Results from the Search for Tidal Disruption Flares in the GALEX Deep Imaging Survey

Source: OAI

ABSTRACT A supermassive black hole in the nucleus of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces and a UV/X-ray flare is emitted by the stream of stellar debris that plunges into the black hole. We initiated a program to take advantage of the UV sensitivity, large volume, and temporal sampling of the Galaxy Evolution Explorer (GALEX) Deep Imaging Survey to search for stellar disruptions in the nuclei of galaxies over a large range of redshifts. Here we present the detection of a luminous UV flare from a quiescent galaxy with the properties of a tidal disruption event.

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    ABSTRACT: Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT All-Sky Survey in several galaxies with no evidence of Seyfert activity in their ground-based optical spectra. These flares had the properties predicted for a tidal disruption of a star by a central supermassive black hole. We report Chandra observations of three of these galaxies taken a decade after their flares that reveal weak nuclear X-ray sources that are from 240 to 6000 times fainter than their luminosities at peak, supporting the theory that these were special events and not ongoing active galactic nucleus (AGN) variability. The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the (t-t_D)^(-5/3) decay predicted for the fall-back phase of a tidal disruption event, but only if ROSAT was lucky enough to catch the event exactly at its peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also consistent with (t-t_D)^(-5/3). In the H II galaxy NGC 5905 we find only resolved, soft X-ray emission that is undoubtedly associated with starburst activity. When accounting for the starburst component, the ROSAT observations of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are consistent with a (t-t_D)^(-5/3) decay by at least a factor of 1000. Although we found weak Seyfert~2 emission lines in Hubble Space Telescope spectra of NGC 5905, indicating that a low-luminosity AGN was present prior to the X-ray flare, we favor a tidal disruption explanation for the flare itself. Comment: 17 pages, 4 figures, to appear in ApJ April 1 2004
    The Astrophysical Journal 02/2004; 604(2). DOI:10.1086/381937 · 6.28 Impact Factor
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    ABSTRACT: We compute rates of tidal disruption of stars by supermassive black holes in galactic nuclei, using downwardly-revised black hole masses from the M-sigma relation. In galaxies with steep nuclear density profiles, which dominate the overall event rate, the disruption frequency varies inversely with assumed black hole mass. We compute a total rate for non-dwarf galaxies that is about a factor ten higher than in earlier studies. Disruption rates are predicted to be highest in nucleated dwarf galaxies, assuming that such galaxies contain black holes. Monitoring of a rich galaxy cluster for a few years could rule out the existence of intermediate mass black holes in dwarf galaxies. Comment: 11 pages, 9 figures, uses emulateapj.sty
    The Astrophysical Journal 05/2003; 600(1). DOI:10.1086/379767 · 6.28 Impact Factor
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    ABSTRACT: We present a Chandra ACIS study of the early-type galaxy NGC 4552. We detect 47 X-ray point sources, most of which are likely LMXBs, within 4 R_e. The brightest X-ray source coincides with the optical/UV/radio center of the galaxy, and shows variability on >1 hr timescales, indicating the possible existence of a LLAGN. The 46 off-center sources and the unresolved point sources contribute about 29% and 20% to the total luminosity of the galaxy, respectively. The corrected cumulative XLF of the off-center sources is best fit by a broken power-law model with a break at L_b=4.4+2.0-1.4 e38 ergs/s. We identified 210 GCs candidates in a HST WFPC2 optical image of the galaxy's central region. Of the 25 off-center LMXBs that fall within the WFPC2 FOV, 10 sources are coincident with a GC. Thus the fraction of the GCs hosting bright LMXBs and the fraction of the LMXBs associated with GCs are 4.8% and 40%, respectively. In the V and I bands, the GCs hosting bright LMXBs are typically 1-2 magnitudes brighter than the GCs with no detected LMXBs. There are about 1.9+-0.4 times as many LMXBs in the red, metal-rich GCs as there are in the blue, metal-poor ones. We find no obvious difference between the luminosity distributions of LMXBs in GCs and in field, but the cumulative spectrum of the LMXBs in GCs tends to be softer than that of the LMXBs in field. We detected 3 X-ray sources that have isotropic luminosities larger than 1e39 erg/s. The one located in the joint Chandra-HST field is found to be associated with a GC. By studying the ACIS spectra we infer that the this may be a candidate black hole system with a mass of 15-135 M_sun. One of the other sources with a luminosity brighter than 1e39 ergs/s reveals temporal variations in brightness on timescales greater than an hour. Comment: Accepted for publication in ApJ
    The Astrophysical Journal 06/2005; 631(2). DOI:10.1086/432636 · 6.28 Impact Factor


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