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.[8]
HD 49798 was discovered in 1964 to be a rare hydrogen-deficient O class subdwarf, and was the brightest known at the time.[9] This was identified as a binary star, but the companion could not be detected visually or spectroscopically.[10]
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.[11] This is detected from the 13-second X-ray pulse, which results from the stellar wind accreting onto the compact object.[8] 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.[8] This system is considered a likely candidate to explode as a type Ia supernova within a few thousand years.[12][13]
^ abcdKudritzki, R. P.; Simon, K. P. (1978). "Non-LTE analysis of subluminous O-star. The hydrogen-deficient subdwarf O-binary HD 49798". Astronomy and Astrophysics. 70: 653. Bibcode:1978A&A....70..653K.
^ abNew X-ray observations of the hot subdwarf binary HD 49798/RX J0648.0–4418, 2021, arXiv:2104.03867
^Jaschek, Mercedes; Jaschek, Carlos (1963). "HD 49798, a New O-Type Subdwarf". Publications of the Astronomical Society of the Pacific. 75 (445): 365. Bibcode:1963PASP...75..365J. doi:10.1086/127968.
^Bisscheroux, B. C.; Pols, O. R.; Kahabka, P.; Belloni, T.; Van Den Heuvel, E. P. J. (1997). "The nature of the bright subdwarf HD 49798 and its X-ray pulsating companion". Astronomy and Astrophysics. 317: 815. Bibcode:1997A&A...317..815B.