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Radio follow-up of the ongoing transient event in NGC 6440

ATEL # 2377; J.C.A. Miller-Jones (NRAO), C.O. Heinke (U Alberta), G.R. Sivakoff (U Virginia), D. Pooley (U Wisconsin), J. Homan (MIT), D. Altamirano (U Amsterdam)
on 9 Jan 2010; 3:40 UT
Password Certification: James C.A. Miller-Jones (jmiller@science.uva.nl)

Subjects: Radio, Globular Clusters, Neutron Stars, Transients

A bright outburst, peaking at about 250 mCrab was recently detected from the globular cluster NGC 6440 (ATel #2360). This globular cluster is known to contain at least two transient X-ray sources (in 't Zand, 1998; Heinke et al., 2009), both of which are accreting millisecond X-ray pulsars (AMXPs; Gavriil et al., 2007; Altamirano et al., 2009), and several candidate quiescent X-ray binaries (Pooley et al., 2002). Owing to the 1 degree spatial resolution of MAXI, the source responsible for the outburst could not be identified, and pointed X-ray follow-up observations were precluded by the proximity of the source to the Sun. We therefore performed a 2h Target of Opportunity observation of NGC 6440 with the VLA at 8.4 GHz to ascertain the source of the bright X-ray emission. The array was in its most compact D configuration. 5 sources were significantly detected within the 11 arcmin field of view, with the following positions and flux densities:
Source RA (J2000) Error (s.sss) Dec (J2000) Error (s.ss) Flux (mJy) Error (mJy)
CX 17 48 52.032 0.060 -20 21 53.31 1.02 0.157 0.024
1 17 48 46.285 0.008 -20 21 36.07 0.14 1.307 0.042
2 17 48 47.943 0.004 -20 19 58.87 0.07 2.473 0.041
3 17 48 49.258 0.027 -20 25 35.29 0.47 0.347 0.037
4 17 48 22.412 0.037 -20 29 09.86 0.57 2.900 0.414
Sources 1, 2, and 3, respectively, correspond to sources 1, 2, and 3 of Knapp et al. (1996).

Source 1 is the only source significantly detected within the cluster half-mass radius. This source is positionally coincident with source 1 of Knapp et al. (1996), albeit a factor of two fainter in flux density than their measurement, taken in 1991 February. However, Source 1 does not correspond to any known X-ray source. The closest is CX8 of Pooley et al. (2002), 6.8 arcsec away. At the positions of the known transient X-ray sources, SAX J1748.9-2021 and NGC 6440 X-2, the measured flux densities were 6.4 and 0.3 microJy/beam respectively, compared to the image noise level of 24 microJy/beam.

We estimate the X-ray luminosity of NGC 6440 by comparing its MAXI 4-10 keV lightcurve to that of the Crab, and using a distance of 8.5 kpc (Ortolani et al. 1994), find a luminosity of (3+/-1)e37 erg/s.

For the above distance, our radio nondetection of the two known AMXPs in the globular cluster corresponds to a 3-sigma upper limit on the radio luminosity of 5.1e28 erg/s. If the current transient is due to one of these sources, their non-detection is a factor of 3.5 below the neutron star radio/X-ray correlation of Migliari & Fender (2006). This radio upper limit, combined with radio detections at higher X-ray luminosities, suggests that the radio/X-ray correlation of Migliari & Fender (2006) does not continue beyond L_x~10e37 erg/s, perhaps due to quenching of the radio emission during a spectral state change, as seen in black hole systems and suggested for Aql X-1 (Tudose et al. 2009). However, with the present data, we cannot definitively identify the source responsible for the current outburst, which could equally be a new transient. Observations at other frequencies are therefore strongly encouraged once the source is in a more favorable position.

The VLA is a facility of the NRAO which is operated by AUI, under cooperative agreement with the NSF.

VLA image of NGC 6440


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