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A New Fast, Soft X-Ray Transient Discovered with the RXTE/ASM Smith, D. A. & Remillard, R. The All-Sky Monitor on RXTE has detected an unusually soft rapid X-ray transient that we designate XTE J1200+521. It was detected in nine consecutive observations, allowing the position to be constrained to a ~30' error box at 90% confidence (including an estimate of systematic error) with the following corners: RA: Decl: (J2000.0) 12h 00m 10s 5d 6'.0 12h 00m 36s 5d 21'.0 12h 00m 10s 5d 36'.0 11h 59m 36s 5d 21'.0 The X-ray transient was first detected on 2003 Mar 18 at 12:08:12 (UTC) at about 300+-40 mCrab in the A band (1.5-3 keV), and 250+-20 mCrab in the sum band (1.5-12 keV). There was no evidence for any x-ray emission from this location during the last previous observation at 10:40:28 (UTC), constraining the onset of the flare to a less than two-hour interval. The transient decayed smoothly during the nine observations, falling to an intensity of 90+-20 mCrab (1.5-12 keV) over thirteen minutes. Examination of the time series data yields no significant evidence for intensity variation on time scales of seconds. When the source was next observed at 13:51:40 (UTC), its sum-band intensity had fallen below a 2-sigma detectibility threshhold of around 50 mCrab (1.5-12 keV, 90 s exposure). The transient flare seems too long to be a normal Type I X-ray burst (or Gamma-Ray Burst) but too short to be one of the multi-hour "superbursts" seen from a few X-ray binaries (e.g. Kuulkers et al., A&A, 2002). It is much softer than other fast X-ray transients seen with the ASM such as V4641 Sgr or CI Cam (Bradt et al., 2000, astro-ph/0003438). It is also at an extremely high galactic latitude, 65 degrees north of the plane. It could be a very rapid stellar flare, such as that seen from AB Dor in only three ASM observations on 1998 Jun 09. A SIMBAD search of the error box reveals an unidentified ROSAT source, 1RXS J120011.2+051056, at 12h 00m 11s.20, +05d 10' 56".5, as well as a nearby, high-proper-motion G5 star, HD 104243, 12h 00m 14s.39, +05d 21' 48".7 (J2000.0). It is not possible from the ASM data to determine if either of these sources is a more likely candidate for the source of the soft X-ray flare. Observations at other wavelengths to determine if either of these sources is currently active are encouraged.
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