arXiv subjects

P. Ramsden

Publications and source records attributed to P. Ramsden.

7 recordsLinked to original sources

arXiv2026

Can tidal disruption event models reliably measure black hole masses?

Tidal disruption event (TDE) light curves are increasingly used to infer the masses of quiescent supermassive black holes ($M_{\rm{BH}}$), offering a powerful probe of low-mass black hole demographics independent of host-galaxy scaling relations. However, the reliability of most semi-analytic TDE models assume full stellar disruption, despite theoretical expectations that partial disruptions dominate the TDE population. In this work we test the robustness of current TDE models using three repeating partial TDEs (rpTDEs), in which the multiple flares produced by the same surviving stellar core must yield consistent black hole masses. We present spectroscopic observations establishing AT 2023adr as a rpTDE, making it the third such spectroscopically confirmed event. We independently model the flares of the three rpTDEs; 2020vdq, 2022dbl, and 2023adr, applying fallback-accretion fits, stream-stream collision scaling relations, luminosity-based empirical relations, and cooling-envelope fits. After accounting for statistical and model-specific systematics, we find that all TDE models generally return self-consistent $M_{\rm{BH}}$ values between flares, and are broadly consistent with host-galaxy $M_{\rm{BH}}$ proxies, recovering $M_{\rm{BH}}$ to within 0.3-0.5 dex. However, the convergence of fallback models towards unphysical stellar masses and impact parameters reveals limitations in the existing fallback model grids. We also show that light curve coverage, particularly in the near-UV, is critical for constraining model parameters. This has direct implications for interpreting the thousands of TDE light curves expected from upcoming surveys such as the Rubin Observatory's Legacy Survey of Space and Time, where from simulations, we find that $M_{\rm{BH}}$ may be underestimated on average by 0.5 dex without additional follow-up.

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arXiv2025

MUSE IFU observations of galaxies hosting Tidal Disruption Events

We present an analysis of twenty tidal disruption event (TDE) host galaxies observed with the MUSE integral-field spectrograph on ESO VLT. We investigate the presence of extended emission line regions (EELRs) and study stellar populations mostly at sub-kpc scale around the host nuclei. EELRs are detected in 5/20 hosts, including two unreported systems. All EELRs are found at z<0.045, suggesting a distance bias and faint EELRs may be missed at higher redshift. EELRs only appear in post-merger systems and all such hosts at z<0.045 show them. Thus, we conclude that TDEs and galaxy mergers have a strong relation, and >45% of post-merger hosts in the sample exhibit EELRs. Furthermore, we constrained the distributions of stellar masses near the central black holes (BHs), using the spectral synthesis code Starlight and BPASS stellar evolution models. The youngest nuclear populations have typical ages of 1 Gyr and stellar masses below 2.5MSun. The populations that can produce observable TDEs around non-rotating BHs are dominated by subsolar-mass stars. 3/4 TDEs requiring larger stellar masses exhibit multi-peaked light curves, possibly implying relation to repeated partial disruptions of high-mass stars. The found distributions are in tension with the masses of the stars derived using light curve models. Mass segregation of the disrupted stars can enhance the rate of TDEs from supersolar-mass stars but our study implies that low-mass TDEs should still be abundant and even dominate the distribution, unless there is a mechanism that prohibits low-mass TDEs or their detection.

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arXiv2025

The case of AT2022wtn: a Tidal Disruption Event in an interacting galaxy

We present the results from our multi-wavelength monitoring campaign of the transient AT2022wtn, discovered by the Zwicky Transient Facility in the nucleus of SDSSJ232323.79+104107.7, the less massive galaxy in an active merging pair with a mass ratio of ~10:1. AT2022wtn shows spectroscopic and photometric properties consistent with a X-ray faint N-strong TDE-H+He with a number of peculiarities. Specifically, a 30-days long plateau at maximum luminosity, a corresponding dip in temperature and the development of a double-horned N III+ He II line profile. Strong and time-evolving velocity offsets in the TDE broad emission lines and the detection of a transient radio emission, indicate the presence of outflows. Overall, the observed properties are consistent with the full disruption of a low-mass star by a ~10$^{6}$ M$_{\odot}$ SMBH followed by an efficient disk formation and the launch of a quasi-spherical reprocessing envelope of fast expanding outflowing material. The observed differences between the He II and the Hydrogen and N III lines can be explained either with a spatial separation of the lines emitting region or with a late-time reveal of shocks from the returning debris streams, as the photosphere recedes. Finally, we present an extensive analysis of the hosting environment and discuss the implications for the discovery of two TDEs in interacting galaxy pairs, finding indication for an over-representation of TDEs in these systems. The AT2022wtn host galaxy properties suggest that it is in the early stages of the merger, therefore we may be witnessing the initial enhanced rate of TDEs in interacting galaxies before the post-starburst phase.

