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ATCA radio non-detection of IGR J17361-4441

ATel #3617; C. Ferrigno (ISDC, Switzerland), J. Stevens (CSIRO Astronomy and Space Science, Australia), E. Bozzo (ISDC, Switzerland), T. M. Belloni (INAF - Brera Observatory, Italy), P. R. den Hartog (Stanford University HEPL, USA), F. Fontani (INAF - Arcetri Observatory, Italy), I. Kreykenbohm (Dr. Karl Remeis-Sternwarte & ECAP, Germany), A. Papitto (University of Cagliari, Italy), J. Rodriguez (CEA, France)
on 31 Aug 2011; 16:24 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

Subjects: Radio, X-ray, Black Hole, Globular Cluster, Transient

Referred to by ATel #: 3627

Following the INTEGRAL discovery of the new hard X-ray transient IGR J17361-4441 in the Globular Cluster NGC 6388 (ATEL #3565, #3566), a target of opportunity observation with the Australian Telescope Compact Array (ATCA) was triggered. ATCA observed the field centered on the reported Swift/XRT position for a total exposure time of 8h in the configuration 6D (maximum baseline of 6 km). The observations were conducted with centre frequencies of 5.5 GHz, 9 GHz, 17 GHz, and 19 GHz, with 2 GHz bandwidth around each using the CABB correlator (Wilson et al 2011, MNRAS, in press). The total integration time was about 5.3 h for the 2 lower frequencies (observed simultaneously) and 2.7 h for the higher frequencies (observed simultaneously).

Natural-weighted images were made from the ATCA data, with fields-of-view (FWHM) of 8.5, 5.2, 2.8, and 2.5 arcmin, and synthesized beam sizes (FWHM) of 4.5×1.8, 2.7×1.1, 1.7×0.6, and 1.5×0.5 arcsec at 5.5, 9, 17, and 19 GHz respectively. No hint of radio emission was found within 4 arcmin from the Swift position of IGR J17361-441 (ATEL #3566). The RMS noise levels achieved in the images were 14.1 μJy, 19.0 μJy, 20.0 μJy, and 21.2 μJy for the 5.5, 9, 17, and 19 GHz frequencies, respectively.

The lack of a radio counterpart in NGC 6388 makes it less likely that the new transient is related to the suspected intermediate massive black hole at the center of the globular cluster (see Nucita et al. 2008, A&A, 478, 736; Cseh et al. 2010, MNRAS, 406, 1049). The current X-ray flux of the source (ATEL #3566) and the ATCA upper limits indeed do not favor a black hole nature for this transient, as its location on the BH fundamental plane (Merloni, 2003, MNRAS, 345, 1057; Kording et al. 2006, A&A, 456, 439) would imply a black hole mass lower than solar.

Assuming that the central BH remained inactive and the X-ray outburst is due to another object, the results of the ATCA observation would give a 3 sigma upper limit on the mass of the of the BH in NGC 6388 of 600 Solar masses, roughly a factor three lower than the value reported by Cseh et al. (2010). This improvement is due to our more stringent upper limits on the radio emission.

A refined analysis of the ATCA data is on-going. We thank the ATCA team, in particular Dr. Philip Edwards, for the very rapid scheduling of our ToO observation and technical assistance.

Further observations at different wavelengths are strongly encouraged. NGC 6388 is presently being monitored daily with Swift.