Increasing activity in T CrB suggests nova eruption is impending
aa r X i v : . [ a s t r o - ph . S R ] S e p Draft version September 28, 2020
Typeset using L A TEX twocolumn style in AASTeX63
Increasing activity in T CrB suggests nova eruption is impending.
Gerardo J. M. Luna,
J. L. Sokoloski,
4, 5
Koji Mukai, and N. Paul M. Kuin CONICET-Universidad de Buenos Aires, Instituto de Astronoma y Fsica del Espacio (IAFE), Av. Inte. Giraldes 2620, C1428ZAA,Buenos Aires, Argentina Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina. Universidad Nacional de Hurlingham, Av. Gdor. Vergara 2222, Villa Tesei, Buenos Aires, Argentina. Columbia Astrophysics Lab 550 W120th St., 1027 Pupin Hall, MC 5247 Columbia University, New York, New York 10027, USA LSST Corporation, 933 North Cherry Ave, Tucson, AZ 85721. CRESST and X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK (Received; Revised; Accepted September 28, 2020)
Submitted to ApJABSTRACTEstimates of the accretion rate in symbiotic recurrent novae (RNe) often fall short of theoreticalexpectations by orders of magnitude. This apparent discrepancy can be resolved if the accumulationof mass by the white dwarf (WD) is highly sporadic, and most observations are performed during lowstates. Here we use a reanalysis of archival data from the Digital Access to a Sky Century @Harvard(DASCH) survey to argue that the most recent nova eruption in symbiotic RN T CrB, in 1946, occurredduring – and was therefore triggered by – a transient accretion high state. Based on similarities in theoptical light curve around 1946 and the time of the prior eruption, in 1866, we suggest that the WDin T CrB accumulates most of the fuel needed to ignite the thermonuclear runaways (TNRs) duringaccretion high states. A natural origin for such states is dwarf-nova like accretion-disk instabilities,which are expected in the presumably large disks in symbiotic binaries. The timing of the TNRs insymbiotic RNe could thus be set by the stability properties of their accretion disks. T CrB is in themidst of an accretion high state like the ones we posit led to the past two nova eruptions. Combinedwith the approach of the time at which a TNR would be expected based on the 80-year intervalbetween the prior two novae (2026 ± Keywords: binaries: symbiotic — accretion, accretion disks — stars: individual: T CrB INTRODUCTIONT Coronae Borealis (T CrB) is the nearest symbioticrecurrent nova; a binary system in which a white dwarf(WD) accretes from its red giant companion through adisk. Twice in the last two centuries, the accreted mate-rial ignited on the surface of the WD via runaway ther-monuclear fusion reactions and produced a nova erup-tion. In T CrB, outbursts were recorded in 1866 and1946, suggesting a recurrence time of ∼
80 years. The
Corresponding author: Gerardo J. M. [email protected] distance as determined from
Gaia data is 806 +33 − pc(Bailer-Jones et al. 2018). As in the other symbioticrecurrent novae RS Oph, V745 Sco, and V3890 Sgr,the WD mass is required to be well above 1 M ⊙ togenerate repeated nova eruptions in less than a cen-tury, and so T CrB is a candidate supernova Ia progen-itor (e.g., Fekel et al. 2000; Shahbaz et al. 1997). Be-ing the nearest known recurrent nova, we expect thenext eruption of T CrB – predicted to happen between2023.6 ± γ -rays to radio wavelengths, pro-viding detailed information about the mass, structure, Luna et al. energetics and perhaps driving mechanism of the out-flow.The nova recurrence time is inversely related to thewhite dwarf mass and the accretion rate. In the case of asystem with the parameters of T CrB, M
W D of 1.2 – 1.37M ⊙ (Belczynski & Mikolajewska 1998; Stanishev et al.2004) and recurrence time of about 80 years, theoreticalmodels by Prialnik & Kovetz (1995) predict that an ac-cumulated mass of 10 − M ⊙ will be needed for a TNR.Over 80 years that implies an average accretion rate ofbetween 10 − M ⊙ yr − and 10 − M ⊙ yr − .But measuring the accretion rate in symbiotic binariesis a difficult task. Although in cataclysmic variablesthe optical brightness can be used as a proxy for theaccretion rate, in symbiotic stars, where the contributionto the optical broadband light of the nebulae and the redgiant are not negligible, optical photometry does notprovide an actual measurement of the accretion rate.In the case of T CrB, Selvelli et al. (1992) andStanishev et al. (2004) estimated the rate of accretiononto the WD ( ˙ M W D ) from spectra obtained with theIUE (International Ultraviolet Explorer) satellite be-tween 1978 until 1990. Including periods of both highand low UV flux between 1978 and 1990, they found anaverage accretion rate of 9.6 × − ( d/ pc ) M ⊙ yr − .By modeling the SED in the UV region, Stanishev et al.