New transient LMXB in the globular cluster NGC 6440

ATel #2139; Craig O. Heinke and Sonia A. Budac (Univ. of Alberta)
on 29 Jul 2009; 22:35 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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We report the discovery of a new transient LMXB in the globular cluster NGC 6440 in our Chandra observations of the cluster. We observed NGC 6440 from July 28, 15:06 to July 29, 05:26 (TT). A bright source can be clearly identified ~15" north of the cluster core, along with several known sources in the cluster core. Using the position of the known transient LMXB CX1 as reported by Pooley et al. 2002, ApJ, 573,184 as an astrometric reference, the transient is located at RA=17:48:52.76(2),Dec=-20:21:24.1(1). The transient saturates the central pixels, producing an X-ray halo and readout streak. Extracting the spectrum from the readout streak, we fit it with an absorbed power-law of photon index 1.7+-0.1, N_H=6.2+0.6e21, and Lx(0.5-10 keV)=1.5e36 ergs/s (for an assumed 8.5 kpc distance). Its position near the core of NGC 6440 makes it nearly certain that the transient is located in this globular cluster. The location of the source is inconsistent with any known X-ray source from prior observations of NGC 6440, and with the known radio pulsars in NGC 6440 (Freire et al. 2008, ApJ, 675, 670). No photons are found in the combined 0.3-8 keV image from two previous observations, allowing a quiescent (2 sigma) upper limit of Lx(0.5-10)< 1e31 ergs/s for an assumed power-law of photon index 2, or Lx(0.5-10) < 2.4e31 ergs/s for a blackbody of kT=0.15 keV. The low quiescent luminosity is similar to those seen in several accreting millisecond X-ray pulsars. We request immediate observations of this new transient in all wavelengths.