Vinokurov A.S., Sarkisyan A.N., Solovyeva Yu.N., Kostenkov A.E., Medvedev A.S., Fabrika S.N. in cooperation with Atapin K.E. (SAO RAS, SAI MSU)

    Analyzing data from the Chandra X-ray Observatory archive, we revealed signatures of an absorption line at ≈8 keV with a width-to-energy ratio of σ/E ≈ 0.2 in the spectrum of the ultraluminous X-ray source NGC5474 X-1 taken during an outburst, when the object's X-ray luminosity reached ≈2×10⁴⁰ erg/s. It has been suggested that this broad line may be a cyclotron resonant scattering feature produced by transitions of electrons between Landau levels in magnetic field. This implies that the accretor in NGC5474 X-1 must be a neutron star with a magnetic field of about 10¹² G. The Hubble Space Telescope data shows that the X-ray outburst of the object was accompanied by more than 13 times increase of its U band flux. Optical observations of NGC5474 X-1 carried out in its faint state have allowed us to impose restrictions on the donor star mass (< 7 solar masses), as well as to reveal a star cluster of about 1 billion years old, the center of which is located at a projected distance of about 2 pc from the source. This cluster may be the parent cluster for NGC5474 X-1, but the presence of stars with an age of ~10 Myr within 300 pc around X-1 does not exclude an accidental projection of the ULX onto the old cluster.

Fig. 1. Left: unfolded spectrum of NGC5474 X-1 extracted from the annular aperture that excludes most of piled events in the Chandra data taken on December 3, 2007. The red line shows the model consisting of a Gaussian absorption line and a power-law continuum modified for interstellar absorption. The bottom panel shows the residuals of the approximation. Right: fluxes (or their upper limits) of the NGC5474 X-1 optical counterpart measured during its outburst and its weak state (two upper and a group of lower points, respectively) using the Hubble Space Telescope images. The lines show some stellar atmosphere models that satisfy the upper limits to the source flux in the observed weak state: the B3 main sequence star, A2 and M0 bright giants.

The work was carried out within the framework of the research plan № AAAA-A21-121011890155-0.

Published:
K. Atapin, A. Vinokurov, A. Sarkisyan, Yu. Solovyeva, A. Kostenkov, A. Medvedev, S. Fabrika. NGC 5474 X-1: a neutron star ULX in an old stellar cluster?, Monthly Notices of the Royal Astronomical Society, 2023, Volume 527, Issue 4, pp. 10185–10200, DOI: 10.1093/mnras/stad3719.