2023 August 22
Get set for the next eruption of the recurrent nova T Coronae Borealis!
The following article appeared in the 2023 August Journal, based on a talk at the June meeting (1). Updates leading to the next eruption of the recurrent nova will appear in the Forum. Observations of this star are encouraged.
The recurrent nova, T Coronae Borealis
T CrB is one of 10 known galactic recurrent novae and has been observed in eruption twice: in 1866 and 1946. The 80 year separation has led to much anticipation of a further eruption soon. However, eruptions of recurrent novae do not occur with specific regularity, so there is uncertainty about when exactly it will erupt. But erupt it will! At its peak it is magnitude 2.
Like all novae, T CrB is a binary system. In this case there is an M4 red giant star in a 227-day orbit round a planet-sized white dwarf. The two stars are 0.54 AU apart. Material is drawn from the red giant due to the strong gravitational field of the white dwarf. It accumulates in an accretion disk around the white dwarf. Some of the material gets funnelled down to the white dwarf’s surface, becomes compacted and heated, and eventually ignites in a spectacular thermonuclear explosion. We see the result as a sudden brightening of the star. Only a tiny fraction of the mass of the binary is lost in the eruption, so that with time the process can repeat itself. In fact, it is thought that all novae recur on some time scale, but for some it could be thousands of years.
In a recent paper in Monthly Notices of the RAS (2), Prof. Bradley E. Schaefer (Department of Physics and Astronomy, Louisiana State University) presents a long-term light curve of T CrB spanning 1842–2022. He notes that the two previous eruptions were identical, and he uses features in the light curve to predict when the next eruption might occur: 2025.5 ±1.3. Then on June 30, with more data to hand, Schaefer revised his prediction to 2024.4 ± 0.3, 2024 February to August (3).
In early 2015, T CrB entered a “super active” state in which it brightened by about three-quarters of a magnitude and became bluer. Intriguingly, it entered a similar super active state in 1938, some 8 years before the eruption. Hence astronomers have been put on warning of an eruption soon. Schaefer points out that that the end of this state in 1945 was a year before the eruption. This was followed by a dip 3 months before the eruption. This dip was noticed by the famous US amateur astronomer, Leslie Peltier, leading him to believe an eruption was imminent (4). This was a remarkable insight which turned out to be correct. Readers of Peltier’s charming autobiography, Starlight Nights (5), will be aware, though, that having monitored T CrB for over 25 years, when his alarm clock woke him on a cold February morning in 1946, he detected symptoms of a cold and returned to bed. That was of course the morning T CrB erupted again! The pre-eruption dip appears to be unique to T CrB and I have begun to refer to it as the “Peltier dip”.
Intriguingly, recent observations show that the super active phase has ended. Not only has the star begun to fade, but also “flickering” (random variations in the light curve over a few minutes) has resumed. This was last seen before the recent super active state. John Toone reported detecting flickering visually of up to 0.2 mag on 2022 November 14 and on April 23 this year (6), way before professional observers (which might point to an even earlier outburst date than Schafer’s).
David Boyd has also reported a change in the spectrum of T CrB. The spectrum is dominated by two features: absorption lines from the red giant and the emission lines from the nebula engulfing the binary system. The nebula comes from the outer atmosphere of the red giant which it is gradually losing. Most of the radiation from the white dwarf is in the UV, hence does not contribute significantly to the visible spectrum, but it excites the gaseous nebula giving rise to hydrogen and helium emission lines. David notes that the integrated flux of the H and He emission lines is at its lowest level since 2016.
So clearly things are in transition in T CrB and it now appears that we are in the final run up to the next eruption, possibly in the early part of next year. However, it should be remembered that although the last two outbursts were identical, it doesn’t mean that the third one will be!
A call for observations: visual, digital and spectroscopic
Vigilance is called for and observers are encouraged to keep a close eye on T CrB. Nightly visual and digital (CCD/CMOS) observations are requested to monitor the current fade and identify the minimum of the pre-eruption Peltier dip, which will be the 3 month warning of the eruption. Comparison star charts of the field are available from the VSS website (7). At the time of writing, T CrB is around 10th magnitude, so a small telescope is required. During the dip it is likely to decline to 12th magnitude briefly.
Time series photometry runs of around 2 hours to monitor flickering would also be useful. Exposures should be less than 30 seconds so that the rapid changes can be characterised, and with random errors <0.01 mag. Ideally a B filter should be used, otherwise V, but failing that unfiltered. Visual observers might also look out for flickering. Spectroscopy would also be useful to monitor the change in the H and He emission as the eruption approaches – and, of course, during the eruption.
Observers are requested to start observing now and report their observations to the VSS databases. In the event of the eruption being detected (e.g., anything brighter than magnitude 7), please report to the BAA VSS Alert email system (8). An alternative is the BAA Forum, though this might result in a delay in the observation being picked up. From UK latitudes, observations are generally possible in the western evening sky until late November and in the morning sky after that. Observers are requested to make extra efforts during this period, thus avoiding a seasonal gap.
It is exciting to anticipate the appearance of a bright nova in our skies in the not-too-distant future. Although T CrB reaches magnitude 2, this is a fast nova and it fades by 3 magnitudes in ~5 days so we still have to hope for good weather. It’s also worth noting that observations should continue after the eruption as a secondary eruption is anticipated about 85 days after the first. But it won’t be nearly as bright: 8th magnitude in 1866 and 1946. This secondary eruption will be much longer at ~90 days.
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References
1. The author gave a talk with the same title at the 2023 June BAA meeting. This has more background information about T CrB and can be accessed via the BAA YouTube Channel
2. Schaefer B.E., Monthly Notices RAS, stad735 (2023)
3. Schaefer B., ATel 16114 (2023).
4. Peltier L., Harvard Announcement Card, 709 (1945).
5. Peltier L. C., Starlight Nights. Harper & Row, New York, p. 192 (1965).
6. Toone J., Variable Star Section Circular 196, June 2023, pg 8 (2023); available for download.
7. Chart
8. BAA VSS Alert
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