Pointing the Schmidt telescope at Calar Alto, a volunteer scientist observing for the European Space Agency has discovered a new type of cataclysmic variable star, the first eclipsing stream-fed intermediate polar binary star. In this kind of “violent couple”, one of the stars (the white dwarf) pulls out matter from its partner (the red dwarf) towards its magnetic poles in huge streams of gas and dust.
Using the Calar Alto Schmidt telescope as part of the Space Situational Awareness Program of the European Space Agency (ESA), Erwin Schwab, an amateur astronomer and volunteer scientist for ESA, was searching for the lost comet P/2012 K3 (Gibbs). In the night from July 26 to 27, 2019, he took series of images from the predicted position in the constellation Sagittarius. The comet was not there… Yet, during the data inspection, Schwab noticed a star in the field, which suddenly disappeared from one picture to the next. The faint variable object was cataloged as J1832.4-1627 from its coordinates on the sky. After further observing nights with the Schmidt and other telescopes, he was able to detect several deep eclipses of the star, which occurred every 9 hours or so.
Erwin Schwab found out that J1832.4-1627 was a new cataclysmic variable star, probably of the very rare DQ Herculis type. This is a binary star system, also called intermediate polar, consisting of a white dwarf – brighter star with an intermediate magnetic field -- and a red dwarf orbiting at close distance. The red dwarf has filled its Roche lobe and matter is being drawn off onto the surface of the white dwarf star through its magnetic poles, creating small periodic variations in brightness. But in addition, as seen from Earth, the brighter white dwarf star is periodically eclipsed by the darker red dwarf.
Schwab and his colleague Paul Breitenstein, a teacher in Münster (Germany) observed J1832.4-1627 continuously in the following two years (2020 and 2021) in order to determine the spectral class of the red dwarf star and to verify whether there exists an accretion disk or not around the white dwarf. Together with Klaus Beuermann, an emeritus professor at the University of Göttingen (Germany) and expert in intermediate polars, further important physical parameters were determined, such as star masses and diameters, temperatures, luminosities, orbit inclination, and magnetic forces. Furthermore, an oscillation of the eclipse ingress and egress times was detected, which indicates that the source of the main emission circles around the white dwarf star’s rotation axis. Finally, Beuermann, Breitenstein and Schwab identify J1832.4-1627 as the first known eclipsing stream-fed intermediate polar, a long-sought type of star system.
Typical for almost all previously known intermediate polars is that the flow of matter first hits an accretion disk before reaching the surface of the white dwarf star. Yet, for J1832.4-1627, there is no evidence for the presence of an accretion disk, which should be visible in the light curve. It is thus a diskless accretor, were the stream of matter pulled out from the red dwarf flows directly to the magnetic poles of the white dwarf star (see artist’s impression).
J1832.4-1627 shows the combination of deeply eclipsing and diskless stream-accreting geometry, which is unique. Beuermann, Breitenstein & Schwab come to the conclusion that ”There exists presently no similar object and its eclipsing nature emphasizes the potential it carries for studies of its evolutionary status and further evolution“. The referee for the accepted paper highlights the importance of this discovery with the following remark ”Only several days before I was asked to review this manuscript, I had been lamenting the fact that no deeply eclipsing, stream-accreting intermediate polars were known. I’m glad that this void has been filled.“
K. Beuermann, P. Breitenstein & E. Schwab "J1832.4-1627, the first eclipsing stream-fed intermediate polar", to be published in Astronomy & Astrophysics DOI: 10.1051/0004-6361/202141585 (preprint available here)
Helmholtz Center for Heavy Ion Research (GSI, Darmstadt)
Erwin Schwab :e.schwab @ gsi.de
Calar Alto Observatory (CAHA, Almeria)
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