arXiv subjects

Ananya Bandopadhyay

Publications and source records attributed to Ananya Bandopadhyay.

14 recordsLinked to original sources

arXiv2026

The Role of Stellar Spin in Repeating Partial Tidal Disruption Events

The repeated tidal stripping of a star by a supermassive black hole, known as a repeating partial tidal disruption event (rpTDE), can give rise to a transient that rebrightens months to years after the first outburst. Among the rpTDE candidates so far observed, some exhibit dimmer peak luminosities during each successive outburst, which is a trend that has not been reproduced from theoretical models when the star survives more than one encounter with the black hole. Here we suggest that this trend can be recovered if the partially disrupted star is initially (i.e., prior to its first mass-stripping event) rapidly rotating, which is expected if the star was placed on its orbit through the Hills breakup of a tidally locked and tight binary. We test this hypothesis with hydrodynamical simulations of high-mass ($\geqslant 1 M_{\odot}$) main sequence stars repeatedly partially disrupted by a $10^6 M_{\odot}$ black hole, and demonstrate that successively dimmer outbursts are indeed recovered for high (tens of percent breakup) and prograde (i.e., aligned with the orbital angular momentum) stellar spins. Our results provide strong indirect evidence for the operation of the Hills mechanism in seeding the stars in rpTDEs.

astro-ph.HE↗

arXiv2026

Repeating Nuclear Transients from Repeating Partial Tidal Disruption Events

Extragalactic nuclear transients that exhibit repeating outbursts can be modeled as the repeated dynamical interaction between bound stars and supermassive black holes (SMBHs). A subset of these transients, with recurrence timescales of months-to-years, have been explained as accretion flares from the repeated tidal stripping of a star by an SMBH, in a repeating partial tidal disruption event (rpTDE). We outline the scope of the rpTDE model and discuss hydrodynamical simulations and analytical predictions for the stability of stars undergoing repeated mass loss, and the long-term evolution of these flares as a function of stellar type and orbital parameters. Our findings demonstrate that high-mass and centrally concentrated stars undergo negligible changes in structure in response to small amounts ($\sim 1-10\% M_\star$) of mass loss, and can survive many mass-stripping encounters with an SMBH. Contrarily, low-mass and less evolved stars are unstable to mass loss, and would be destroyed within a few orbits. We discuss the implications of these results for constraining the stellar type and orbital parameters of observed sources, such as ASASSN-14ko, for which $\gtrsim 20$ flares have been observed, and AT2020vdq, which exhibits a second flare that is brighter than its primary outburst.

astro-ph.HE↗

arXiv2026

The Maximum Gravity Model for partial Tidal Disruption Events: Mass Loss, Peak Fallback Rate and Dependence on Stellar Properties

A star entering the tidal sphere of a supermassive black hole (SMBH) can be partially stripped of mass, resulting in a partial tidal disruption event (TDE). Here we develop an analytical model for properties of these events, including the peak fallback rate, $\dot{M}_{\rm peak}$, the time at which the peak occurs, $t_{\rm peak}$, and the amount of mass removed from the star, $ΔM$, for any star and any pericenter distance associated with the stellar orbit about the black hole. We compare the model predictions to 1276 hydrodynamical simulations of partial TDEs of main-sequence stars by a $10^6 M_\odot$ SMBH. The model yields $t_{\rm peak}$ predictions that are in good agreement (to within tens of percent) with the numerical simulations for any stellar mass and age. The agreement for $\dot{M}_{\rm peak}$ is weaker due to the influence of self-gravity on the debris stream dynamics, which remains dynamically important for partial TDEs; the agreement for $\dot{M}_{\rm peak}$ is, however, to within a factor of $\sim 2-3$ in the majority of cases considered, with larger differences for low-mass stars ($M_\star \lesssim 0.5 M_\odot$) on grazing orbits with small mass loss. We show that partial TDE lightcurves for disruptions caused by $\sim 10^6M_\odot$ SMBHs can span $\sim 20-100$ day peak timescales, whereas grazing encounters of high-mass stars with high-mass SMBHs can yield longer peak timescales ($t\gtrsim 1000$ days), associated with some observed transients. Our model provides a significant step toward an analytical prescription for TDE lightcurves and luminosity functions.

