Late time X-ray, IR and radio observations of the tidal disruption event galaxy NGC 5905

Abstract

NGC 5905 is one of the few galaxies with no prior evidence for an active galactic nucleus (AGN) in which an X-ray flare was detected by the ROSAT All Sky Survey (RASS) in 1990–1991. Follow-up studies showed that the X-ray flare was due to the tidal disruption of a star by the massive black hole in the center of the galaxy. In this study we present analysis of late-time follow-up observations of NGC 5905 using Chandra archival data, Spitzer archival data, GMRT 1.28 GHz radio observations and VLA 3 GHz and 8 GHz archival data. The X-ray image shows no compact source that could be associated with an AGN. Instead, the emission is extended—likely due to nuclear star formation. The total measured X-ray luminosity from this extended emission region is comparable to the X-ray luminosity determined from the 2002 Chandra observations and is a factor of 200 less than the peak of the X-ray flare observed in 1990. Diffuse X-ray emission was detected close to the circum-nuclear star forming ring. The Spitzer 2006 mid-infrared spectrum also shows strong evidence of nuclear star formation but no clear AGN signatures. The semi-analytical models of Tommasin et al. (Astrophys. J. 709:1257, 2010) together with the measured [OIV]/[NeII] line ratio suggest that at most only 5.6 % of the total IR Flux at 19 μm is being contributed by the AGN. The GMRT 1.28 GHz observations reveal a nuclear source. In the much higher resolution VLA 3 GHz map, the emission has a double lobed structure of size 2.7′′ due to the circumnuclear star forming ring. The GMRT 1.28 GHz peak emission coincides with the center of the circumnuclear ring.

We did not detect any emission in the VLA 8 GHz (1996) archival data. Instead we give upper limits to the radio afterglow of the tidal disruption event (TDE) using 3 σ upper limits where σ is the map noise. The 3 σ limits at 1.28 GHz, 3 GHz and 8 GHz are 0.17 mJy, 0.09 mJy and 0.09 mJy, respectively. Our studies thus show that (i) NGC 5905 has a declining X-ray flux consistent with a TDE, (ii) the IR flux is dominated by nuclear star formation, (iii) the nuclear radio emission observed from the galaxy is due to circumnuclear star formation, (iv) no compact radio emission associated with a radio afterglow from the TDE is detected.