arXiv:2009.11902v1 [astro-ph.SR] 24 Sep 2020 [email protected] orsodn uhr ead .M Luna M. J. Gerardo author: Corresponding in nTCB ubrt eercre n16 and 1866 of in time recorded recurrence were a outbursts suggesting CrB, erup- 1946, T a In ther- runaway produced via tion. and WD reactions the fusion of surface mate- monuclear accreted the the on centuries, a ignited two rial through last the companion in giant Twice dwarf red white disk. its a from which accretes in (WD) system binary a nova; recurrent 1 OIE-nvria eBeo ie,Isiuod Astrono de Instituto Aires, Buenos de CONICET-Universidad ooa oels( r)i h ers symbiotic nearest the is CrB) (T Borealis Coronae T yee sn L using 2020 Typeset 28, September version Draft 4 oubaAtohsc a 5 10hS. 07PpnHl,M Hall, Pupin 1027 St., W120th 550 Lab Astrophysics Columbia 7 ihteapoc ftetm twihaTRwudb xetdbased expected years. few be Keywords: next would the TNR in t CrB a past T which the in (2026 to occurring at novae led TNR two time posit prior ac the we the ones their of between the of approach like properties state the stability high binaries. the with accretion symbiotic by an in set dwarf-n of disks be is midst thus large states could thermonu presumably such RNe the the for symbiotic ignite in origin to expected natural in needed are A eruption, fuel which prior the states. the of high of most accretion time accumulates the hig CrB and accretion T transient 1946 in a around – curve by A light triggered Digital sy optical therefore in the was eruption nova from and recent – data most observa archival during the most that of and argue reanalysis to sporadic, a survey highly (DASCH) use is we (WD) Here dwarf discrepancy white states. apparent the This by mass magnitude. of of orders by expectations 8 eateto hsc,Uiest fMrln,BlioeC Baltimore Maryland, of University Physics, of Department ulr pc cec aoaoy nvriyCleeLond College University Laboratory, Science Space Mullard siae fteaceinrt nsmitcrcretnve(RNe novae recurrent symbiotic in rate accretion the of Estimates 6 RSTadXryAtohsc aoaoy AAGdadSpa Goddard NASA Laboratory, Astrophysics X-ray and CRESST 3 nvria ainld ulnhm v dr egr 2222 Vergara Gdor. Av. Hurlingham, de Nacional Universidad A 2 ead .M Luna, M. J. Gerardo T nvria eBeo ie,Fcla eCeca xca y Exactas Ciencias de Facultad Aires, Buenos de Universidad 1. E X iais yboi crto,aceindss—sas niiul T individual: stars: — disks accretion accretion, — symbiotic binaries: INTRODUCTION twocolumn nraigatvt nTCBsget oaeuto simpen is eruption nova suggests CrB T in activity Increasing 5 STCroain 3 ot hryAe usn Z85721. AZ Tucson, Ave, Cherry North 933 Corporation, LSST tl nAASTeX63 in style Rcie;Rvsd cetdSpebr2,2020) 28, September Accepted Revised; (Received; ,2 3 2, 1, ∼ 0yas The years. 80 .L Sokoloski, L. J. ± ) h urn crto ihsaeicesstelklho fa of likelihood the increases state high accretion current the 3), unsArs Argentina Aires, Buenos umte oApJ to Submitted ABSTRACT ayFiadlEpco(AE,A.It.Grle 60 C1428 2620, Giraldes Inte. Av. (IAFE), Espacio del Fsica y ma ,5 4, 27Clmi nvriy e ok e ok107 USA 10027, York New York, New University, Columbia 5247 C n omuyS ay okn,Sre H N,UK 6NT, RH5 Surrey Dorking, Mary, St Holmbury on, ut,10 ilo ice atmr,M 15,USA 21250, MD Baltimore, Circle, Hilltop 1000 ounty, oiMukai, Koji iigdtie nomto bu h as structure, mass, the about information detailed viding hn togyfrom strongly shine from determined as distance h Dms srqie ob elaoe1 above well be Sgr, to V3890 required and is Sco, mass V745 WD Oph, the RS novae recurrent eteuto fTCB–peitdt apnbetween happen the to predicted expect – we CrB 2023.6 nova, T of recurrent eruption known next nearest the ing uy n oTCBi addt uenv aprogen- cen- Ia a candidate a than (e.g., is less itor CrB T in so eruptions and tury, nova repeated generate ( alrJnse l 2018 al. et Bailer-Jones eFih etr rebl,M 07,USA 20771, MD Greenbelt, Center, Flight