(2004) obtained a high-state accretion rate (from 1980to 1988) of 1.1 × − ( d/ pc ) M ⊙ yr − and a low-state accretion rate (from 1978 to 1980 and 1988-1990)of 1.5 × − ( d/ pc ) M ⊙ yr − . It should be notedthat the high-state identified by Stanishev et al. (2004)did not reach B-magnitudes as bright as the ones dis-cussed here (see Sect. 3).Additional constraints on ˙ M onto the WD in T CrBcome from the fact that the boundary layer is typicallyoptically thin, making it a hard X-ray source in qui-escence, with the strength of the hard X-ray emissiondirectly related to ˙ M (Luna et al. 2008; Kennea et al.2009). To date, this is a unique feature among all knownsymbiotic recurrent novae. Suzaku observations in2006 allowed us to measure ˙ M q = 0.7 × − ( d/ pc ) M ⊙ yr − (Luna et al. 2019), where ˙ M q is the accretionrate measured between nova eruptions. Clearly both UVand X-ray measurements indicate that ˙ M q was low since1979 until 2006 when compared with the predicted av-erage accretion rate necessary to trigger a nova outburstevery 80 years. OBSERVATIONS.We base the findings described below on publicly avail-able data from two archives – the DASCH (Digital Ac-cess to a Sky Century @Harvard; Grindlay et al. 2009) project to digitize the Harvard Astronomical Photo-graphic Plate collection, which provides a photometricdatabase with a baseline of about 100 years; and V -and B -band observations from the archive of the Amer-ican Association of Variable Star Observers (AAVSO).Details about the DASCH photometric pipeline can befound in Tang et al. (2013). The DASCH database con-tains unflagged observations of T CrB from April 1901through May 1989 with non-uniform sampling. RESULTS.3.1.
Optical high state leading up to the 1946 novaeruption
By querying the DASCH and searching for photomet-ric observations of T CrB previous to the 1946 eruption,we found clear evidence in the B -band light curve foran optical bright state that started in 1938 and lastedabout 7 years, until about 1945, about one year beforethe nova eruption, and then continued after the novaevent. Figure 1 shows the DASCH light curve (bluedots). This light curve confirms the phenomenon men-tioned in an abstract by Schaefer (2014a), who describedfinding such a high state associated with both the 1866and 1946 nova eruptions. However, the DASCH datarevealed that the high state which continued after theeruption for several years had been missed in the pre-vious studies. The visual AAVSO data suggested thata minor precursor event occurred before each eruptionthat lasted approximately one year. But our examina-tion of the AAVSO light curve revealed that its coveragearound the time of the two novae was too sparse to re-veal the full high state that is evident in the DASCHdata. The DASCH light curve also shows that after ∼ Similarity to the current high state
An on-going optical brightening event that started inearly 2014 is extremely similar to the 1938-1945 highstate, and the current high state is driven by an in-crease in ˙ M W D . The current optical bright state reachedits maximum (B ∼
10) in April 2016 and has continuedsince then at an average brightness of B ∼ XMM -Newton observation in January 2017 allowed usto detect a soft X-ray component from the boundarylayer, which had become mostly optically thick, and tomeasure the accretion rate of above 6.6 × − (d/806pc) M ⊙ yr − (Luna et al. 2018). In March 2018, an-other XMM -Newton observation showed that ˙ M mighthave decreased to about 6 × − (d/806 pc) M ⊙ yr − CrB
XMM -Newton spectrum using an alternative modelwith higher absorbing column suggests the ˙ M could alsohave remained high. In Figure 1, we show the DASCHlight curve, shifted by 80 years and superimposed uponthe AAVSO B-magnitude light curve from the current“super-active” state. The correspondence between theshape of the initial rise of both events is remarkable.Based on the similar optical behavior in 1938 and 2014,and that both brightenings took place several years be-fore expected nova events, it is likely that previous tothe 1946 eruption, the accretion increased to the 10 − M ⊙ yr − level as it did in 2014. B AAVSO 2004-nowDASCH 1924-1967Schaefer (2010)
Figure 1.
