Stellar Tidal Disruption Events Research Articles

Exploring Low-mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of the JWST, Roman Space Telescope, and LSST Surveys

The James Webb Space Telescope (JWST) has recently uncovered the presence of low-luminosity active galactic nuclei (AGNs) at z = 4–11. Spectroscopic observations have provided estimates of the nuclear black hole (BH) masses for these sources, extending the low-mass boundary down to M • ∼ 106–7 M ⊙. Despite this breakthrough, the observed lowest mass of BHs is still ≳1–2 orders of magnitude heavier than the predicted mass range of their seed population, thereby leaving the initial mass distribution of massive BHs poorly constrained. In this paper, we focus on UV-to-optical (in the rest frame) flares of stellar tidal disruption events (TDEs) embedded in low-luminosity AGNs as a tool for exploring low-mass BH populations with ≲104–6 M ⊙. We provide an estimate of the TDE rate over z = 4–11, associated with the properties of JWST-detected AGN host galaxies, and we find that deep and wide survey programs with JWST and the Roman Space Telescope (RST) can detect and identify TDEs up to z ≃ 4–7. The predicted detection numbers of TDEs at z > 4 in 1 yr are NTDE∼2–10(0.2–2) for the JADES-Medium (and COSMOS-Web) survey with JWST and NTDE∼2–10(8–50) for the deep (and wide) tiers of the High Latitude Time Domain Survey with RST. We further discuss survey strategies for hunting for transient high-redshift TDEs in wide-field surveys with RST, as well as a joint observation campaign with the Vera C. Rubin Observatory for enhancing the detection number. The high-redshift TDE search will give us a unique opportunity to probe the mass distribution of early BH populations.

Establishing accretion flares from supermassive black holes as a source of high-energy neutrinos

ABSTRACT The origin of cosmic high-energy neutrinos remains largely unexplained. For high-energy neutrino alerts from IceCube, a coincidence with time-variable emission has been seen for three different types of accreting black holes: (1) a gamma-ray flare from a blazar (TXS 0506+056), (2) an optical transient following a stellar tidal disruption event (TDE; AT2019dsg), and (3) an optical outburst from an active galactic nucleus (AGN; AT2019fdr). For the latter two sources, infrared follow-up observations revealed a powerful reverberation signal due to dust heated by the flare. This discovery motivates a systematic study of neutrino emission from all supermassive black hole with similar dust echoes. Because dust reprocessing is agnostic to the origin of the outburst, our work unifies TDEs and high-amplitude flares from AGN into a population that we dub accretion flares. Besides the two known events, we uncover a third flare that is coincident with a PeV-scale neutrino (AT2019aalc). Based solely on the optical and infrared properties, we estimate a significance of 3.6σ for this association of high-energy neutrinos with three accretion flares. Our results imply that at least ∼10 per cent of the IceCube high-energy neutrino alerts could be due to accretion flares. This is surprising because the sum of the fluence of these flares is at least three orders of magnitude lower compared to the total fluence of normal AGN. It thus appears that the efficiency of high-energy neutrino production in accretion flares is increased compared to non-flaring AGN. We speculate that this can be explained by the high Eddington ratio of the flares.

Small-scale radio jets and tidal disruption events: a theory of high-luminosity compact symmetric objects

ABSTRACT Double lobe radio sources associated with active galactic nuclei represent one of the longest studied groups in radio astronomy. A particular subgroup of double radio sources comprises the compact symmetric objects (CSOs). CSOs are distinguished by their prominent double structure and subkpc total size. It has been argued that the vast majority of high-luminosity CSOs (CSO 2s) represent a distinct class of active galactic nuclei with its own morphological structure and lifecycle. In this work, we present theoretical considerations regarding CSO 2s. We develop a semi-analytic evolutionary model, inspired by the results of large-scale numerical simulations of relativistic jets, that reproduces the features of the radio source population. We show that CSO 2s may be generated by finite energy injections and propose stellar tidal disruption events as a possible cause. We find that tidal disruption events of giant branch stars with masses ≳1 M⊙ can fuel these sources and discuss possible approaches to confirming this hypothesis. We predict that if the tidal disruption scenario holds, CSO 2s with sizes less than 400 pc should outnumber larger sources by more than a factor of 10. Our results motivate future numerical studies to determine whether the scenarios we consider for fuelling and source evolution can explain the observed radio morphologies. Multiwavelength observational campaigns directed at these sources will also provide critical insight into the origins of these objects, their environments, and their lifespans.