ce ± il ee,Beo ie,Argentina. Aires, Buenos Tesei, Villa , auae,Beo ie,Argentina. Aires, Buenos Naturales, ( 1 ee ta.2000 al. et Fekel cafr2019 Schaefer boi NTCB n14,occurred 1946, in CrB, T RN mbiotic a ersle fteaccumulation the if resolved be can tt.Bsdo iiaiisi the in similarities on Based state. h v ieaceinds instabilities, accretion-disk like ova ,7 6, cs oaSyCnuy@Harvard Century Sky a to ccess n .Pu .Kuin M. Paul N. and fe alsoto theoretical of short fall often ) in r efre uiglow during performed are tions la uaas(Ns during (TNRs) runaways clear 86 esgetta h WD the that suggest we 1866, rto ik.TCBi nthe in is CrB T disks. cretion onv rpin.Combined eruptions. nova wo h iigo h Nsin TNRs the of timing The γ ry ordowvlnts pro- wavelengths, radio to -rays nte8-erinterval 80-year the on n prxmtl 06–to – 2026 approximately and ) .A nteohrsymbiotic other the in As ). ; hha ta.1997 al. et Shahbaz CrB ding. Gaia aai 806 is data 8 M − +33 .Be- ). ZAA, 31 ⊙ pc to 2 Luna et al. energetics and perhaps driving mechanism of the out- project to digitize the Harvard Astronomical Photo- flow. graphic Plate collection, which provides a photometric The nova recurrence time is inversely related to the database with a baseline of about 100 years; and V - mass and the accretion rate. In the case of a and B-band observations from the archive of the Amer- system with the parameters of T CrB, MW D of 1.2 – 1.37 ican Association of Observers (AAVSO). M⊙ (Belczynski & Mikolajewska 1998; Stanishev et al. Details about the DASCH photometric pipeline can be 2004) and recurrence time of about 80 years, theoretical found in Tang et al. (2013). The DASCH database con- models by Prialnik & Kovetz (1995) predict that an ac- tains unflagged observations of T CrB from April 1901 −6 cumulated mass of 10 M⊙ will be needed for a TNR. through May 1989 with non-uniform sampling. Over 80 years that implies an average accretion rate of −8 −1 −7 −1 between 10 M⊙ yr and 10 M⊙ yr . 3. RESULTS. But measuring the accretion rate in symbiotic binaries 3.1. Optical high state leading up to the 1946 nova is a difficult task. Although in cataclysmic variables eruption the optical brightness can be used as a proxy for the By querying the DASCH and searching for photomet- accretion rate, in symbiotic stars, where the contribution ric observations of T CrB previous to the 1946 eruption, to the optical broadband light of the nebulae and the red we found clear evidence in the B-band light curve for giant are not negligible, optical photometry does not an optical bright state that started in 1938 and lasted provide an actual measurement of the accretion rate. about 7 years, until about 1945, about one year before In the case of T CrB, Selvelli et al. (1992) and the nova eruption, and then continued after the nova Stanishev et al. (2004) estimated the rate of accretion event. Figure 1 shows the DASCH light curve (blue onto the WD (M˙ ) from spectra obtained with the W D dots). This light curve confirms the phenomenon men- IUE (International Ultraviolet Explorer) satellite be- tioned in an abstract by Schaefer (2014a), who described tween 1978 until 1990. Including periods of both high finding such a high state associated with both the 1866 and low UV flux between 1978 and 1990, they found an −9 2 −1 and 1946 nova eruptions. However, the DASCH data average accretion rate of 9.6×10 (d/806pc) M⊙ yr . revealed that the high state which continued after the By modeling the SED in the UV region, Stanishev et al. eruption for several years had been missed in the pre- (2004) obtained a high-state accretion rate (from 1980 −8 2 −1 vious studies. The visual AAVSO data suggested that to 1988) of 1.1×10 (d/806pc) M⊙ yr and a low- a minor precursor event occurred before each eruption state accretion rate (from 1978 to 1980 and 1988-1990) −9 2 −1 that lasted approximately one