EX Hydrae is in outburst

The American Association of Variable Star Observers (who tweet here) has issued a Special Notice, indicating that the star EX Hydrae is in outburst. The star, which outbursts randomly, has grown in magnitude from ~14 to 9.4. EX Hydrae outbursts are usually short lived, on the order of days and so CCD and visual estimates of its magnitude are requested. The coordinates for the star are:

Right Ascension: 12 52 24.22 , Declination: -29 14 56.0 Epoch: J2000

A spotter’s chart with the magnitudes of nearby stars listed is here.

EX Hydrae has variability in a number of ways. It is a system with a star of mass 0.4-0.7 solar masses and one of mass 0.07-0.10 solar masses. The two stars are an ordinary long lived small dwarf star and a white dwarf star (the remains of the core of a larger star that blew off its outer layers at the end of its life), which is slowly taking material from its companion. The two are located around 65 parsecs from us and orbit each other with a period of 98 minutes. Their orientation is such that they get in front of one another, creating a small but measurable eclipse.

The white dwarf component is an ‘intermediate polar’ meaning it has a weak magnetic field. A polar class white dwarf has a strong magnetic field that prevents the formation of a full accretion disc, preferring to channel matter along field lines to the poles. An intermediate polar has a weak field, which means an accretion disc is prevented from forming out to a certain distance from the star by the field, but from this point onwards, the disc forms as normal. Material falling into the poles gets accelerated and releases radiation. This radiation beams out, rather like in a pulsar, and is recorded in much the same way. This creates a 67 minute repeating signal that shows the rotation rate of the white dwarf.

Every so often (and there is presently no way of knowing when this will happen, it has happened as little as a few days after previously erupting up to several years) the mass loading will be sufficient to reignite fusion on the surface of the white dwarf, creating, through the thermal properties of a white dwarf, heat and radiation pressure that blasts away excess material to form a short-lived nebula. It is one of these dwarf novae that is creating the outburst visible now.

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