A Unified Theory of Jetted Tidal Disruption Events: From Promptly Escaping Relativistic to Delayed Transrelativistic Jets

Only a tiny fraction ∼1% of stellar tidal disruption events (TDEs) generate powerful relativistic jets evidenced by luminous hard X-ray and radio emissions. We propose that a key property responsible for both this surprisingly low rate and a variety of other observations is the typically large misalignment ψ between the orbital plane of the star and the spin axis of the supermassive black hole (SMBH). Such misaligned disk/jet systems undergo Lense–Thirring precession together about the SMBH spin axis. We find that TDE disks precess sufficiently rapidly that winds from the accretion disk will encase the system on large scales in a quasi-spherical outflow. We derive the critical jet efficiency η > η crit for both aligned and misaligned precessing jets to successfully escape from the disk wind ejecta. As η crit is higher for precessing jets, less powerful jets only escape after alignment with the SMBH spin. Alignment can occur through magneto-spin or hydrodynamic mechanisms, which we estimate occur on typical timescales of weeks and years, respectively. The dominant mechanism depends on η and the orbital penetration factor β. Hence, depending only on the intrinsic parameters of the event {ψ, η, β}, we propose that each TDE jet can either escape prior to alignment, thus exhibiting an erratic X-ray light curve and two-component radio afterglow (e.g., Swift J1644+57), or escape after alignment. Relatively rapid magneto-spin alignments produce relativistic jets exhibiting X-ray power-law decay and bright afterglows (e.g., AT2022cmc), while long hydrodynamic alignments give rise to late jet escape and delayed radio flares (e.g., AT2018hyz).

Partial stellar tidal disruption events and their rates

ABSTRACT Tidal disruption events (TDEs) of stars operated by massive black holes (MBHs) will be detected in thousands by upcoming facilities such as the Vera Rubin Observatory. In this work, we assess the rates of standard total TDEs, destroying the entire star, and partial TDEs, in which a stellar remnant survives the interaction, by solving 1D Fokker–Planck equations. Our rate estimates are based on a novel definition of the loss cone whose size is commensurate with the largest radius at which partial disruptions can occur, as motivated by relativistic hydrodynamical simulations. Our novel approach unveils two important results. First, partial TDEs can be more abundant than total disruptions by a factor of a few to a few tens. Second, the rates of complete stellar disruptions can be overestimated by a factor of a few to a few tens if one neglects partial TDEs, as we find that many of the events classified as total disruptions in the standard framework are in fact partial TDEs. Accounting for partial TDEs is particularly relevant for galaxies harbouring a nuclear stellar cluster featuring many events coming from the empty loss cone. Based on these findings, we stress that partial disruptions should be considered when constraining the luminosity function of TDE flares; accounting for this may reconcile the theoretically estimated TDE rates with the observed ones.