astro-ph.HE↗

arXiv2025

Comprehensive X-ray Observations of the Exceptional Ultra-long X-ray and Gamma-ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-ray Afterglow Properties

GRB 250702B is an exceptional transient that produced multiple episodes of luminous gamma-ray radiation lasting for $>25$ ks, placing it among the class of ultra-long gamma-ray bursts (GRBs). However, unlike any known GRB, the \textit{Einstein Probe} detected soft X-ray emission up to 24 hours before the gamma-ray triggers. We present comprehensive X-ray observations of the transient's afterglow obtained with the Neil Gehrels Swift Observatory, the Nuclear Spectroscopic Telescope Array, and the Chandra X-ray Observatory between 0.5 to 65 days (observer frame) after the initial high-energy trigger. The X-ray emission decays steeply as $\sim t^{-1.9}$, and shows short timescale X-ray variability ($ΔT/T < 0.03$) in both Swift and NuSTAR, consistent with flares superposed on an external shock continuum. Serendipitous detections by the Swift Burst Alert Telescope (BAT) out to $\sim$0.3 days and continued NuSTAR variability to $\sim$2 days imply sustained central engine activity; including the precursor, the required engine duration is $\gtrsim 3$ days. Afterglow modeling favors the combination of forward and reverse shock emission in a wind-like ($k \approx 2$) environment. These properties, especially the long-lived engine and early soft X-ray emission, are difficult to reconcile with a collapsar origin, and GRB 250702B does not fit neatly with canonical ultra-long GRBs or relativistic tidal disruption events (TDEs). A hybrid scenario in which a star is disrupted by a stellar-mass black hole (a micro-TDE) provides a plausible explanation, although a relativistic TDE from an intermediate-mass black hole remains viable.

astro-ph.HE↗

arXiv2025

Predicting the Properties of the Fallback Rate from Tidal Disruption Events: Investigating the Maximum Gravity Model

A star destroyed by the tidal field of a supermassive black hole (SMBH) in a tidal disruption event (TDE) gives rise to a luminous flare. TDEs are being detected at an ever-increasing rate, motivating the need for accurate models of their lightcurves. The ``maximum gravity'' (MG) model posits that a star is completely destroyed when the tidal field of the SMBH exceeds the maximum self-gravitational field within the star, $g_{\rm max}$, and predicts the peak fallback rate $\dot{M}_{\rm peak}$ and the time to peak $t_{\rm peak}$. Here we perform hydrodynamical simulations of the complete disruption of 24 stars with masses ranging from $0.2-5.0 M_\odot$, at different stages of their main sequence evolution, to test the predictions of this model. We find excellent agreement between the MG model predictions and our simulations for stars near the zero-age main sequence, while the predictions are less accurate (but still within $\sim 35-50\%$ of the simulation results) for highly evolved stars. We also generalize the MG model to incorporate the Paczy{ń}ski-Wiita potential to assess the impact of strong-gravity effects -- which are especially important for deep encounters that are required to completely destroy evolved and centrally concentrated stars -- and find good agreement with recent works that include relativistic gravity. Our results demonstrate that this model provides accurate constraints on the peak timescale of TDE lightcurves and their correlation with black hole mass.

astro-ph.HE↗

arXiv2025

The Double Tidal Disruption Event AT 2022dbl Implies That at Least Some "Standard" Optical TDEs are Partial Disruptions

Flares produced following the tidal disruption of stars by supermassive black holes can reveal the properties of the otherwise dormant majority of black holes and the physics of accretion. In the past decade, a class of optical-ultraviolet tidal disruption flares has been discovered whose emission properties do not match theoretical predictions. This has led to extensive efforts to model the dynamics and emission mechanisms of optical-ultraviolet tidal disruptions in order to establish them as probes of supermassive black holes. Here we present the optical-ultraviolet tidal disruption event AT 2022dbl, which showed a nearly identical repetition 700 days after the first flare. Ruling out gravitational lensing and two chance unrelated disruptions, we conclude that at least the first flare represents the partial disruption of a star, possibly captured through the Hills mechanism. Since both flares are typical of the optical-ultraviolet class of tidal disruptions in terms of their radiated energy, temperature, luminosity, and spectral features, it follows that either the entire class are partial rather than full stellar disruptions, contrary to the prevalent assumption, or that some members of the class are partial disruptions, having nearly the same observational characteristics as full disruptions. Whichever option is true, these findings could require revised models for the emission mechanisms of optical-ultraviolet tidal disruption flares and a reassessment of their expected rates.

