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Late Time X-ray, IR and Radio Observations of the Tidal Disruption Event Galaxy NGC~5905

by Harsha Raichur (Nordita & Raman Research Institute)

122:026 ()


NGC 5905 is one of the few galaxies with no prior evidence for a Type-I AGN in which an X-ray flare was detected by the ROSAT All Sky Survey (RASS) in 1990-91. 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 centre 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 1280 MHz radio observations and VLA 3 GHz radio data. The X-ray image shows no compact source that could be associated with an active galactic nucleus (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 AGN signatures. The GMRT 1280 MHz observations reveal a nuclear source that has a size similar to the circum-nuclear star forming ring. When a UV cutoff is applied, the corresponding high resolution map has a peak close to the optical nucleus of the galaxy. However, in the much higher resolution VLA 3 GHz map, the emission has a double lobed structure of size 2.7^". It is probably due to a ring or thick disk of hot gas associated with the circum-nuclear star forming ring. The GMRT 1280 MHz peak emission coincides with the centre of the circum-nuclear ring. We could not detect any compact emission that could be a radio afterglow from the tidal disruption event (TDE). At 3 GHz the map noise gives an upper limit of 32 uJy and at 1.28 GHz the limit is 206 uJy for the radio afterglow of the TDE. Our studies thus show that (i) NGC 5905 has a declining X-ray flux consistent with a TDE; (ii) the nuclear radio emission observed from the galaxy is due to circum-nuclear star formation; (iii) No compact emission associated with a radio afterglow from the TDE was detected; we can only give the upper limits as stated above