year. But our examina- of 1.5×10 (d/806pc) M⊙ yr . It should be noted tion of the AAVSO light curve revealed that its coverage that the high-state identified by Stanishev et al. (2004) around the time of the two novae was too sparse to re- did not reach B-magnitudes as bright as the ones dis- veal the full high state that is evident in the DASCH cussed here (see Sect. 3). data. The DASCH light curve also shows that after ∼7 Additional constraints on M˙ onto the WD in T CrB years of “super-active” state, T CrB faded and almost come from the fact that the boundary layer is typically reached the pre-“super-active” brightness for about one optically thin, making it a hard X-ray source in qui- year, after which time the nova eruption occurred. escence, with the strength of the hard X-ray emission ˙ directly related to M (Luna et al. 2008; Kennea et al. 3.2. Similarity to the current high state 2009). To date, this is a unique feature among all known An on-going optical brightening event that started in symbiotic recurrent novae. Suzaku observations in −9 2 early 2014 is extremely similar to the 1938-1945 high 2006 allowed us to measure M˙ q = 0.7×10 (d/806pc) −1 state, and the current high state is driven by an in- M⊙ yr (Luna et al. 2019), where M˙ q is the accretion crease in M˙ . The current optical bright state reached rate measured between nova eruptions. Clearly both UV W D its maximum (B∼10) in April 2016 and has continued and X-ray measurements indicate that M˙ q was low since since then at an average brightness of B∼10.5 (referred 1979 until 2006 when compared with the predicted av- to as a “super-active” state by Munari et al. 2016). An erage accretion rate necessary to trigger a nova outburst XMM-Newton observation in January 2017 allowed us every 80 years. to detect a soft X-ray component from the boundary layer, which had become mostly optically thick, and to 2. OBSERVATIONS. measure the accretion rate of above 6.6×10−8 (d/806 2 −1 We base the findings described below on publicly avail- pc) M⊙ yr (Luna et al. 2018). In March 2018, an- able data from two archives – the DASCH (Digital Ac- other XMM-Newton observation showed that M˙ might −9 2 −1 cess to a Sky Century @Harvard; Grindlay et al. 2009) have decreased to about 6×10 (d/806 pc) M⊙ yr T CrB 3

(Zhekov & Tomov 2019), although a fit to the 2018 by Schaefer (2014b), to our knowledge, the DASCH data XMM-Newton spectrum using an alternative model show this clearly for the first time. with higher absorbing column suggests the M˙ could also We propose that both high states play a significant have remained high. In Figure 1, we show the DASCH role in the further development toward the nova erup- light curve, shifted by 80 years and superimposed upon tion. Outside these states, the quiescent accretion rate −9 −1 the AAVSO B-magnitude light curve from the current typically seems to be about 10 M⊙ yr ; at this level “super-active” state. The correspondence between the it would not be possible to reach the required ignition −6 shape of the initial rise of both events is remarkable. mass of a few 10 M⊙ in approximately 80 years. If the Based on the similar optical behavior in 1938 and 2014, accretion rate during the high states is about 5×10−9 −1 −8 −1 and that both brightenings took place several years be- M⊙ yr to 5×10 M⊙ yr , then an order of mag- −6 fore expected nova events, it is likely that previous to nitude estimation yields that about 10 M⊙ could be the 1946 eruption, the accretion increased to the 10−8 accreted onto the WD in 20 years, providing a large frac- −1 M⊙ yr level as it did in 2014. tion of the ignition mass. We emphasize however that the nature of the post-eruption high state is unknown, and thus its connection to a period of increased accretion rate is only speculative. 7 AAVSO 2004-now By comparing both “super-active” states, we see that DASCH 1924-1967 Schaefer (2010) the current state reached the peak earlier than expected 8 if the recurrence time is exactly 80 years (we note how- ever that most recurrent novae do not recur precisely 9 periodically; Schaefer 2010). Moreover, the plateau of the current “super-active” state is about half a mag- B 10 nitude fainter than the 1938-1945 state, and thus the fading to the quiescent, pre-eruption state, could have

