Rapid onset of a Comptonisation zone in the repeating tidal disruption event XMMSL2 J140446.9-251135

TL;DR

This study presents XMMSL2 J1404-2511 as a tidal disruption event in a quiescent host with an unusually rapid transition from a thermal disc to a warm Comptonising corona, forming within roughly $10$–$30$ days and persisting through a $\sim100$-day plateau before a steep decay in X-ray/bolometric luminosity by about a factor of $500$ over $230$ days. Using multiwavelength data (X-ray, optical/UV, radio) and spectral modelling (COMPBB/SIMPL/TDEDISCSPEC), the authors constrain a corona with $kT_e\sim$ a few keV and $\tau\sim$ a few, and infer a black hole mass around $M_{\rm BH}\approx 4\times10^{6}\,M_{\odot}$, with an accreted mass of $\sim0.003\,M_\odot$ and a bolometric peak near $L_{\rm bol}\sim8\times10^{43}$ erg s$^{-1}$. The discovery of repeated flares in eROSITA data supports a scenario of multiple partial disruptions, while a comparison with optically selected TDEs suggests coronal X-ray emission is more common in X-ray selected events, inviting further long-term monitoring to disentangle intrinsic differences from selection effects. Together, these results place strong constraints on the timescales and physical conditions required to form and dissipate a warm corona during TDE evolution, with implications for accretion physics around supermassive black holes.

Abstract

We report here on observations of a tidal disruption event, XMMSL2 J1404-2511, discovered in an XMM-Newton slew, in a quiescent galaxy at z=0.043. X-ray monitoring covered the epoch when the accretion disc transitioned from a thermal state, with kT~80 eV, to a harder state dominated by a warm, optically-thick corona. The bulk of the coronal formation took place within 7 days and was coincident with a temporary drop in the emitted radiation by a factor 4. After a plateau phase of ~100 days, the X-ray flux of XMMSL2 J1404-2511 decayed by a factor 500 within 230 days. We estimate the black hole mass in the galaxy to be $M_{BH}=4\pm{2}\times10^{6}$ solar masses and the peak X-ray luminosity $L_{X}\sim6\times10^{43}$ ergs/s. The optical/UV light curve is flat over the timescale of the observations with $L_{opt}\sim 2\times10^{41}$ ergs/s. We find that TDEs with coronae are more often found in an X-ray sample than in an optically-selected sample. Late-time monitoring of the optical sample is needed to test whether this is an intrinsic property of TDEs or is due to a selection effect. From the fast decay of the X-ray emission we consider that the event was likely due to the partial stripping of an evolved star rather than a full stellar disruption, an idea supported by the detection of two further re-brightening episodes, two and four years after the first event, in the SRG/eROSITA all-sky survey.

Figures (13)

• Figure 1: Finder chart of XMMSL2 J1404-2511 based on an r' band image from the Legacy DR10 survey performed on DECam on the Blanco CT10 telescope. Red circle is the XMM- Newton slew error circle of 8 arcseconds radius and the white circle is the Swift-UVOT enhanced position with an error of 1.9 arcseconds.
• Figure 2: Optical spectrum of 2MASX 1404-25 taken with the New Technology Telescope (NTT) on 2018-03-08 as part of the ePESSTO program.
• Figure 3: Host galaxy photometry based on GALEX Martin05, DECaLS Dey19, and WISE Wright10Mainzer11. We also include the post-peak aperture photometry in the UVOT B- and U-band. We show samples from the posterior distribution of population synthesis galaxy models Conroy09Johnson17, following the method outlined in vanVelzen21. The predicted magnitudes (including UVOT W1, M2, and W2) are shown with open circles.
• Figure 4: Light curve of XMMSL2 J1404-2511 in the Swift-UV filters (upper) and in V and G filters from the ASAS-SN Sky Patrol v2.0 project Hart23Shappee14 (lower) where the vertical green lines indicate the dates of the three flares seen in X-ray data. Magnitudes represent the host galaxy light, which dominates in the optical filters, plus light from the TDE which dominates in the UV.
• Figure 5: Observed soft X-ray (0.2-2 keV) flux light curve of XMMSL2 J1404-2511 during 2018 and 2019. XMM-Newton points are shown as red circles and Swift-XRT points as blue diamonds or a blue downward triangle for the upper limit. Fluxes have been calculated using the spectral model COMPBB (see Tab. \ref{['tab:specfits']}).