The Absence of Radio Emission from the Globular Cluster G1

Details The Absence of Radio Emission from the Globular Cluster G1 The Absence of Radio Emission from the Globular Cluster G1

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21924701w&link_type=abstract

American Astronomical Society, AAS Meeting #219, #247.01

Astronomy and Astrophysics

Astronomy

Scientific paper

The globular cluster G1 in M31 has been suggested as a good candidate to host an intermediate-mass black hole. An excess of dark mass at the cluster center was inferred from studies of the stellar dynamics, and the subsequent detection of both X-ray and radio emission from positions consistent with the cluster core suggested the presence of an accreting black hole. From the ratio of radio to X-ray luminosities, the black hole mass was believed to fall in the range 500-19,000 solar masses, although a variable stellar-mass X-ray binary could not be ruled out owing to the non-simultaneity of the radio and X-ray observations. We therefore made strictly-simultaneous observations in 2011 with Chandra and the Expanded Very Large Array (EVLA) to determine the ratio of radio to X-ray luminosities. The EVLA was in its A configuration, providing a FWHM resolution of 0.4 arcsec near 6 GHz. While the X-ray emission was consistent with the previously-reported level of about 2x1036 ergs/s (2-10 keV), no radio emission was detected from the cluster to a 5-sigma upper limit of 8 microJy/beam, about 3.5 times lower than the previously-reported radio detection in 2006. We discuss two possible explanations for the dramatic radio non-detection, namely that the radio source in G1 is time-variable, or that the previous 4.5-sigma source was an artifact, possibly caused by the mix of VLA and EVLA electronics in use in 2006. Although our simultaneous measurements in 2011 can constrain the mass of a central black hole through the empirical radio--X-ray--mass relation for accreting systems, the simplest explanation is that the X-ray emission from G1 arises from a low-mass X-ray binary. The NRAO is a facility of the NSF operated under cooperative agreement by AUI.

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