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MAXI J1733-222: Swift identification as likely 4U 1730-22 in first outburst since 1972 discovery

ATel #14686; J. A. Kennea (PSU), A. Bahramian (MSU), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (Nagoya U.), M. Serino (RIKEN), H. Negoro (Nihon U.)
on 8 Jun 2021; 15:37 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Neutron Star

Referred to by ATel #: 14688, 14693, 14694, 14757, 14769, 14896

MAXI J1733-222 was detected by MAXI (ATEL #14683) on June 7th, 2021 at 19:43UT. It was speculated to be either a new transient or GRB. Follow-up observations of this region by Swift were taken starting June 8th, 2021 at 08:53:33UT, consisting of 7 PC mode observations with an exposure time of 500s. Inside the MAXI error region we find a bright point source at the following location: RA/Dec (J2000) = 263.4889, -22.0307 which is equivalent to:

RA(J2000) = 17h 33m 57.33s,
Dec(J2000) = -22d 01m 50.4s

with an uncertainty of 3.5 arc seconds radius (90% confidence). This position lies 16.4 arc minutes from the MAXI reported localization, inside the MAXI error circle. We therefore conclude that this source is the counterpart of MAXI J1733-222.

We note that this source is consistent with the location of the X-ray transient 4U 1730-22 (e.g. Cominsky et al. 1978), which entered outburst in 1972 and has been inactive since. A purported quiescent X-ray counterpart, CXOU J173357.5-220156, was reported by Tomsick et al (2017). This Chandra sources lies 7 arc seconds from the XRT position, although we note that the Swift XRT position does not include usual UVOT correction of the systematic errors. Further Swift observations will hopefully refine the position, and resolve this inconsistency. The XRT position is not consistent with any other catalogued X-ray or optical source, including in the observations reported by Tomsick et al (2017).

We therefore conclude that MAXI J1733-222 is likely a new outburst seen from the transient 4U 1730-22, the first observed since it’s initial detection by Uhuru in 1972.

Spectral analysis of the XRT PC mode data shows that the data are well modeled by an absorbed power-law model, with N_H = 0.73 +/- 0.22 x 10^22 cm^-2, and photon index 2.1 +/- 0.3. Average flux during the 500s exposure was 1 x 10^-10 erg/s/cm^2 (0.3 - 10 keV). Fitting a blackbody model to the spectrum gives a blackbody temperature of 0.76 +/- 0.07 keV. The power-law model is favored.

There is no evidence of time variability in the short exposure. However, the source has faded significantly since detected detection by MAXI at a level of 59 +/- 29 mCrab, consistent with the non-detection of the source by MAXI on the next transit.

Further observations to monitor the flux and obtain a higher quality localization with Swift have been requested.