescriptions may be required depending on whether a neutron star or BH is formed.

astro-ph.HE↗