A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5

SDSS J120136.02+300305.5 was detected in an XMM-Newton slew from June 2010 with a flux 56 times higher than an upper limit from ROSAT, corresponding to Lx~3x10^44 ergs/s. It has the optical spectrum of a quiescent galaxy (z=0.146). Overall the X-ray flux has evolved consistently with the canonical t^-5/3 model, expected for returning stellar debris from a tidal disruption event, fading by a factor ~300 over 300 days. In detail the source is very variable and became invisible to Swift between 27 and 48 days after discovery, perhaps due to self-absorption. The X-ray spectrum is soft but is not the expected tail of optically thick thermal emission. It may be fit with a Bremsstrahlung or double-power-law model and is seen to soften with time and declining flux. Optical spectra taken 12 days and 11 months after discovery indicate a deficit of material in the broad line and coronal line regions of this galaxy, while a deep radio non-detection implies that a jet was not launched during this event.

💡 Research Summary

The paper presents a comprehensive multi‑wavelength study of an X‑ray flare discovered in the XMM‑Newton slew survey in June 2010, associated with the galaxy SDSS J120136.02+300305.5 (z = 0.146). Prior ROSAT observations only yielded an upper limit, making the XMM‑Newton detection—approximately 56 times brighter than the ROSAT limit—a striking outburst with an inferred 0.2–2 keV luminosity of ~3 × 10⁴⁴ erg s⁻¹. Optical spectroscopy classifies the host as a quiescent galaxy lacking any prior signs of nuclear activity.

The authors assembled a dense X‑ray light curve using Swift/XRT, XMM‑Newton, and Chandra observations. The flare follows the canonical t⁻⁵/³ decline expected for the fallback of stellar debris after a tidal disruption event (TDE), fading by a factor of ~300 over roughly 300 days. Notably, the source vanished from Swift’s view between 27 and 48 days post‑discovery, a behavior the authors attribute to transient self‑absorption or a temporary increase in the optical depth of the nascent accretion flow.

Spectral analysis reveals that the emission is soft but does not conform to a simple black‑body tail from an optically thick, thermal disk. Instead, the data are better described by either a bremsstrahlung model or a double‑power‑law representation, the latter consisting of a steep low‑energy component and a flatter high‑energy tail. As the flux declines, the spectrum systematically softens, suggesting cooling of the emitting region or a reduction in high‑energy photon production as the accretion rate drops.

Two optical spectra obtained 12 days and 11 months after the X‑ray discovery show a striking deficit of both broad‑line region (e.g., Hα, Hβ) and coronal‑line emission (