Swift observations show MAXI J0051-736 is a new outburst of SMC X-2
ATel #8091; J. A. Kennea, D. N. Burrows (PSU), M. J. Coe, A. J. Bird (Southampton) G. Vasilopoulos, F. Haberl (MPE), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), P. Curran (CEA-Saclay), K. Yamaoka (AGU), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 24 Sep 2015; 14:15 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Transient
At 02:00UT on September 24th, 2015, Swift began a target-of-opportunity of the error region of MAXI J0051-736 (Negoro et al., ATEL #8088). We performed a 7-point tiling observation in Photon Counting mode (PC), with 500s exposure per tile in order to ensure coverage of the MAXI error circle. We find a bright point source inside the MAXI error region at the following location: RA/Dec (J2000) = 13.63945, -73.68362, which is equivalent to:
RA (J2000): 00h 54m 33.47s
Dec (J2000): -73d 41' 01.0''
with an estimated error of 2 arc-seconds radius (90% containment). This position lies 11.1 arc-minutes from the center of the MAXI error region, and is therefore consistent with being the the counterpart to MAXI J0051-736. This source is also consistent with position of 2MASS J00543343-7341012, which is classified as a B0 type star, and is suggested to be the optical counterpart of SMC X-2 (e.g. Schmidtke et al., 2006, AJ, 132, 971). Therefore, we conclude that MAXI J0051-736 is a new outburst of the Be/X-ray binary system SMC X-2, the first seen since 2000. We note that the Swift position represents the most accurate measured X-ray position of SMC X-2, and confirms the optical counterpart suggested by Schmidtke et al (2006).
In PC mode data the source is bright and piled up, with a pile-up corrected count rate of ~5.5 c/s. Unfortunately the ~2.5s frame time of PC mode data precludes using the data to confirm the presence of the 2.37s pulsar period of SMC X-2. We have requested follow-up observations in Windowed Timing mode to confirm the presence of this period.
The time-averaged spectrum of the PC mode data shows the source to be hard, well fit with a power-law with photon index of 0.61 +/- 1.5. The measured NH is 2 +/- 1 x 1021 cm-2, roughly consistent with the expected Galactic Value of 4.7 x 1021 cm-2 (Willingale et al. 2013). The 0.3-10 keV flux is 5.4 +/- 0.5 x 10-10 erg cm-2 s-1, corresponding to a luminosity of 2.4 x 1038 erg s-1 at the SMC distance of 62 kpc.
The source is also found in the Swift BAT hard X-ray transient monitor. Â It is first detected in a 4-day integration beginning 2015 Sep 5 (MJD 57270) and continues to be detected, with an average rate of 0.003 +/- 0.0009 ct s-1 cm-2 (15-50 keV; ~15 mCrab) during the 4-day period beginning 2015 Sep 21. SMC X-2 has been added to the transient monitor catalog and will continue to be monitored.