11 already started. Although the AAVSO light curve does not show a significant decline in the optical brightness,

12 both X-ray softness ratio and Hα flux indicate a secular decline in the accretion rate since 2016 (Luna et al., in 55000. 60000. 65000. prep MJD .). We predict that T CrB is within 3-6 years of its next thermonuclear runaway. Figure 1. DASCH B-magnitude light curve (blue dots) and In cataclysmic variables, a single disk instability dwarf B-magnitude data (black dots) from Table 12 in Schaefer nova outburst cannot cause the WD to accumulate (2010), covering the 1924 to 1967 period, which includes enough mass to significantly fuel a nova eruption. For the pre and post-eruption activity and AAVSO B-magnitude example, Cannizzo (1993) modeled the disk instabil- light curve covering the 2004-now “super-active” state (red ity outburst in SS Cyg and found that about 6×10−10 dots). The DASCH and Schaefer (2010) light curves were −1 shifted by 80 years (see Sect. 4). Vertical dashed line marks M⊙ yr is stored in the disk during quiescence, which the 1946.1 eruption shifted by +80 years. The match of is then all or partially dumped into the WD during the initial rise between the current and previous brighten- a long-lasting (about 50 days long) outburst. The ing is remarkable. The shaded area shows the pre and post- aforementioned theoretical models by Prialnik & Kovetz eruption periods during which T CrB was in a high accretion (1995) show that the ignition mass for a nova eruption −6 rate state (see Section 4). in 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 can store a significant amount of −6 mass (about 10 M⊙; Wynn 2008), the WD has the po- 4. DISCUSSION. tential to assemble enough mass after a disk instability to trigger a nova eruption. The historical light curves show that T CrB experi- Our results thus suggest a scenario in which the WDs ences two active states between nova eruptions: one is in symbiotic recurrent novae accumulate most of the ig- the so-called “super-active” state (Munari et al. 2016), nition mass during sporadic high states. Hints of such which occurs between ∼8 and ∼1 year before the nova episodes were already noticed by Nelson et al. (2011) eruption and another one, less noticeable, which starts in their analysis of quiescent X-ray data of the symbi- about 200 days after the nova eruption and lasts for otic recurrent nova RS Oph. Because of their faintness about 8 years. Although previously noted in an abstract 4 Luna et al. when not experiencing nova eruptions, the data on the other symbiotic recurrent novae (V3890 Sgr, V745 Sco, and perhaps V2487 Oph) are too scarce for us to place constraints on low-amplitude high states like the one for T CrB we report here. With the ignition mass on the WDs in symbiotic recurrent novae most likely to be reached during an accretion high state, states such as the one that T CrB is currently in then become indica- tors of an impending nova eruption.

ACKNOWLEDGMENTS We 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 for partial support from NSF grants AST-0407380, AST- 0909073, and AST-1313370. GJML is a member of the CIC-CONICET (Argentina) and acknowledge support from grants ANPCYT-PICT 0901/2017 and CONICET- NSF International Cooperation Grant 2016. NPMK ac- knowledges support from the UK Space Agency. JLS acknowledges support from NSF award AST-1616646.

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