New X-ray transient outburst in Terzan 5

ATel #3714; D. Altamirano (U. Amsterdam), N. Degenaar (U. Amsterdam), C. O. Heinke (U. Alberta), J. Homan (MIT), D. Pooley (Sam Houston State U.), G. R. Sivakoff (U. Alberta), R. Wijnands (U. Amsterdam)
on 26 Oct 2011; 15:51 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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We have been monitoring the globular cluster Terzan 5 with weekly RXTE observations. Our observation from 4:57 to 5:28 UT on Oct. 26, 2011 found an RXTE PCA intensity of 0.008(2) Crab, vs. typical quiescent intensity of 0.002(1) Crab (2-16 keV). No indications of pulsations are visible yet, but the countrate is only 43 cts/s/PCU including background. Swift/BAT daily-averaged flux measurements (http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/EXO1745-248/ ) confirm the flux increase, reaching 0.0058(8) cts/cm^2/s (15-50 keV), vs. typical quiescent flux of +-0.001 cts/cm^2/s.

This could be a new outburst of EXO 1745-248 (Markwardt & Swank 2000, IAUC # 7454 ; Wijnands et al. 2005, ApJ, 618, 883), or of IGR J17480-2446 (Atel #2919,#2922,#2929,#2933,#2937,#2946,#2952,#2974 ), or of one of the numerous other quiescent X-ray binaries in Terzan 5 (Heinke et al. 2006, ApJ, 651, 1098). Identifying which X-ray binary is undergoing outburst is critical for understanding the outburst behavior of X-ray binaries, and for studying the cooling of their crusts after outbursts (e.g. Degenaar & Wijnands 2011, MNRAS, 414, L50). Swift, RXTE and Chandra observations have been requested to identify the source's position and measure its X-ray characteristics. We encourage rapid observations with X-ray and radio instruments (due to the high N_H and cluster environment, optical/IR observations are very difficult).