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New NGC 6440 transient declining rapidly

ATEL # 2143; C. O. Heinke, S. A. Budac (Univ. of Alberta), D. Altamirano, M. Linares, R. Wijnands (Univ. of Amsterdam), H. N. Cohn, P. M. Lugger (Indiana Univ.), T. E. Strohmayer (NASA/GSFC), M. Servillat and J. E. Grindlay (Harvard Univ.)
on 1 Aug 2009; 0:08 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Craig Heinke (cheinke@virginia.edu)

Subjects: X-ray, Request for Observations, Binaries, Globular Clusters, Neutron Stars, Transients

The new transient LMXB in NGC 6440 (CXOGlb J174852.7-202124; Atel #2139) is declining rapidly. RXTE observed NGC 6440 on July 30 (18:47 to 19:26 TT). The PCA top-layer spectrum is well fit with an absorbed power-law of photon index 2.2+-0.1, with N_H set to 6e21 cm^-2 (as found by Chandra). The inferred 0.5-10 keV luminosity is 4e35 ergs/s, a decline by a factor of 3.5 from the Chandra observation ~2 days earlier. Swift observed on July 31 (4:55 to 6:22 TT). An absorbed power-law fit with N_H fixed finds a photon index 2.1+-0.4 and L_X of 3.3+-0.6e34 ergs/s, approaching quiescence.

Galactic Center bulge scans with the RXTE PCA indicated possible low-level activity at the beginning of June (~3 mCrab). A Swift XRT observation June 4 found Lx=1.5e34 ergs/s at position RA=17h48m52.7s, Dec=-20:21:24, error 5" (consistent with the new transient), then a June 11 Swift XRT observation found Lx~6e33 ergs/s, consistent with emission from the other cluster sources. A measurement of 7 mCrab by the RXTE/PCA bulge scan on July 28 was the first sign of this outburst. Thus the new transient appears to have undergone two outbursts, both faint and short. We encourage ground-based optical/IR observations within the next few days.

We thank Jean Swank and the Swift team for rapidly scheduling the RXTE and Swift observations respectively, and Craig Markwardt for making the Galactic Center Bulge Scan results rapidly available.


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