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)

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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.