arXiv:1807.01304v1 [astro-ph.HE] 3 Jul 2018 asv ht wr,wt M with dwarf, white massive a ussi 86ad14,we trahdmgiue sbright as out- magnitudes -type reached it recorded V when had 1946, and CrB 1866 in T bursts companion. hydrogen-ri the of from amount a critical terial (WD) a dwarf i.e. of white accretion , the after on surface giant runaway eruption thermonuclear red strong a systems, a by these triggered is In system. companion binary the symbiotic where novae current (e. plasma 2014). al. thermal et thin Suleimanov optically 1993; Popham an & of Narayan that obser threshol be the this will and below radiation spectrum while own blackbody-like, its be boundar to will the thick spectrum which optical above be will threshold layer a predicts Theory fading. ioaesa19;Saihve l 04.Ulk nms sy most in Unlike 2004). al. et Stanishev 1998; Mikolajewska setisl hog rgtnn noptical in brightening man can a rate through accretion the itself o in isfest depth change t sudden optical high A the layer. its determines boundary to rate due accretion energies The i X-ray perature. region, in availa This often the layer. , ac- of cretion half boundary the approximately the radiates and disk, as Keplerian disk a known accretion is Keplerian object a creting between interface The Introduction 1. uy4 2018 4, July srnm Astrophysics & Astronomy = rmtccag ntebudr ae ntesmitcrecur symbiotic the in layer boundary the in change Dramatic ,bcmn ae-y beti h otensy thosts It sky. northern the in object naked-eye a becoming 3, ooa oels( r)i n ftefu nw re- known four the of one is CrB) (T Borealis Coronae T .J .Luna, M. J. G. hl h pia u eandhg;( high; remained flux optical the while pcrmbcm uhsfe n rgt e,blackbody-li new, bright, a and softer much became spectrum t tutr rmtcly ehv inse uhcag f change such mo witnessed the XMM have reaches We material dramatically. which at structure rate its the in increase sudden A al 04( B-ban 2014 and V early (AAVSO) Observers Variable of Association e words. b Key 8 about observed that to event instability. similar a be could which 11 10 9 8 7 6 5 4 3 1 2 Argentina NF-Ittt iAtosc pzaeeFsc omc,Via Cosmica, Fisica e Spaziale Astrofisica di Istituto - INAF nvriyCleeLno,MladSaeSineLaborato Science Space Mullard London, College University eateto hsc n srnm,Uiest fPittsbu of University Astronomy, and Physics of Department oubaAtohsc a 5 10hS. 07PpnHl,M Hall, Pupin 1027 St., W120th 550 Lab Astrophysics Columbia eateto hsc,Uiest fMrln,BlioeC Baltimore Maryland, of University Physics, of Department RSTadXryAtohsc aoaoy AAGdadSpa Goddard NASA Laboratory, Astrophysics X-ray and CRESST nvria ainlAtr arth,A.Calchaqu´ı 620 Av. Jauretche, Arturo Nacional Universidad OIE-nvria eBeo ie,Isiuod Astrono de e-mail: Argentina Instituto Aires, Buenos Aires, C1428ZAA, Buenos de CONICET-Universidad nvria eBeo ie,Fcla eCeca xca y Exactas Ciencias de Facultad Aires, Buenos de Universidad nttt eCeca srnoia,d aTer e Espa del y Tierra la Astron´omicas, de Ciencias de Instituto eatmnod ´sc srnmı,Uiesddd aSe La de Astronom´ıa, Universidad F´ısica y de Departamento -Newton, [email protected] ∆ iais yboi crto,aceindss–X-rays: – disks accretion accretion, – symbiotic binaries: V ≈ Swift .) ( 1.5): 1 , 2 , 3 us lr eecp (BAT) Telescope Alert Burst .Mukai, K. , i h adXryeiso sse ihBTams aihd ( vanished; almost BAT with seen as emission X-ray hard the ) aucitn.tcrb˙submitted˙to˙arxiv no. manuscript WD = 1.2-1.37 4 , 5 iii oaTCrneBorealis. Coronae T nova .L Sokoloski, L. J. h Vflxicesdb tlatafco f4 vrtequiesc the over 40 of factor a least at by increased flux UV the ) / M Vada