The Mass Fallback Rate of the Debris in Relativistic Stellar Tidal Disruption Events

Highly energetic stellar tidal disruption events (TDEs) provide a way to study black hole characteristics and their environment. We simulate TDEs with the Phantom code in a general relativistic and Newtonian description of a supermassive black hole’s gravity. Stars, which are placed in parabolic orbits with different β parameters, are constructed with the stellar evolution code MESA and therefore have realistic stellar density profiles. We study the mass fallback rate of the debris and its dependence on β, stellar mass, and age as well as the supermassive black hole’s spin and the choice of the gravity description. We calculate the peak value , time to peak t peak, duration of the super-Eddington phase t Edd, time during which , early rise-time τ rise, and late-time slope n ∞. We recover the trends of , t peak, τ rise, and n ∞ with β, stellar mass, and age which were obtained in previous studies. We find that t Edd, at a fixed β, scales primarily with the stellar mass, while scales with the compactness of stars. The effect of the SMBH’s rotation depends on the orientation of its rotational axis relative to the direction of the stellar motion in the initial orbit. Encounters in prograde orbits result in narrower curves with higher , while the opposite occurs for retrograde orbits. We find that disruptions, at the same pericenter distance, are stronger in a relativistic tidal field than in a Newtonian one. Therefore, relativistic curves have higher , and shorter t peak and t Edd.

Extreme accretion events: TDEs and changing‐look AGN

AbstractWe present a review of the topics of X‐ray stellar tidal disruption events (TDEs) and changing‐look active galactic nuclei (AGN). Stars approaching a supermassive black hole (SMBH) can be tidally disrupted and accreted. TDEs were first discovered in the X‐ray regime and appear as luminous, giant‐amplitude flares from inactive galaxies. The early X‐ray observations with ROSAT also established the extreme X‐ray spectral softness of these events with temperatures of order 50–100 eV that continues to be seen in the majority of more recently identified events. While the majority of X‐ray TDEs has been identified from inactive galaxies and some showed the highest amplitudes of variability recorded from galaxy cores (amplitudes exceeding factors of 1000–6000), a small fraction of AGN has been found to be highly variable as well. In AGN, this so‐called changing‐look phenomenon often comes with a strong change in the optical broad emission lines, leading to Seyfert‐type changes between class 1 and class 2. These two forms of activity represent the extremes of variability among active and quiescent galaxies, and have opened up a new window to understanding accretion physics under extreme conditions. Finally, we introduce the term “frozen‐look AGN” to describe systems that show constant line emission despite dramatic changes in the observed ionizing continuum. These systems are best explained by strong changes of absorption along our line‐of‐sight.

A Possible 250 s X-Ray Quasi-periodicity in the Fast Blue Optical Transient AT2018cow

The fast blue optical transients (FBOTs) are a new population of extragalactic transients of unclear physical origin. A variety of mechanisms has been proposed including failed supernova explosion, shock interaction with a dense medium, young magnetar, accretion onto a compact object and stellar tidal disruption event, but none is conclusive. Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of ∼250 s (at a significance level of 99.76%) in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data. The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope, with observations covering from 6 to 37 days after the optical discovery, though the significance level is lower (94.26%). This suggests that the quasi-periodic oscillation (QPO) frequency may be stable over at least 1.1 × 104 cycles. Assuming the ∼250 s QPO to be a scaled-down analog of that typically seen in stellar mass black holes, a black hole mass of ∼103–105 solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with a stellar tidal disruption by a black hole of ∼104 solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.

Transient Radio Emission from Low-redshift Galaxies at z < 0.3 Revealed by the VLASS and FIRST Surveys

We present the discovery of a sample of 18 low-redshift (z < 0.3) galaxies with transient nuclear radio emission. These galaxies are not detected or are weakly detected in the Faint Images of the Radio Sky at Twenty cm survey, performed from 1993–2009, but have brightened significantly in radio flux (by a factor of ≳5) in the epoch I (2017–2019) observations of the Very Large Array Sky Survey (VLASS). All 18 galaxies have been detected in VLASS epoch II observations, from 2020–2021, from which the radio flux has been found to evolve slowly (with variability amplitudes of ≳40%) over a period of about 3 yr. 15 galaxies have been observed in the Rapid ASKAP Continuum Survey, and a flat or inverted spectral slope between 888 MHz and 3 GHz is found. Based on the Sloan Digital Sky Survey spectra taken before the radio brightening, 14 of the 18 galaxies can be classified as LINERs or normal galaxies with weak or no nuclear activity. Most galaxies are red and massive, with more than half having central black hole masses above 108M⊙. We find that only one galaxy in our sample displays an optical flare lasting for at least two months, with a long decay in the infrared light curve that can be explained as the dust-heated echo emission of a central optical flare, such as a stellar tidal disruption event. We discuss several possibilities for the transient radio emission and conclude that it is likely associated with a newborn radio jet triggered by short sporadic fueling of a supermassive black hole. Such a scenario can be tested with further multifrequency radio observations of these sources, via measuring their radio flux variability and spectral evolution.