astro-ph.HE↗

arXiv2025

Repeated Tidal Interactions between Stars and Supermassive Black Holes: Mass Transfer, Stability, and Implications for Repeating Partial Tidal Disruption Events

Stars orbiting supermassive black holes can generate recurring accretion flares in repeating partial tidal disruption events (TDEs). Here we develop an efficient formalism for analyzing the time-dependent response of a star to the removal of a fraction ($\lesssim 10\%$) of its mass. This model predicts that mass loss results in a decrease in the average density of low-mass ($\lesssim 0.7 M_{\odot}$) stars. Contrarily, higher-mass stars exhibit an increase in their average density, such that the change is more pronounced for larger mass losses, and stars with masses $\sim 1.5-2 M_{\odot}$ experience the largest such increase. We predict that the final energy of the star post-mass-loss (i.e., the ``surviving core'') is effectively given by the binding energy of the original star interior to the radius from which mass is removed, i.e., the final core energy is agnostic to the process that removes the mass and -- as a corollary -- tidal heating is comparatively insignificant. We find excellent agreement between our predictions and one-dimensional Eulerian simulations of a star undergoing mass loss, and three-dimensional Lagrangian simulations of partial TDEs. We conclude that 1) partially disrupted stars are not significantly heated via tidal dissipation, 2) evolved and moderately massive ($\gtrsim 1.5 M_{\odot}$) stars can most readily survive many repeated stripping events, and 3) progressively dimmer flares -- observed in some repeating partial TDE candidates -- could be explained by the increase in the density of the star post-mass-loss.

astro-ph.HE↗

arXiv2024

Repeating nuclear transients from repeating partial tidal disruption events: reproducing ASASSN-14ko and AT2020vdq

Some electromagnetic outbursts from the nuclei of distant galaxies have been found to repeat on months-to-years timescales, and each of these sources can putatively arise from the accretion flares generated through the repeated tidal stripping of a star on a bound orbit about a supermassive black hole (SMBH), i.e., a repeating partial tidal disruption event (rpTDE). Here we test the rpTDE model through analytical estimates and hydrodynamical simulations of the interaction between a range of stars, which differ from one another in mass and age, and an SMBH. We show that higher-mass ($\gtrsim 1 M_{\odot}$), evolved stars can survive many ($\gtrsim 10-100$) encounters with an SMBH while simultaneously losing $few \times 0.01 M_{\odot}$, resulting in accretion flares that are approximately evenly spaced in time with nearly the same amplitude, quantitatively reproducing ASASSN-14ko. We also show that the energy imparted to the star via tides can lead to a change in its orbital period that is comparable to the observed decay in the recurrence time of ASASSN-14ko's flares, $\dot{P}\simeq-0.0026$. Contrarily, lower-mass and less-evolved stars lose progressively more mass and produce brighter accretion flares on subsequent encounters for the same pericenter distances, leading to the rapid destruction of the star and cessation of flares. Such systems cannot reproduce ASASSN-14ko-like transients, but are promising candidates for recreating events such as AT2020vdq, which displayed a second and much brighter outburst compared to the first. Our results imply that the lightcurves of repeating transients are tightly coupled with stellar type.

astro-ph.HE↗

arXiv2024

A Potential Second Shutoff from AT2018fyk: An updated Orbital Ephemeris of the Surviving Star under the Repeating Partial Tidal Disruption Event Paradigm