X-ray an and UV Arancibia, ⊙ Blznk & (Belczynski / -a eecp (XRT) Telescope X-Ray hma- ch l ac- ble 10 6 the d are s the f ABSTRACT , em- rtefis iei h yboi eurn oaTCB u ana Our CrB. T nova recurrent symbiotic the in time first the or .Nelson, T. ved 11 m- ecmoetapae.W ugs htteotclbrighten optical the that suggest We appeared. component ke ,F aea unsArs Argentina Aires, Buenos Varela, F. 0, g., titra ato naceinds,ie h onaylaye boundary the i.e. disk, accretion an of part internal st as i IAECNCT,A.Ep˜aSr11,J42S,SnJu San J5402DSP, 1512, Espa˜na Sur Av. (ICATE-CONICET), cio fr h otrcn hroula ubrti 96 sdue is 1946, in outburst thermonuclear recent most the efore n aaidctsta uiga pia rgtnn vn th event brightening optical an during that indicates data d y g,Ptsug,P 15260 PA Pittsburgh, rgh, ea v itra 20 aSrn,Chile. Serena, La 1200, Cisternas Av. rena, - n .E Nu˜nez, E. N. and ut,10 ilo ice atmr,M 15,USA 21250, MD Baltimore, Circle, Hilltop 1000 ounty, .L af 5,I916Plro Italy Palermo, I-90146 153, Malfa La U. y omuyS.Mr,Drig H N,U.K. 6NT, RH5 Dorking, Mary, St. Holmbury ry, auae,Beo ie,Argentina Aires, Buenos Naturales, lr eecp BT erl ta.20;Kne ta.2009 al. with et CrB Kennea T 2004; of Observations al. et Gehrels (BAT; Telescope Alert inds htetnsott h iclrsto aisR radius circularisation the to out extends that disk tion ikbfr ecigteW ufc.Teln ria period orbital accretion long an The into surface. point WD 227.5687 the of L1 reaching the before through disk a proceeds and thus 1998), star Mikolajewska donor cretion & 1999) (Belczynski Schmid Roche-lobe (M¨urset & its M4III fills the CrB T in biotics, ob eetdwt the with detected eno bright be and hard to sometimes emission X-ray with stars otic (s L1 point 2008). Wynn internal in Lagrangian mate- 5 the transfered eq. through the passed had that when momentum rial angular same the with 10 rto ikbudr ae Ln ta.20;Ikeize a et Iłkiewicz 2008; al. et (Luna a 2016). layer by th boundary from described plasma disk be thermal cretion could thin optically spectrum absorbed, X-ray highly hot, hard the that showed cafr(04 ugssta r rgtndb bu 1 about by ( brightened years CrB several V T in that mag suggests (2014) Schaefer paper. forthcoming a in discussed 27Clmi nvriy e ok e ok107 USA 10027, York New York, New University, Columbia 5247 C ´ ayFıiadlEpco IF) v ne ¨iads262 G¨uiraldes Inte. Av. (IAFE), Espacio, del F´ısica y m´ıa eFih etr rebl,M 07,USA 20771, MD Greenbelt, Center, Flight ce binaries 12 ntrso t -a pcrm r soeo v symbi- five of one is CrB T spectrum, X-ray its of terms In h xasiehsoia pia ih uv opldby compiled curve light optical historical exhaustive The 7 m ..adsac rmteW hthsaKpeinorbit Keplerian a has that WD the from distance a i.e. cm; .Kuin, P. Suzaku / lrVoe pia eecp UO)adAmerican and (UVOT) Telescope Optical UltraViolet ± .09dy Fkle l 00 mle naccre- an implies 2000) al. et (Fekel days 0.0099 , 8 11 RXTE .Segreto, A. ii h R -a u erae significantly decreased flux X-ray XRT the ) n recent and elGhesSwift Gehrels Neil ∼ RXTE )bfr ohrcre recurrent recorded both before 8) 9 .Cusumano, G. n au;ad( and value; ent NuSTAR and Suzaku bevtoswl be will observations bevtr Burst Observatory ,cnchange can r, n20 n 2009 and 2006 in 9 iv tsatdin started at h X-ray the ) .Jaque M. n event, ing © oadisk a to yi of lysis S 2018 ESO rent an, 0, circ ac- e ugh ee c- ≈ ). l. 1 G. J. M. Luna et al.: Dramatic changes in T CrB. novae outburst. The origin and frequency of these brightening 2.3. Optical/UV photometry. events is unknown, but they perhaps indicate a change in accre- tion flow onto the WD. Since the onset of the