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RXTE detects X-ray bursts from Circinus X-1

ATel #2643; M. Linares (MIT), P. Soleri (Groningen), A. Watts, D. Altamirano, M. Armas-Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, M. van der Klis, A. Patruno, R. Wijnands, Y. Yang (Amsterdam), P. Casella (Southampton), N. Rea (ICE-IEEC)
on 26 May 2010; 20:01 UT
Credential Certification: Manuel Linares (linares@mit.edu)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient

Referred to by ATel #: 2650, 2651, 2653, 2671, 2674, 2699

After the recent report of X-ray re-brightening (ATel #2608), RXTE has observed the peculiar neutron star X-ray binary Cir X-1 eleven times during the last two weeks (May 11-25, 2010). We report the detection of nine X-ray bursts in RXTE-PCA data, 25 years after the first -and the only previous- detection of X-ray bursts from this source (Tennant et al. 1986, MNRAS, 219, 871). At least one of the nine bursts has all the characteristics of a type I (thermonuclear) X-ray burst. Only three type I X-ray bursts had been previously identified from this source (Tennant et al. 1986, MNRAS, 221, 27).

Nakajima et al. (ATel #2608) reported a sudden (within ~3 days) re-brightening of Cir X-1 (up to an intensity of ~0.3 Crab on May 8th) after a ~2 yr-long period of very low activity. RXTE observations reveal a total of nine X-ray bursts between May 15th and 25th, showing that Cir X-1 entered a bursting state for the first time since 1985 (Tennant et al. 1986). These bursts show different profiles, durations between ~30 and ~60 s and peak burst (2.5-25.0 keV) net count rates ranging between ~100 and ~450 c/s/PCU. On May 20th RXTE detected three consecutive bursts with wait times of ~31 and ~28 minutes. Despite apparent deviations from a purely thermal spectrum, all three May-20 bursts feature typical type I X-ray burst light curves and some evidence of spectral softening along the tail. The spectrum of the latest burst on May 25th is well fitted with an absorbed black body model and shows clear cooling along the burst tail, characteristic of type I (thermonuclear) X-ray bursts (see http://space.mit.edu/~linares/CirX1_10052505625.jpg ). We carried out a preliminary search for burst oscillations and found no significant detections.

RXTE monitoring also reveals that the persistent/non-burst emission has been decaying during the last two weeks, from ~0.27 Crab on May 11th down to ~0.02 Crab on May 23d (near periastron according to radio ephemeris in Nicolson 2007, Atel #985). The latest RXTE observation on May 25th shows the source again at a higher average intensity: ~0.16 Crab. We note that the flux is highly variable on time-scales shorter than an RXTE observation, as is common for this source.

We thank the RXTE team for scheduling the observations.

Time-resolved spectroscopy May 25 burst.