The tidal disruption event (TDE) AT2018dyk/ASASSN-18UL showed a rapid dimming event 500 days after discovery, followed by a re-brightening roughly 700 days later. It has been hypothesized that this behavior results from a repeating partial TDE (rpTDE), such that prompt dimmings/shutoffs are coincident with the return of the star to pericenter and rebrightenings generated by the renewed supply of tidally stripped debris. This model predicted that the emission should shut off again around August of 2023. We report AT2018fyk's continued X-ray and UV monitoring, which shows an X-ray (UV) drop in flux by a factor of 10 (5) over a span of two months, starting 14 Aug 2023. This sudden change can be interpreted as the second emission shutoff, which 1) strengthens the rpTDE scenario for AT2018fyk, 2) allows us to constrain the orbital period to a more precise value of 1306$\pm$47 days, and 3) establishes that X-ray and UV/optical emission track the fallback rate onto this SMBH -- an often-made assumption that otherwise lacks observational verification -- and therefore the UV/optical lightcurve is powered predominantly by processes tied to X-rays. The second cutoff implies that another rebrightening should happen between May-Aug 2025, and if the star survived the second encounter, a third shutoff is predicted to occur between Jan-July 2027. Finally, low-level accretion from the less bound debris tail (which is completely unbound/does not contribute to accretion in a non-repeating TDE) can result in a faint X-ray plateau that could be detectable until the next rebrightening.

astro-ph.HE↗

arXiv2024

Measuring Neutron Star Radius with second and third generation Gravitational Wave Detector Networks

The next generation of ground-based interferometric gravitational wave detectors will observe mergers of black holes and neutron stars throughout cosmic time. A large number of the binary neutron star merger events will be observed with extreme high fidelity, and will provide stringent constraints on the equation of state of nuclear matter. In this paper, we investigate the systematic improvement in the measurability of the equation of state with increase in detector sensitivity by combining constraints obtained on the radius of a $1.4 \, \mathrm{M}_{\odot}$ neutron star from a simulated source population. Since the measurability of the equation of state depends on its stiffness, we consider a range of realistic equations of state that span the current observational constraints. We show that a single 40km Cosmic Explorer detector can pin down the neutron star radius for a soft, medium and stiff equation of state to an accuracy of 10m within a decade, whereas the current generation of ground-based detectors like the Advanced LIGO-Virgo network would take $\mathcal{O}(10^5)$ years to do so for a soft equation of state.

astro-ph.HE↗

arXiv2024

Characterizing Gravitational Wave Detector Networks: From A$^\sharp$ to Cosmic Explorer

Gravitational-wave observations by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo have provided us a new tool to explore the Universe on all scales from nuclear physics to the cosmos and have the massive potential to further impact fundamental physics, astrophysics, and cosmology for decades to come. In this paper we have studied the science capabilities of a network of LIGO detectors when they reach their best possible sensitivity, called A#, given the infrastructure in which they exist and a new generation of observatories that are factor of 10 to 100 times more sensitive (depending on the frequency), in particular a pair of L-shaped Cosmic Explorer observatories (one 40 km and one 20 km arm length) in the US and the triangular Einstein Telescope with 10 km arms in Europe. The presence of one or two A# observatories in a network containing two or one next generation observatories, respectively, will provide good localization capabilities for facilitating multimessenger astronomy and precision measurement of the Hubble parameter. Two Cosmic Explorer observatories are indispensable for achieving precise localization of binary neutron star events, facilitating detection of electromagnetic counterparts and transforming multimessenger astronomy. Their combined operation is even more important in the detection and localization of high-redshift sources, such as binary neutron stars, beyond the star-formation peak, and primordial black hole mergers, which may occur roughly 100 million years after the Big Bang. The addition of the Einstein Telescope to a network of two Cosmic Explorer observatories is critical for accomplishing all the identified science metrics. For most metrics the triple network of next generation terrestrial observatories are a factor 100 better than what can be accomplished by a network of three A# observatories.

gr-qc↗

arXiv2023

The Peak of the Fallback Rate from Tidal Disruption Events: Dependence on Stellar Type