current optical During each visit with Swift we also obtained UVOT exposures brightening that started in early 2014, referred to as a ”super- with the UVM2 (λ2246 Å, FWHM=498 Å) filter. The UVOT active” state, Munari et al. (2016) found that the luminosity of was constructed using the uvotproduct tool. Most the ionizing source has increased, leading to a strengthening of UVOT observations taken during the current optical brighten- high-ionization emission lines such as He II λ4686Å and the ing are saturated with Vega-magnitudes brighter than about 10. nebular radiation, which now overwhelms the contin- However, we were able to measure source magnitudes using the uum. Munari et al. (2016), however, only presented observations readout streak, as detailed by Page et al. (2013). Observations through December 2015. The light curve from the AAVSO in- with the Optical Monitor (OM) onboard XMM-Newton with the dicates that the maximum brightness was reached around 4-5 V, U, B, W1 filters were also saturated. April 2016 (see Fig. 1). T CrB was persistently detected with the We also collected multi- photometric observations in Swift/BAT since Swift launch in late 2004, until late 2014 when the V and B bands from the American Association of Variable in the span of 4 100-day bins, the BAT 14–50 keV flux declined Star Observers (AAVSO) to study the X-ray data in the context from ∼4 mCrab to ∼2 mCrab, then exhibited a sudden drop to 0 of the optical state. The observations covered a period of ∼4200 (within 1 sigma) in the following time bin. days. In this article we study the super-active state, focusing on the XMM-Newton observation taken about 300 days after the optical maximum. Based on the behavior of the high-energy emission 3. Analysis and results. and the nature of the T CrB system, we propose that the super- active state is due to a disk-instability. In Section 2 we describe The XRT flux reached its lowest value since the launch of Swift our dataset while in Section 3 we present the results from the during our observing campaign that started in 2017 January. spectral and timing analysis. Section 4 presents our interpreta- Along with the X-ray fading, the XRT spectra have clearly soft- tion. ened, with a soft component dominating the spectra at energies less than about 1 keV. T CrB increased its brightness in UV significantly, from about 15 to brighter than approximately 9.8 UVM2 mag. The upper panel in Figure 1 shows the resulting 2. Observations. optical, X-ray and UV light curves. On 2017 February 23, well into the super-active state, our 2.1. Swift deep observation with XMM-Newton showed that the EPIC spectra (Fig. 2) are obviously complex, and can be divided into On January 18th 2017 we started a Swift XRT+UVOT moni- three energy ranges. Above 3 keV, there is a highly absorbed toring campaign, with observations first every week, then ev- component with prominent emission line complex in the 6–7 ery two weeks, and during the last , once every month. The keV range, indicative of optically thin, thermal origin. This is Swift/XRT was operated in Photo Counting mode. We extracted likely the same δ-type component seen in T CrB in its normal source and background count rates from each observation from state (Kennea et al. 2009; Luna et al. 2013). Below ∼0.7 keV, a circular region with a radius of 20 pixels (≈47′′) centered on the spectra are dominated by a soft, unabsorbed component. A T CrB SIMBAD coordinates (α=15h 59m 30.16s; δ=+25◦ 55′ blackbody provide an good description of this region. Photons 12.6′′). We accounted for the presence of dead