Decoding the X-Ray Flare from MAXI J0709–159 Using Optical Spectroscopy and Multiepoch Photometry

Abstract

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/Gas Slit Camera observations in soft and hard X-rays from MAXI J0709–159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 1037 erg s−1 (MAXI), which got reduced to 1032 erg s−1 (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from the 2.01 m Himalayan Chandra Telescope and the 2.34 m Vainu Bappu Telescope (VBT) facilities in India, on 2022 February 1 and 2. In addition to Hα emission, we found emission lines of He i in the optical spectrum of this star. By comparing our spectrum of the object with those from the literature we found that He i lines show variability. Using photometric studies we estimate that the star has an effective temperature of 20,000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favors it to be evolving off the main sequence in the color–magnitude diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disk. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.

Discovery of a new supergiant fast X-ray transient MAXI J0709-159 associated with the Be star LY CMa

“…From the best-fit model, the absorbed and absorption-corrected 2-10 keV fluxes were estimated to be 6 ± 1 × 10 −13 and 6 +3 −1 × 10 −13 erg cm −2 s −1 , respectively. These spectral parameters are consistent with those given by Bhattacharyya et al (2022) which utilized the same NuSTAR data.…”

“…The EW of the Hα line was estimated to be −23 Å with an uncertainty of ∼ 15 %. Bhattacharyya et al (2022) also performed optical follow-up observations and reported that the Hα EW was −17.6 Å on February 2 and −16.9 Å on February 3. The results suggest that the EW might change in these 3 days.…”

“…In fact, optical follow-up observations confirmed the broad Hα line, which indicates the Be circumstellar disk (section 6). Bhattacharyya et al (2022) proposed that LY CMa would be an evolved Be star located between main sequence stars and supergiants on the optical color-magnitude diagram.…”

“…In Gaia Early Data Release 3, the distance is estimated to be D = 3.03 +0.31 −0.27 kpc (Bailer-Jones et al 2021) 2 . From optical follow-up observations and archival data analysis, Bhattacharyya et al (2022) suggested that LY CMa would be an evolved Be star, rather than a main sequence star or a supergiant. A radio observation was carried out on 2022 January 31 with the MerrKAT radio telescope, but no significant emission was detected at 1.28 GHz and the 3-σ upper limit was estimated to be 57 µJy (Rhodes et al 2022) Finally, the Swift ToO observation was carried out on 2022 February 23.…”

Discovery of a new supergiant fast X-ray transient MAXI J0709−159 associated with the Be star LY Canis Majoris

“…In Gaia Early Data Release 3, the distance is estimated to be D = 3.03 +0.31 −0.27 kpc (Bailer-Jones et al 2021). 2 From optical follow-up observations and archival data analysis, Bhattacharyya et al (2022) suggested that LY CMa could be an evolved Be star, rather than a main sequence star or a supergiant. A radio observation was carried out on 2022 January 31 with the MeerKAT radio telescope, but no significant emission was detected at 1.28 GHz and the 3σ upper limit was estimated to be 57 μJy (Rhodes et al 2022) Finally, the Swift ToO observation was carried out on 2022 February 23.…”