4U 1730-22 (CXOU J173357.5-220156) was detected in outburst by INTEGRAL for the first time since 1972
ATel #14870; Sergei A. Grebenev (Space Research Institute, Moscow)
on 23 Aug 2021; 22:39 UT
Credential Certification: Sergei Grebenev (sergei@hea.iki.rssi.ru)
Subjects: Infra-Red, Optical, X-ray, Request for Observations, Neutron Star
Referred to by ATel #: 14896
A short outburst of the LMXB transient 4U 1730-22 was detected by the IBIS/ISGRI telescope during recent INTEGRAL observations of the Galactic center field. The source has not been active since 1972, when a single ~200 day outburst was detected by the Uhuru satellite (Cominsky et al. 1978; Forman et al. 1978). The origin of the source was recognized by Chandra in 2004 (Tomsick et al. 2007) which detected its quiescent soft X-ray emission (the source CXOU J173357.5-220156) and proved its spectrum to be described by a neutron star atmosphere model with an effective temperature of about 130 eV.
The transient source was first detected (with S/N=7.3) in the IBIS/ISGRI mosaic image measured in the 20-80 keV energy band on August 12-24, 2021. The best fit position of the source, R.A.=263.520 deg, Decl.=-22.022 deg (equinox 2000.0), was 1.8' off that of CXOU J173357.5-220156 (it is within the error box). Our further analysis has shown that the maximum of the source activity was observed between August 12, 20:25 and August 13, 03:17 (UTC) when its mean flux was equal to 31+/-3 mCrab and the confidence of detection was S/N=10.1.
We encourage further observations of this unique transient with a very long quiescent cycle.