Searching for ultra-light bosons and constraining black hole spin distributions with stellar tidal disruption events

Stars that pass close to the supermassive black holes located in the center of galaxies can be disrupted by tidal forces, leading to flares that are observed as bright transient events in sky surveys. The rate for these events to occur depends on the black hole spins, which in turn can be affected by ultra-light bosons due to superradiance. We perform a detailed analysis of these effects and show that searches for stellar tidal disruptions have the potential to uncover the existence of ultra-light bosons. In particular, we find that upcoming stellar tidal disruption rate measurements by the Vera Rubin Observatory’s Legacy Survey of Space and Time can be used to either discover or rule out bosons with masses ranging from 10−20 to 10−18 eV. Our analysis also indicates that these measurements may be used to constrain a variety of supermassive black hole spin distributions and determine if close-to maximal spins are preferred.

Discovery of late-time X-ray flare and anomalous emission line enhancement after the nuclear optical outburst in a narrow-line Seyfert 1 Galaxy

CSS J102913+404220 is an atypical narrow-line Seyfert 1 galaxy with an energetic optical outburst occurring co-spatially with its nucleus. We present a detailed analysis of multi-wavelength photometric and spectroscopic observations of this object covering a period of a decade since outburst. We detect mid-infrared (MIR) flares delayed by about two months relative to the optical outburst and with an extremely high peak luminosity of L4.6 μm &gt; 1044 erg s−1. The MIR peak luminosity is at least an order of magnitude higher than any known supernovae explosions, suggesting the optical outburst might be due to a stellar tidal disruption event (TDE). We find late-time X-ray brightening by a factor of ≳30 with respect to what is observed about 100 days after the optical outburst peak, followed by a flux fading by a factor of ∼4 within two weeks, making it one of the active galactic nuclei (AGNs) with extreme variability. Despite the dramatic X-ray variability, there are no coincident strong flux variations in optical, UV, and MIR bands. This unusual variability behavior has been seen in other highly accreting AGNs and could be attributed to absorption variability. In this scenario, the decrease in the covering factor of the absorber with accretion rate could cause the X-ray brightening, possibly induced by the TDE. Most strikingly, while the UV/optical continuum remains almost unchanged with time, an evident enhancement in the flux of the Hα broad emission line is observed about a decade after the nuclear optical outburst, which is an anomalous behavior never seen in any other AGN. Such an Hα anomaly could be explained by the replenishment of gas clouds and excitation within the broad line region (BLR) that perhaps originates from its interaction with outflowing stellar debris. Our results highlight the importance of the late-time evolution of a TDE, which can affect the accreting properties of the AGN, as suggested by recent simulations.

