Search for Faint Black Hole Transients: Results for EXO1846-031

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Session 116 -- Globular Clusters and Compact Objects
Oral presentation, Thursday, 12, 1995, 10:00am - 11:30am

[116.07] Search for Faint Black Hole Transients: Results for EXO1846-031

J. Grindlay, D. Barret, P. Bloser, M. Garcia, J. McClintock, P. Zhao (CfA), S. Zhang, A. Harmon (MSFC), W. Paciesas (Univ. Alabama), E. Ford, P. Kaaret, M. Tavani (Columbia)

Black hole candidates (BHCs) are increasingly found as luminous hard x-ray sources, with spectra exending up to 100-200 keV, which undergo transient outbursts and appear as "X-ray novae" for durations of typically 10-100 days. The BATSE detectors on CGRO constitute a powerful all-sky monitor and have detected the brightest of these, with peak fluxes typically 1-3 times that of the Crab, at a rate of 1-2/year. Optical counterparts of these suggest they are typically within 3-5 kpc. The CfA-BATSE team has initiated a survey for much fainter BHCs in an effort to better constrain the total population of BH binaries in the Galaxy. Our search will extend down to transients with peak outbursts of about 0.1 Crab, which should then occur at a total rate of about 10/year in the Galaxy. We are studying 7 historical BHC transients to both search for recurrent outbursts as well as new (faint) transients in their adjoining fields. In the field for the BHC and ultra-soft source EXO1846-031 (cf. Parmar et al 1993, Astron. Ap., 279, 179), we have found in collaboration with the Columbia team a recurrent outburst in September 1992. Further analysis has revealed three other outbursts, with recurrence times of typically 300-400 days, outburst durations of about 10-30 days, and peak fluxes of about 0.1 Crab. The most recent outburst (beginning about September 23, 1994, and discovered on October 13) was searched for an optical counterpart with moderately deep I-band imaging with the 1.2m telescope at the Whipple Observatory on UT 14.15 October. No optical counterpart brighter than I = 21.5 in the EXOSAT error circle (Parmar et al) was detected, suggesting the source is either very distant or heavily absorbed or that this outburst decayed more quickly. Results will be presented for both the x-ray data, including spectra, and the optical search for this still unidentified BHC. A general progress report for the Faint Black Hole Transient Survey will also be provided.

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