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arXiv2024

Double "acct": a distinct double-peaked supernova matching pulsational pair-instability models

We present multi-wavelength data of SN2020acct, a double-peaked stripped-envelope supernova (SN) in NGC2981 at ~150 Mpc. The two peaks are temporally distinct, with maxima separated by 58 rest-frame days, and a factor of 20 reduction in flux between. The first is luminous (M$_{r}$ = -18.00 $\pm$ 0.02 mag), blue (g - r = 0.27 $\pm$ 0.03 mag), and displays spectroscopic signatures of interaction with hydrogen-free circumstellar material. The second peak is fainter (M$_{r}$ = -17.29 $\pm$ 0.03 mag), and spectroscopically similar to an evolved stripped-envelope SNe, with strong blended forbidden [Ca II] and [O II] features. No other known double-peak SN exhibits a light curve similar to that of SN 2020acct. We find the likelihood of two individual SNe occurring in the same star-forming region within that time to be highly improbable, while an implausibly fine-tuned configuration would be required to produce two SNe from a single binary system. We find that the peculiar properties of SN2020acct match models of pulsational pair instability (PPI), in which the initial peak is produced by collisions of shells of ejected material, shortly followed by a terminal explosion. Pulsations from a star with a 72 M$_{\odot}$ helium core provide an excellent match to the double-peaked light curve. The local galactic environment has a metallicity of 0.4 Z$_{\odot}$, a level where massive single stars are not expected retain enough mass to encounter the PPI. However, late binary mergers or a low-metallicity pocket may allow the required core mass. We measure the rate of SN 2020acct-like events to be $<3.3\times10^{-8}$ Mpc$^{-3}$ yr$^{-1}$ at z = 0.07, or <0.1% of the total core-collapse SN rate.

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arXiv2024

Quasi-periodic X-ray eruptions years after a nearby tidal disruption event

Quasi-periodic Eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs), undergoing instabilities or interacting with a stellar object in a close orbit. It has been suggested that this disk could be created when the SMBH disrupts a passing star, implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs, and two observed TDEs have exhibited X-ray flares consistent with individual eruptions. TDEs and QPEs also occur preferentially in similar galaxies. However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. Here we report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 hours from AT2019qiz, a nearby and extensively studied optically-selected TDE. We detect and model the X-ray, ultraviolet and optical emission from the accretion disk, and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs.

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arXiv2024

The fast transient AT 2023clx in the nearby LINER galaxy NGC 3799 as a tidal disruption of a very low-mass star

We present an extensive analysis of the optical and UV properties of AT2023clx, the closest TDE to date, that occurred in the nucleus of the interacting LINER galaxy, NGC3799 (z=0.01107). After correcting for the host reddening (E(B-V) = 0.179 mag), we find its peak absolute g-band magnitude to be -18.03{+/-}0.07 mag, and its peak bolometric luminosity to be L=(1.57{+/-}0.19)x10^43 erg/s. AT2023clx displays several distinctive features: first, it rose to peak within 10.4{+/-}2.5 days, making it the fastest rising TDE to date. Our SMBH mass estimate of M_BH ~ 10^6 Msol rules out the possibility of an intermediate-mass BH as the reason for the fast rise. Dense spectral follow-up reveals a blue continuum that cools slowly and broad Balmer and HeII lines as well as weak HeI 5876,6678 emission features that are typically seen in TDEs. The early, broad (width ~ 15000 km/s) profile of Ha matches theoretical expectations from an optically thick outflow. A flat Balmer decrement (~ 1.58) suggests that the lines are collisionally excited rather than being produced via photoionisation, in contrast to typical active galactic nuclei. A second distinctive feature, seen for the first time in TDE spectra, is a sharp, narrow emission peak at a rest wavelength of ~ 6353 A. This feature is clearly visible up to 10d post-peak; we attribute it to clumpy material preceding the bulk outflow, which manifests as a high-velocity component of Ha (-9584 km/s). Its third distinctive feature is the rapid cooling during the first ~ 20 days after peak, reflected as a break in the temperature evolution. Combining these findings, we propose a scenario for AT2023clx involving the disruption of a very low-mass star (<=0.1 Msol) with an outflow launched in our line of sight and with disruption properties that led to efficient circularisation and prompt accretion disc formation, observed through a low-density photosphere.

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arXiv2023

AT2022aedm and a new class of luminous, fast-cooling transients in elliptical galaxies

We present the discovery and extensive follow-up of a remarkable fast-evolving optical transient, AT2022aedm, detected by the Asteroid Terrestrial impact Last Alert Survey (ATLAS). AT2022aedm exhibited a rise time of $9\pm1$ days in the ATLAS $o$-band, reaching a luminous peak with $M_g\approx-22$ mag. It faded by 2 magnitudes in $g$-band during the next 15 days. These timescales are consistent with other rapidly evolving transients, though the luminosity is extreme. Most surprisingly, the host galaxy is a massive elliptical with negligible current star formation. X-ray and radio observations rule out a relativistic AT2018cow-like explosion. A spectrum in the first few days after explosion showed short-lived He II emission resembling young core-collapse supernovae, but obvious broad supernova features never developed; later spectra showed only a fast-cooling continuum and narrow, blue-shifted absorption lines, possibly arising in a wind with $v\approx2700$ km s$^{-1}$. We identify two further transients in the literature (Dougie in particular, as well as AT2020bot) that share similarities in their luminosities, timescales, colour evolution and largely featureless spectra, and propose that these may constitute a new class of transients: luminous fast-coolers (LFCs). All three events occurred in passive galaxies at offsets of $\sim4-10$ kpc from the nucleus, posing a challenge for progenitor models involving massive stars or massive black holes. The light curves and spectra appear to be consistent with shock breakout emission, though usually this mechanism is associated with core-collapse supernovae. The encounter of a star with a stellar mass black hole may provide a promising alternative explanation.

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