Host galaxy properties of quasi-periodically erupting X-ray sources

Context.Quasi-periodic X-ray eruptions (QPEs) are a recently discovered phenomenon, the nature of which remains unclear. Based on their discovery in active galactic nuclei (AGNs), explanations related to an AGN accretion disk or potentially a stellar tidal disruption event (TDE) have been put forward. Alternatives, including highly unequal mass compact object binaries, have also been proposed to explain their properties.Aims.We perform a systematic study of the five known QPE host galaxies with the aim of providing new insights as to their nature.Methods.We analysed new and archival medium resolution optical spectroscopy of the QPE hosts. We measured emission (and absorption) line fluxes, their ratios, and equivalent widths (EWs) to locate the QPE hosts on diagnostic diagrams. We also measured the velocity dispersion of the stellar absorption lines to estimate their black hole masses.Results.All QPE host galaxies show emission lines in their optical spectra. Based on their ratios and EWs, we find evidence for the presence of an AGN in all sources, including those previously reported as passive. We measure velocity dispersions between 36 and 90 km s−1, implying the presence of low mass (105−6.7 M⊙) black holes, consistent with literature findings. Finally, we find a significant over-representation (two out of the five sources, or a factor of 13−10+13) of quiescent Balmer strong (post-starburst) galaxies among QPE hosts.Conclusions.The presence of a narrow line region consistent with an AGN in all QPE host galaxies implies that a long-lived accretion flow likely plays an integral part in the QPE phenomenon. The strong over-representation of quiescent Balmer strong galaxies among QPE hosts can be naturally explained in both the TDE and interacting extreme mass ratio inspiral hypotheses.

Stellar Tidal Disruption Events with Abundances and Realistic Structures (STARS): Library of Fallback Rates

Abstract We present the STARS library, a grid of tidal disruption event (TDE) simulations interpolated to provide the mass fallback rate (dM/dt) to the black hole for a main-sequence star of any stellar mass, stellar age, and impact parameter. We use a one-dimensional stellar evolution code to construct stars with accurate stellar structures and chemical abundances, then perform tidal disruption simulations in a three-dimensional adaptive-mesh hydrodynamics code with a Helmholtz equation of state, in unprecedented resolution: from 131 to 524 cells across the diameter of the star. The interpolated library of fallback rates is available on GitHub (github.com/jamielaw-smith/STARS_library) and version 1.0.0 is archived on Zenodo; one can query the library for any stellar mass, stellar age, and impact parameter. We provide new fitting formulae for important disruption quantities ( ) as a function of stellar mass, stellar age, and impact parameter. Each of these quantities varies significantly with stellar mass and stellar age, but we are able to reduce all of our simulations to a single relationship that depends only on stellar structure, characterized by a single parameter , and impact parameter β. We also find that, in general, more centrally concentrated stars have steeper dM/dt rise slopes and shallower decay slopes. For the same ΔM, the dM/dt shape varies significantly with stellar mass, promising the potential determination of stellar properties from the TDE light curve alone. The dM/dt shape depends strongly on stellar structure and to a certain extent stellar mass, meaning that fitting TDEs using this library offers a better opportunity to determine the nature of the disrupted star and the black hole.

X-ray flares from the stellar tidal disruption by a candidate supermassive black hole binary

Optical transient surveys have led to the discovery of dozens of stellar tidal disruption events (TDEs) by massive black hole in the centers of galaxies. Despite extensive searches, X-ray follow-up observations have produced no or only weak X-ray detections in most of them. Here we report the discovery of delayed X-ray brightening around 140 days after the optical outburst in the TDE OGLE16aaa, followed by several flux dips during the decay phase. These properties are unusual for standard TDEs and could be explained by the presence of supermassive black hole binary or patchy obscuration. In either scenario, the X-rays can be produced promptly after the disruption but are blocked in the early phase, possibly by a radiation-dominated ejecta which leads to the bulk of optical and ultraviolet emission. Our findings imply that the reprocessing is important in the TDE early evolution, and X-ray observations are promising in revealing supermassive black hole binaries.

Cosmic Distributions of Stellar Tidal Disruptions by Massive Black Holes at Galactic Centers