A star completely destroyed in a tidal disruption event (TDE) ignites a luminous flare that is powered by the fallback of tidally stripped debris to a supermassive black hole (SMBH) of mass $M_{\bullet}$. We analyze two estimates for the peak fallback rate in a TDE, one being the "frozen-in" model, which predicts a strong dependence of the time to peak fallback rate, $t_{\rm peak}$, on both stellar mass and age, with $15\textrm{ days} \lesssim t_{\rm peak} \lesssim 10$ yr for main sequence stars with masses $0.2\le M_{\star}/M_{\odot} \le 5$ and $M_{\bullet} = 10^6M_{\odot}$. The second estimate, which postulates that the star is completely destroyed when tides dominate the maximum stellar self-gravity, predicts that $t_{\rm peak}$ is very weakly dependent on stellar type, with $t_{\rm peak} = \left(23.2\pm4.0\textrm{ days}\right)\left(M_{\bullet}/10^6M_{\odot}\right)^{1/2}$ for $0.2\le M_{\star}/M_{\odot} \le 5$, while $t_{\rm peak} = \left(29.8\pm3.6\textrm{ days}\right)\left(M_{\bullet}/10^6M_{\odot}\right)^{1/2}$ for a Kroupa initial mass function truncated at $1.5 M_{\odot}$. This second estimate also agrees closely with hydrodynamical simulations, while the frozen-in model is discrepant by orders of magnitude. We conclude that (1) the time to peak luminosity in complete TDEs is almost exclusively determined by SMBH mass, and (2) massive-star TDEs power the largest accretion luminosities. Consequently, (a) decades-long extra-galactic outbursts cannot be powered by complete TDEs, including massive-star disruptions, and (b) the most highly super-Eddington TDEs are powered by the complete disruption of massive stars, which -- if responsible for producing jetted TDEs -- would explain the rarity of jetted TDEs and their preference for young and star-forming host galaxies.

astro-ph.HE↗

arXiv2023

Cosmic Explorer: A Submission to the NSF MPSAC ngGW Subcommittee

Gravitational-wave astronomy has revolutionized humanity's view of the universe, a revolution driven by observations that no other field can make. This white paper describes an observatory that builds on decades of investment by the National Science Foundation and that will drive discovery for decades to come: Cosmic Explorer. Major discoveries in astronomy are driven by three related improvements: better sensitivity, higher precision, and opening new observational windows. Cosmic Explorer promises all three and will deliver an order-of-magnitude greater sensitivity than LIGO. Cosmic Explorer will push the gravitational-wave frontier to almost the edge of the observable universe using technologies that have been proven by LIGO during its development. With the unprecedented sensitivity that only a new facility can deliver, Cosmic Explorer will make discoveries that cannot yet be anticipated, especially since gravitational waves are both synergistic with electromagnetic observations and can reach into regions of the universe that electromagnetic observations cannot explore. With Cosmic Explorer, scientists can use the universe as a laboratory to test the laws of physics and study the nature of matter. Cosmic Explorer allows the United States to continue its leading role in gravitational-wave science and the international network of next-generation observatories. With its extraordinary discovery potential, Cosmic Explorer will deliver revolutionary observations across astronomy, physics, and cosmology including: Black Holes and Neutron Stars Throughout Cosmic Time, Multi-Messenger Astrophysics and Dynamics of Dense Matter, New Probes of Extreme Astrophysics, Fundamental Physics and Precision Cosmology, Dark Matter and the Early Universe.

astro-ph.IM↗

arXiv2023

Detectability of Sub-Solar Mass Neutron Stars Through a Template Bank Search

We study the detectability of gravitational-wave signals from sub-solar mass binary neutron star systems by the current generation of ground-based gravitational-wave detectors. We find that finite size effects from large tidal deformabilities of the neutron stars and lower merger frequencies can significantly impact the sensitivity of the detectors to these sources. By simulating a matched-filter based search using injected binary neutron star signals with tidal deformabilities derived from physically motivated equations of state, we calculate the reduction in sensitivity of the detectors. We conclude that the loss in sensitive volume can be as high as $78.4 \%$ for an equal mass binary system of chirp mass $0.17 \, \textrm{M}_{\odot}$, in a search conducted using binary black hole template banks. We use this loss in sensitive volume, in combination with the results from the search for sub-solar mass binaries conducted on data collected by the LIGO-Virgo observatories during their first three observing runs, to obtain a conservative upper limit on the merger rate of sub-solar mass binary neutron stars. Since the discovery of a low-mass neutron star would provide new insight into formation mechanisms of neutron stars and further constrain the equation of state of dense nuclear matter, our result merits a dedicated search for sub-solar mass binary neutron star signals.

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