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MAXI/GSC detection of an outburst from GX 304-1

ATEL # 2297; T. Yamamoto (RIKEN), S.Nakahira (Aoyama U.), N.Kawai (Tokyo Tech), H.Negoro (Nihon U.), K.Makishima, M.Sugizaki, (RIKEN), M.Nakajima (Nihon U.), K. Sugimori (Tokyo Tech), M.Matsuoka (JAXA) and the MAXI team
on 15 Nov 2009; 3:31 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Nobuyuki Kawai (nkawai@phys.titech.ac.jp)

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

On November 14 with MAXI/GSC, we detected an X-ray source with an increasing intensity at a position consistent with GX 304-1, a transient X-ray binary pulsar with a Be star companion. With the examination of the earlier data, we find that the current outburst had already started on November 11. The preliminary daily average flux (2-20 keV) of the source was 25 mCrab on November 11, 44 mCrab on November 12, and 54 mCrab on November 13 (uncertainty +/-5 mCrab).

We note that the RXTE/ASM light curve (http://xte.mit.edu/asmlc/srcs/gx304-1.html) and the Swift/BAT light curve (http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/GX304-1/) shows that GX304-1 underwent an outburst peaking at around July 10, 2009. The start of the current outburst is consistent with its recurrence period of 132.5 days, and predicts the peak at around November 19. These consecutive outbursts may indicate that GX 304-1 entered an active phase after the extended low activity since early 1980's. In the long-term RXTE/ASM light curve, the outburst in July 2009 was the first major activity of GX 304-1 since 1996, although a detection of the source had been reported by INTEGRAL in 2008 June-July at a 20-40 keV intensity of 3-7 mCrab (ATEL #1613).

MAXI is currently in the commissioning phase. We are planning to start distributing the light curves in December 2009.


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