Abstract Stars can be consumed (either tidally disrupted or swallowed whole) by massive black holes (MBHs) at galactic centers when they move into the vicinity of the MBHs. In this study, we investigate the rates of stellar consumption by central MBHs and their cosmic distributions, including the effects of triaxial galaxy shapes in enhancing the reservoir of low-angular-momentum stars and incorporating realistic galaxy distributions. We find that the enhancement in the stellar consumption rates due to triaxial galaxy shapes can be significant, by a factor of ∼3 for MBH mass – and up to more than one order of magnitude for . Only for are the stellar consumption rates significantly higher in galaxies with steeper inner surface brightness profiles. The average (per galaxy) stellar consumption rates correlate with central MBH masses positively for and negatively for . The volumetric stellar tidal disruption rates are for MBHs in the mass range 105– at z = 0; and the volumetric stellar consumption rates by MBHs with higher masses are , which can be the stellar tidal disruption rate if the high-mass BHs are extremely spinning Kerr BHs or the rate of being swallowed if those BHs are Schwarzschild ones. The volumetric stellar consumption rates decrease with increasing redshift, and the decrease is relatively mild for – and stronger for higher . Most of the stellar tidal disruption events (TDEs) at z = 0 occur in galaxies with mass , and about 1%–2% of the TDEs can occur in high-mass galaxies with .

A Mid-infrared Flare in the Active Galaxy MCG-02-04-026: Dust Echo of a Nuclear Transient Event

Abstract We report the discovery of a mid-infrared (MIR) flare using Wide field Infrared Survey Explorer data in the center of the nearby Seyfert 1.9 galaxy MCG-02-04-026. The MIR flare began in the first half of 2014, peaked around the end of 2015, and faded in 2017. During these years, energy of more than 7 × 1050 erg was released in the infrared, and the flare’s MIR color was generally turning red. We detected neither optical nor ultraviolet (UV) variation corresponding to the MIR flare based on available data. We explained the MIR flare using a dust echo model in which the radiative transfer is involved. The MIR flare can be well explained as thermal reradiation from dust heated by UV–optical photons of a primary nuclear transient event. Although the transient event was not seen directly owing to dust obscuration, we can infer that it may produce a total energy of at least ∼1051 erg, most of which was released in less than ∼3 yr. The nature of the transient event could be a stellar tidal disruption event by the central supermassive black hole (SMBH), or a sudden enhancement of the existing accretion flow onto the SMBH, or a supernova that was particularly bright.

Compact Radio Emission from Nearby Galaxies with Mid-infrared Nuclear Outbursts

Abstract We present 5.5 GHz observations with the Very Large Array of a sample of nearby galaxies with energetic nuclear outbursts at mid-infrared (MIR) bands. These observations reach a uniform depth down to a median rms of ∼10 μJy, representing one of the most sensitive searches for radio emission associated with nuclear transients. We detect radio emission in 12 out of 16 galaxies at a level of &gt;5σ, corresponding to a detection rate of 75%. Such a high detection is remarkably different from previous similar searches in stellar tidal disruption events. The radio emission is compact and not resolved for the majority of sources on scales of ≲0.″5 (&lt;0.9 kpc at z &lt; 0.1). We find that the possibility of the star formation contributing to the radio emission is low, but an active galactic nucleus (AGN) origin remains a plausible scenario, especially for sources that show evidence of AGN activity in their optical spectra. If the detections could represent radio emission associated with a nuclear transient phenomenon such as a jet or outflow, we could use the blast wave model by analogy with the gamma-ray burst afterglows to describe the evolution of radio light curves. In this context, the observations are consistent with a decelerating jet with an energy of ∼1051–52 erg viewed at 30°–60° off-axis at later times, suggesting that powerful jets may be ubiquitous among MIR-burst galaxies. Future continuous monitoring observations will be crucial to decipher the origin of radio emission through detections of potential flux and spectral evolution. Our results highlight the importance of radio observations to constrain the nature of nuclear MIR outbursts in galaxies.

