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ATEL # 2387; Jeroen Homan
on 18 Jan 2010; 0:17 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Jeroen Homan (email@example.com)
Subjects: X-ray, Request for Observations, Black Holes, Transients
Referred to by ATEL #: 2391, 2396, 2424
A comparison of publicly available Swift/BAT 15-50 keV and MAXI/GSC 1.5-4.0 keV light curves of the recently discovered X-ray transient and possible black hole candidate XTE J1752-223 indicates that the source is currently undergoing a transition from a spectrally hard to a spectrally soft X-ray state. Since the discovery of the source in late October 2009 (ATel #2258) until early January 2010 the low- and high-energy light curves had similar shapes, suggesting that the source showed little spectral evolution and had remained in the hard state (as deduced from RXTE observations; ATel #2261).
The Swift/BAT 15-50 keV light curve of XTE J1752-223 shows a slow decay starting around January 4, followed by a more rapid decline after January 12; count rates on January 17 were ~60% lower than on January 4. The MAXI/GSC 1.5-4.0 keV light curve of the source, on the other hand, reveals a slow increase starting around January 4; count rates on January 15 were ~25% higher than on January 4.
The observed anti-correlated changes in the low- and high-energy light curves are typical for spectral state transitions in X-ray binaries. Transitions from hard to soft spectral X-ray states, such as the one currently happening in XTE J1723-223, typically take several weeks to complete and are often accompanied by ejection events that are observable as radio flares. We note that XTE J1752-223 has already been detected with ATCA in early November 2009 (ATel #2278), when the source was still in the hard state. Further radio observations, as well as observations at other wavelengths (when visibility windows open again), are strongly encouraged.
Swift/BAT light curve: http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/XTEJ1752-223/
MAXI/GSC light curve: http://maxi.riken.jp/top/index.php?cid=000000000001&lc_star_dir_id=00000000062