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
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.