Electromagnetic transients and gravitational waves from white dwarf disruptions by stellar black holes in triple systems

ABSTRACT Mergers of binaries comprising compact objects can give rise to explosive transient events, heralding the birth of exotic objects that cannot be formed through single-star evolution. Using a large number of direct N-body simulations, we explore the possibility that a white dwarf (WD) is dynamically driven to tidal disruption by a stellar-mass black hole (BH) as a consequence of the joint effects of gravitational wave (GW) emission and Lidov–Kozai oscillations imposed by the tidal field of an outer tertiary companion orbiting the inner BH–WD binary. We explore the sensitivity of our results to the distributions of natal kick velocities imparted to the BH and WD upon formation, adiabatic mass loss, semimajor axes and eccentricities of the triples, and stellar-mass ratios. We find rates of WD–tidal disruption events (TDEs) in the range 1.2 × 10−3 − 1.4 Gpc−3 yr−1 for z ≤ 0.1, rarer than stellar TDEs in triples by a factor of ∼3–30. The uncertainty in the TDE rates may be greatly reduced in the future using GW observations of Galactic binaries and triples with LISA. WD–TDEs may give rise to high-energy X-ray or gamma-ray transients of duration similar to long gamma-ray bursts but lacking the signatures of a core-collapse supernova, while being accompanied by a supernova-like optical transient that lasts for only days. WD–BH and WD–NS binaries will also emit GWs in the LISA band before the TDE. The discovery and identification of triple-induced WD–TDE events by future time domain surveys and/or GWs could enable the study of the demographics of BHs in nearby galaxies.

A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy

Abstract We present X-ray and radio observations of the Fast Blue Optical Transient CRTS-CSS161010 J045834−081803 (CSS161010 hereafter) at t = 69–531 days. CSS161010 shows luminous X-ray (L x ∼ 5 × 1039 erg s−1) and radio (L ν ∼ 1029 erg s−1 Hz−1) emission. The radio emission peaked at ∼100 days post-transient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blast wave. CSS161010 launched a mildly relativistic outflow with velocity Γβc ≥ 0.55c at ∼100 days. This is faster than the non-relativistic AT 2018cow (Γβc ∼ 0.1c) and closer to ZTF18abvkwla (Γβc ≥ 0.3c at 63 days). The inferred initial kinetic energy of CSS161010 (E k ≳ 1051 erg) is comparable to that of long gamma-ray bursts, but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ( ). This is consistent with the lack of observed γ-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at ∼150 Mpc in a dwarf galaxy with stellar mass M * ∼ 107 M ⊙ and specific star formation rate sSFR ∼ 0.3 Gyr−1. This mass is among the lowest inferred for host galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally located intermediate-mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare (rate &lt; 0.4% of the local core-collapse supernova rate) H-rich transients that can launch mildly relativistic outflows.

Multiwavelength Study of an X-Ray Tidal Disruption Event Candidate in NGC 5092

Abstract We present multiwavelength studies of a transient X-ray source, XMMSL1 J131952.3+225958, associated with the galaxy NGC 5092 at z = 0.023 detected in the XMM-Newton SLew survey (XMMSL). The source brightened in the band by a factor of in 2005 as compared with previous flux limits and then faded by a factor of &gt;200 as observed with XMM-Newton in 2013 and with Swift in 2018. At the flaring state the X-ray spectrum can be modeled with a blackbody at a temperature of ∼60 eV and an overall luminosity of erg s−1. A UV flare and optical flare were also detected with the Galaxy Evolution Explorer and the Sloan Digital Sky Survey, respectively, within several months of the X-ray flare, whose nonstellar UV–optical spectrum can be described with a blackbody at a temperature of K and a luminosity of erg s−1. Interestingly, mid-infrared monitoring observations of NGC 5092 with the Wide-field Infrared Survey Explorer 5–13 yr later show a continuous flux decline. These dramatic variability properties, from the X-ray through UV and optical to infrared, appear to be orderly, suggestive of a stellar tidal disruption event (TDE) by a massive black hole, confirming the postulation by Kanner et al. This TDE candidate belongs to a rare sample with contemporaneous bright emission detected in the X-ray, UV, and optical, which are later echoed by dust-reprocessed light in the mid-infrared. The black hole has a mass of 10 , residing in a galaxy that is dominated by a middle-aged stellar population of 2.5 Gyr.