NuSTAR identified MAXI J0709-159 as a Be X-ray binary with LY CMa

ATel #15193; H. Negoro (Nihon U.), T. Mihara (RIKEN), S. Pike, B. Grefenstette (Caltech), W. Iwakiri (Chuo, U.), M. Nakajima (Nihon U.), and K. Gendreau (NASA/GSFC)
on 1 Feb 2022; 02:00 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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NuSTAR ToO observations of MAXI J0709-159 (ATel #15178, #15181, #15188) were performed from 00:21 to 11:21 UT on 2022 January 29 about 4 days after the MAXI detection of two bright flares.

NuSTAR successfully detected the source at the position consistent with that of the Be star HD 54786 (aka LY CMa) within the systematic uncertainty of the NuSTAR position accuracy (~12", Lansbury et al. 2017, ApJ, 836, 99). The FPMA and FPMB spectra are well represented by an absorbed power-law model with a photon index of 2.0 +/- 0.3 and a equivalent hydrogen column density of 4.5 (+13.7/-4.5) e22. The unabsorbed 2-10 keV flux is 5 +/- 1 e-13 erg/cm2/s, corresponding to 2.5e33 erg/s taking account of the 6.4 kpc distance to HD 54786.

NICER also observed the source quasi-simultaneously. The source count rate was consistent with background as was expected from the above NuSTAR result.

The observed flux is lower than those of the two flares detected by MAXI by about four orders of magnitude. The decay timescale is faster than those of SFXTs, which typically decay by 2 to 3 orders of magnitudes a week (e.g., Romano et al. 2013, AdSpR, 52, 1593). The source has not been listed in any X-ray source catalog (e.g., The Swift-XRT point source catalog 1SXPS (Evans et al. 2014, ApJ, 210, 8), and The 7-year MAXI/GSC source catalog 3MAXI (Hori et al. 2018, ApJS, 235, 7)). Thus, these results imply that we have observed new SFXT-like activity in the MAXI J0709-159 and HD 54786 system. We also note that the power-law energy spectrum with a photon index of about 2 at a luminosity of about 1e33 erg/s is consistent with that of Be neutron star binaries in a quescent state (e.g., Mukherjee & Paul, 2005, A&A, 431, 667). Pulsations have not yet been detected from the source.

Further followup observations are encouraged.
We would like to thank the NuSTAR team for organizing the prompt observations.