NuSTAR follow-up observation of MAXI J0655-013 reveals 1129s pulsations

ATel #15495; M. Shidatsu (Ehime U.), S. Pike (Caltech), T. Mihara (RIKEN), M. Sugizaki (NAOC), H. Negoro, M. Nakajima (Nihon U.), K. Murata (Tokyo Tech)
on 4 Jul 2022; 01:00 UT
Credential Certification: Megumi Shidatsu (shidatsu.megumi.wr@ehime-u.ac.jp)

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We performed a ToO observation of the new X-ray transient MAXI J0655-013 (ATel #15442, #15443) with NuSTAR on 2022 June 21 UT 22:31 - June 22 UT 21:21 with a net exposure of ~ 44 ks. The source was detected near the edge of the field of view and the chip gap of the detectors. The estimated source position is (RA, Dec) = (6:55:14.6,-1:27:49) = (103.8108, -1.4636) with a systematic uncertainty of ~2'. The relatively large uncertainty originates in attitude determination; the main star tracker (Camera Head Unit; CHU 4) on the X-ray optics bench was not available due to the proximity of the Sun. In estimating the source position, we used the FPMB data compiled when the three star trackers on the satellite bus (CHU 1, 2, and 3) are all available. We detected pulsations with a 1129.1 +/- 0.3 s period. The 1-day periodicity found in the MAXI observations (ATel #15453) is likely to be produced by sampling these pulsations with the scan interval of MAXI. No significant eclipse events were seen in the NuSTAR observation. The time averaged spectrum in 3-50 keV can be described with a cutoff power-law model with Gamma = 0.3 +/- 0.1, Ecut = 13 +/- 1 keV, and a 3-50 keV flux of (3.0 +/- 0.1) * 1e-9 erg/s/cm^2 ((3.3 +/- 0.1) * 1e-9 erg/s/cm^2 in 0.01-100 keV). The flux and the spectral profile are consistent with those of MAXI observations. These results confirm that the source is a binary X-ray pulsar, as suggested by ATel #15453. In the VizieR database, there are tens of known optical sources located within the 2' radius from the NuSTAR source position. There is also an X-ray source, 2SXPS J065512.4-012855 (also known as SRGA J065513.5-012846, ~0.3 mCrab), ~1' apart from the NuSTAR position. Its optical counterpart was suggested to be the 12.5 mag long-period variable star V520 Mon (Pavlinsky et al. 2022), which is likely to be a classical Be star at 4.1 kpc (Vioque et al. 2020).