HD 49798 explained

HD 49798 is a binary star in the constellation Puppis about 650 parsecs (circa 2119 light-years) from Earth. It has an apparent magnitude of 8.3, making it one of the brightest known O class subdwarf stars.^[1]

HD 49798 was discovered in 1964 to be a rare hydrogen-deficient O class subdwarf, and was the brightest known at the time.^[2] This was identified as a binary star, but the companion could not be detected visually or spectroscopically.^[3] The X-ray source RX J0648.0-4418 was discovered close to HD 49798's location in the sky. Only the space telescope XMM-Newton was able to identify the source. It is a white dwarf with about 1.3 solar masses, in orbit about HD 49798 and rotating once every 13 seconds.^[4] This is detected from the 13-second X-ray pulse, which results from the stellar wind accreting onto the compact object.^[1] It has been proposed that the white dwarf is surrounded by a debris disk. In this model, the material of the disk would be funneled onto the poles of the dwarf via the magnetic field, which would explain the observed X-ray pulses.^[1] This system is considered a likely candidate to explode as a type Ia supernova within a few thousand years.^{[5] [6]}

Notes and References

1. 2205.09982. 10.1051/0004-6361/202243536. X-ray pulsar HD 49798: A contracting white dwarf with a debris disk?. 2022. Chen. Wen-Cong. Astronomy & Astrophysics. 662. A79. 2022A&A...662A..79C. 248965211.
2. 1963PASP...75..365J. HD 49798, a New O-Type Subdwarf. Publications of the Astronomical Society of the Pacific. 75. 365. Jaschek. Mercedes. Jaschek. Carlos. 1963. 445. 10.1086/127968.
3. 1970MNRAS.150..215T. The spectroscopic orbit of the O-type subdwarf HD 49798. Monthly Notices of the Royal Astronomical Society. 150. 2. 215–225. Thackeray. A. D.. 1970. 10.1093/mnras/150.2.215. free.
4. 1997A&A...317..815B. The nature of the bright subdwarf HD 49798 and its X-ray pulsating companion. Astronomy and Astrophysics. 317. 815. Bisscheroux. B. C.. Pols. O. R.. Kahabka. P.. Belloni. T.. Van Den Heuvel. E. P. J.. 1997.
5. 2012NewAR..56..122W. 1204.1155. Progenitors of type Ia supernovae. New Astronomy Reviews. 56. 4. 122. Wang. Bo. Han. Zhanwen. 2012. 10.1016/j.newar.2012.04.001. 118740933.
6. 2015RAA....15.1813L. 1504.01461. Is the X-ray pulsating companion of HD 49798 a possible type Ia supernova progenitor?. Research in Astronomy and Astrophysics. 15. 11. 1813. Liu. Dong-Dong. Zhou. Wei-Hong. Wu. Cheng-Yuan. Wang. Bo. 2015. 10.1088/1674-4527/15/11/004. 118683249.