A new X-ray transient in the globular cluster Terzan 5

ATel #4242; R. Wijnands, D. Altamirano (University of Amsterdam), C. O. Heinke, G. R. Sivakoff (University of Alberta), D. Pooley (Sam Houston State University, Eureka Scientific)
on 7 Jul 2012; 19:21 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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As part of our program (PI: Altamirano) using Swift/XRT to monitor several globular clusters to detect transient behavior of X-ray binaries in those clusters, we obtained a 1.2 ksec observation of Terzan 5 on 6 July 2012. During this observation, we detect a clear excess of photons which was not present during previous Swift observations (i.e., 26 June 2012, 30 June 2012).

The Swift/XRT unenhanced position (J2000) of this new source is

RA = 17h 48m 05.24s
Dec = -24d 46' 38.4s"

with an error of 3.7" (90% confidence) and the enhanced position is

RA = 17h 48m 05.37s
Dec = -24d 46' 37.5s"

with an error of 2.5", using the online Swift tools ( http://www.swift.ac.uk/user_objects ).

This position is inconsistent (>10" away) with the positions of the two known bright X-ray transients in this cluster (Heinke et al. 2003, ApJ 590, 809; Pooley et al. 2011, ATel #3743), but it is consistent with a faint source detected during previous Chandra observations of Terzan 5 performed in 2000, 2003, and 2009-2012 (CX 2 in Heinke et al. 2006, ApJ 651, 1098). This source was suggested to be a transient neutron-star low-mass X-ray binary in its quiescent state, and the detection of an outburst from this source strongly supports this interpretation. However, we do note that the XRT position is also consistent with an even fainter Chandra source (CX 45 of Heinke et al. 2006). Of course, the presently active source could also be a new source previously undetected in archival Chandra observations. A Chandra observation during the current active state of the source would determine conclusively which source is in outburst.

The source spectrum could be fitted with an absorbed power-law model with a column density of 1.6+0.9-0.6 E22 cm-2 (which is consistent with that of CX 2; Heinke et al. 2006) and a photon index of 2.1+/-0.8. The 0.3-10.0 keV observed flux is 5.4+1.8-1.2 E-12 erg/cm2/s and the 0.3-10 keV unabsorbed flux is 1.4+2.5-0.5 e-11 erg/cm2/s. For a distance of 5.5 kpc (Ortolani et al. 2007, A&A, 470, 1043), this would result in 0.3-10 keV absorbed and unabsorbed X-ray luminosities of ~2e34 erg/s and ~5e34 erg/s, respectively. This is ~10 times brighter than the quiescent luminosity of CX 2 (Heinke et al. 2003, 2006) again demonstrating that the source is in outburst. The current X-ray luminosity is still very low which either indicates that the source is a very-faint X-ray transient with peak outburst luminosities below 1e36 erg/s (see Wijnands et al. 2006, A&A 449, 1117, for a discussion) or we observe the beginning of a brighter outburst. Approved follow-up Swift/XRT observations during the current active state of the source will determine the peak brightness of this source.

We thank the Swift teams at Penn State and Leicester.