DASCH B-magnitude light curve (blue dots) andB-magnitude data (black dots) from Table 12 in Schaefer(2010), covering the 1924 to 1967 period, which includesthe pre and post-eruption activity and AAVSO B-magnitudelight curve covering the 2004-now “super-active” state (reddots). The DASCH and Schaefer (2010) light curves wereshifted by 80 years (see Sect. 4). Vertical dashed line marksthe 1946.1 eruption shifted by +80 years. The match ofthe initial rise between the current and previous brighten-ing is remarkable. The shaded area shows the pre and post-eruption periods during which T CrB was in a high accretionrate state (see Section 4).4.
DISCUSSION.The historical light curves show that T CrB experi-ences two active states between nova eruptions: one isthe so-called “super-active” state (Munari et al. 2016),which occurs between ∼ ∼ before the novaeruption and another one, less noticeable, which startsabout 200 days after the nova eruption and lasts forabout 8 years. Although previously noted in an abstract by Schaefer (2014b), to our knowledge, the DASCH datashow this clearly for the first time.We propose that both high states play a significantrole in the further development toward the nova erup-tion. Outside these states, the quiescent accretion ratetypically seems to be about 10 − M ⊙ yr − ; at this levelit would not be possible to reach the required ignitionmass of a few 10 − M ⊙ in approximately 80 years. If theaccretion rate during the high states is about 5 × − M ⊙ yr − to 5 × − M ⊙ yr − , then an order of mag-nitude estimation yields that about 10 − M ⊙ could beaccreted onto the WD in 20 years, providing a large frac-tion of the ignition mass. We emphasize however thatthe nature of the post-eruption high state is unknown,and thus its connection to a period of increased accretionrate is only speculative.By comparing both “super-active” states, we see thatthe current state reached the peak earlier than expectedif the recurrence time is exactly 80 years (we note how-ever that most recurrent novae do not recur preciselyperiodically; Schaefer 2010). Moreover, the plateau ofthe current “super-active” state is about half a mag-nitude fainter than the 1938-1945 state, and thus thefading to the quiescent, pre-eruption state, could havealready started. Although the AAVSO light curve doesnot show a significant decline in the optical brightness,both X-ray softness ratio and H α flux indicate a seculardecline in the accretion rate since 2016 (Luna et al., inprep .). We predict that T CrB is within 3-6 years of itsnext thermonuclear runaway.In cataclysmic variables, a single disk instability dwarfnova outburst cannot cause the WD to accumulateenough mass to significantly fuel a nova eruption. Forexample, Cannizzo (1993) modeled the disk instabil-ity outburst in SS Cyg and found that about 6 × − M ⊙ yr − is stored in the disk during quiescence, whichis then all or partially dumped into the WD duringa long-lasting (about 50 days long) outburst. Theaforementioned theoretical models by Prialnik & Kovetz(1995) show that the ignition mass for a nova eruptionin a 1.1 M ⊙ WD (as in SS Cyg) is on average 10 − M ⊙ .On the other hand, in symbiotics, where a large and un-stable accretion disk can store a significant amount ofmass (about 10 − M ⊙ ; Wynn 2008), the WD has the po-tential to assemble enough mass after a disk instabilityto trigger a nova eruption.Our results thus suggest a scenario in which the WDsin symbiotic recurrent novae accumulate most of the ig-nition mass during sporadic high states. Hints of suchepisodes were already noticed by Nelson et al. (2011)in their analysis of quiescent X-ray data of the symbi-otic recurrent nova RS Oph. Because of their faintness Luna et al. when not experiencing nova eruptions, the data on theother symbiotic recurrent novae (V3890 Sgr, V745 Sco,and perhaps V2487 Oph) are too scarce for us to placeconstraints on low-amplitude high states like the onefor T CrB we report here. With the ignition mass onthe WDs in symbiotic recurrent novae most likely to bereached during an accretion high state, states such asthe one that T CrB is currently in then become indica-tors of an impending nova eruption.ACKNOWLEDGMENTSWe acknowledge with thanks the variable star obser-vations from the AAVSO International Database con-tributed by observers worldwide and used in this re-search. The DASCH project at Harvard is grateful forpartial support from NSF grants AST-0407380, AST-0909073, and AST-1313370. GJML is a member of theCIC-CONICET (Argentina) and acknowledge supportfrom grants ANPCYT-PICT 0901/2017 and CONICET-NSF International Cooperation Grant 2016. NPMK ac-knowledges support from the UK Space Agency. JLSacknowledges support from NSF award AST-1616646.REFERENCES
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