Conclusive evidence of Cir X-1 as an accreting neutron star
ATel #2653; A. Papitto (INAF-OAC), E. Bozzo (ISDC), A. D'Aì, R. Iaria, T. Di Salvo (Univ. Palermo), L. Burderi (Univ. Cagliari)
on 30 May 2010; 00:22 UT
Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)
Subjects: X-ray, Binary, Neutron Star, Transient
Following the source brightening reported in ATel #2608, a
monitoring campaign of Cir X-1 has begun, involving observations of
RXTE (ATel #2643) and SWIFT (ATel #2650, ATel #2651). The detections
of type I X-ray bursts by RXTE (ATel #2643), from the 1 deg field of
view around the position of Cir X-1, strongly suggested the
identification of a neutron star as the compact object in this
peculiar binary system. While no type-I X-ray burst was observed by
Swift/XRT between 2010 May 27.827 and May 28.158 (ATel #2650), two bursts
were detected by Swift/XRT on May 28.506 (ATel #2651) and on
May 28.844. The sky region from which the burst possibly came from was
thus restricted to the 23.6arcmin x few arcsec field of view of a
Swift/XRT observation performed in Windowed Timing mode.
In order to
definitely confirm the nature of the compact object in Cir X-1, we
requested a Swift/XRT observation to be carried out in Photon Counting
(PC) mode, to have a clear image of the sky region and assess the
presence of possible contaminating other X-ray sources. PC mode
observation started on 2010 May 29.709. We report that, besides Cir
X-1, no other obvious source is detected
in the FOV covered by the observations during which previous bursts
were detected.
This clearly indicates Cir X-1 as the source of the observed bursts,
and, consequently, a neutron star as the compact object in Cir X-1.
Moreover, this conclusion is also supported by the recurrence time of
the bursts observed by RXTE. RXTE detected three bursts during an
uninterrupted observation starting on 2010 May 20.084, and lasting 5.2 ks
(ATel #2643). The second burst lags the first one by ~1650s, which is
compatible with the expected recurrence time in the case of complete
burning of He into elements of the iron group (~1.6MeV/nucleon
released, Wallace & Woosley 1981), given the burst decay timescale
(tau=16+/-1 s), and the observed ratio between the peak flux of the
second burst and the persistent count rate (1.88+/-0.03).
We note that all the reported epochs are UTC, and not barycentred.
Further Swift observations of Cir X-1 are already planned and ongoing.
We thank the Swift team for promptly scheduling this Target of
Opportunity Observation.