Recurrent Nova RS Ophiuchi in outburst!

The recurrent nova RS Ophiuchi is in outburst for the first time since 2006!

Other known outbursts were observed in 1985, 1967, 1958, 1933, and 1898.

RS Oph outbursts since 1933

The current outburst is shown at upper right of the plot that includes outbursts from 1933 onward.

Around 15 visual observations of the current outburst had been recorded in the AAVSO International Database and the nova had reached around magnitude 4.6 at the time I started writing.

The nova will be visible to the unaided eye from some locations and easily visible in binoculars from suburban locations (at least here in South Australia).

Only a handful of recurrent novae are known and the time between outbursts tends to be measured on a timescale of decades, averaging 20 years for RS Oph but varying +/- 10 or so years.

These systems involve a giant star (in this case, a slow irregular variable with a period of a few hundred days) and a white dwarf star pair in which mass is transferred from the giant to the white dwarf, forming an accretion disk which eventually undergoes a runaway thermonuclear reaction.

Example of a Cataclysmic Variable (RS Oph) from
Artist’s impression of RS Oph (from APOD)

RS Oph is around 5000 light years distant and so of course, the event we now see happened around 3000 BC.

The magnitude of the RS Oph binary star system increases rapidly from magnitude 10 or 11 to around 4.5 in the space of day, taking around 100 days to return to the baseline in a characteristic decay curve.

Within 2 days it will diminish by around 1 magnitude and by around 2 magnitudes within 5 days.

So, clear skies would be nice! The local forecast does not look great for Adelaide over the next few days.

It was cloudy by the time I read the outburst notification tonight and first started writing this post, but the sky cleared at around 1 am!

I observed the nova low in the west through 7×50 binoculars and estimated its magnitude to be 4.5 (Julian Date 2459436.16237 which is almost 15:54 UT or 01:24 Australian Central Standard Time):

The last month or so of observations. My magnitude 4.5 observation is shown in the cross-hairs.
The last few days of observations. My magnitude 4.5 observation is shown in the cross-hairs.

The 43 and 46 comparison stars (at about 5 and 11 o’clock with respect to the nova) on the following AAVSO finder chart were used for the estimate:

The finder chart needs to be rotated clockwise somewhat more than 90 degrees but less than 180 degrees to match the orientation of the sky shown in the Stellarium image below:

Note the position of the three roughly magnitude 3.5 stars in a straight line above the nova which is below and to the right of the middle of these stars (in the Stellarium image), with Scorpius at far left. These 3 stars occupy much of the finder chart near the nova (which is above and to the right of the middle star on the chart).

While every variable star type has its own interesting qualities, I find novae to be the most fascinating sub-type of cataclysmic variables and probably of all variable types because of the power and unpredictability involved. They are a reminder that we live in a violent, hostile yet endlessly fascinating universe, that we are, as Bernard Lovell said: In the Centre of Immensities.

It will be interesting to see whether the brightness has now peaked. If previous outbursts are anything to go by, it may have. But we’ll see! Novae and their much rarer cousins, recurrent novae, can really keep me up late at night (it’s now 2:55am here)!

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