SRG/ART-XC and Swift/XRT detection of the on-going outburst from RX J1718.4-4029
ATel #17390; I. . Mereminskiy(1), A. Semena(1), A. Lutovinov(1), S. Molkov(1), A. Tkachenko(1) (1:IKI RAS, Moscow)
on 14 Sep 2025; 08:34 UT
Credential Certification: Andrey Semena (san@iki.rssi.ru)
Subjects: X-ray, Neutron Star, Transient
During the on-going all-sky survey, on Sept.13, 2025, the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory detected an X-ray source at the position of RX J1718.4-4029 - known type-I burster, notable for its stable and very low accretion rate. Observed X-ray flux was 3.1(+/-0.9)x10^-11 erg cm-2 s-1 in the 4-12 keV band, a factor of 5 brighter than its quiescent flux as seen in previous all-sky surveys.
We initiated rapid follow-up observation with Swift/XRT in order to obtain better localization and the source was observed on Sept.14, 2025, source is clearly detected at a mean count rate of 0.3 cts/s in 0.3-10 keV band. We performed initial spectral analysis, applying a simple absorbed power-law model - both the absorption column (NH~(1.7(-0.7,+0.9))x1022 cm-2) and the spectral slope (2.1(-0.5,+0.6)) are consistent with previous data. The observed flux is about 1.9 (-0.4,+0.1)x10-11 erg cm-2 s-1 in the 0.5-10 keV band, similar to what was observed during a short outburst in 2008 (in 't Zand et al., 2009).
Given the puzzling nature of the source an urgent X-ray, NIR and radio observations are encouraged while the active state of the outburst lasts.
The Mikhail Pavlinsky ART-XC telescope is the X-ray telescope on board the SRG observatory which was launched from the Baikonur cosmodrome on 13 July 2019 by the Proton rocket. We thank the Swift team for their rapid response and excellent support.
