[vsnet-alert 11362] X-ray transient in NGC 6440

[Taichi Kato]

   Could someone check NGC 6440 for a potential optical transient?

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The Astronomer's Telegram                   http://www.astronomerstelegram.org 
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ATEL #2139                                                           ATEL #2139

Title:          New transient LMXB in the globular cluster NGC 6440
Author: Craig O. Heinke and Sonia A. Budac (Univ. of Alberta)
Queries:        heinke at ualberta.ca
Posted: 29 Jul 2009;  22:35  UT
Subjects:       X-ray, Request for Observations, Binaries, Globular Clusters,
                Neutron Stars, Transients

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.