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Title:
Discovery of an optical counterpart to the hyperluminous X-ray source in ESO 243-49
Authors:
Soria, Roberto; Hau, George K. T.; Graham, Alister W.; Kong, Albert K. H.; Kuin, N. Paul M.; Li, I-Hui; Liu, Ji-Feng; Wu, Kinwah
Publication:
eprint arXiv:0910.1356
Publication Date:
10/2009
Origin:
ARXIV
Keywords:
Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Cosmology and Extragalactic Astrophysics
Comment:
5 pages, submitted to MNRAS Letters. Contact R Soria for higher-resolution figures
Bibliographic Code:
2009arXiv0910.1356S

Abstract

The existence of black holes of masses ~ 10^3-10^4 Msun has important implications for the formation and evolution of star clusters and galaxies. The strongest candidate so far is the hyperluminous X-ray source HLX1, apparently located in the S0-a galaxy ESO 243-49, but the lack of an identifiable optical counterpart had hampered its interpretation. Using the Magellan telescope, we discovered an unresolved optical source with R = (24.6 +/- 0.2) mag and V = (25.4 +/- 0.3) mag within the X-ray error circle. This implies an X-ray/optical flux ratio ~ 1000. Taking the same distance as ESO 243-49, we obtain an intrinsic brightness M_R = (-10.2 +/- 0.3) mag. With the combined optical and X-ray measurements, we put constraints on the nature of HLX1. We rule out a foreground star and a background AGN. A foreground accreting neutron star is unlikely but cannot be completely ruled out. We also examined the properties of the host galaxy by combining Swift/UVOT observations with stellar population modelling. We found that the overall emission from ES0 243-49 is dominated by a ~ 2-5 Gyr old stellar population, but the far-UV emission is mostly due to ongoing star-formation at a rate of ~ 0.03 Msun/yr. There is a 15% excess above the mean in the far-UV emission North East of the nucleus, towards HLX1, which we interpret as evidence of more recent or intense star formation in that region. The brightness of the optical counterpart is comparable to that of massive globular clusters. We suggest that HLX1 could be an accreting intermediate mass black hole in a star cluster. The cluster could also be the stripped nucleus of a dwarf galaxy as it passed through ESO 243-49, an event which might have caused the current episode of star formation along its trail.
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