columns on the are also detected in the intermediate energy range (0.7–3 keV). CCDs using the tool xrtlccorr. We fit the spectra in two different ways. In one, all components are absorbed by interstellar absorp- 2.2. XMM-Newton tion and local, partial covering absorption that blocks 99.7% of the emission (modelA in Table 1,see also Fig. 2).The soft X-ray 5 We observed T CrB on 2017 February 23 (through a DDT time spectra consist of blackbody-like emission (with Tbb=4×10 K) request) using the EPIC camera in Full Window mode, with the from a region smaller than the surface of the WD, with a spheri- 7 2 mediumfilter, for 53.8ks andthe OM camera in fast-mode.After cal surfacearea of 4.2×10 km (inthecaseofaWDwith1.2 M⊙ 8 removing intervals with high flaring background, the net expo- and RWD=5×10 cm, this represents approximately 13% of the sure time reduced to 38.3 ks. Source, background spectra and surface of the WD). The hard (0.6. E . 10 keV) spectra were light curves where extracted from circular regions of 32 and 42 consistent with a multi-temperature, cooling flow, with maxi- arcsec radii, respectively, with the source region centeredon T mum temperature kTmax=12.9 keV. An alternative to this two- CrB SIMBAD coordinates and the background in a source-free components model, where only the δ-component is partially- region of the same CCD. We used the RMFGEN and ARFGEN to covered by the absorber yielded a blackbody-componentspheri- build the redistribution matrices and ancillary responses. The re- cal surface area of 1.5×105 km2 (model A’). sulting X-rayspectra were groupedwith a minimumof 25 counts We also studied an alternative model, with three separate per energy bin. For timing analysis, we converted photon ar- thermal components, a soft blackbody-like component, a hard rival times to the Solar system barycenter using the SAS task optically thin thermal plasma component and a medium en- barycen. ergy optically thin thermal plasma component to take into ac- We emphasize that both grades distribution and offset maps count photons in the ∼0.7–3 keV region. The soft, optically thick indicate that the Swift and XMM-Newton observations were not and medium-energy, optically thin components are modified by affected by optical loading on the X-ray detector, and the softest mostly interstellar absorption while the hard, cooling flow is af- X-rays in the spectra are real. Very bright optical sources tend to fected by full and partial covering absorbers. Although in terms 2 create spurious X-rays photons, or change the grades and ener- of χν(0.95/222 d.o.f) this model is also acceptable, parameters gies of X-ray photons in the case of moderately optically bright such as the absorbing column densities or the covering fraction, sources, and we have verified that this is not the case for T CrB. are unconstrained. The black-body emitting area in this case was

2 G. J. M. Luna et al.: Dramatic changes in T CrB. XMM 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 9

10

11 Magnitude

12

6

4

2

Flux Density 0

[mCrab (14-50 keV)] -2

15

10

5 Softness Ratio 0

0.09

0.06

0.03

XRT c/s [0.3-10 keV] [0.3-10 c/s XRT 0.00

10

12 UVM2 14

100.00

10.00

1.00

0.10

Log (2-10/0.3-2) [keV] Log (2-10/0.3-2) 0.01 54000. 55000. 56000. 57000. 58000. MJD Fig. 1. (a): AAVSO V- (red dots) and B-band (blue dots) light curves covering the period late 2004-early 2018. Vertical, dashed line shows the time of maximum optical brightness. Black, solid line shows a curve from a moving average with 228 days (the ) mean. (b) Swift BAT 14–50 keV light curve with 100 days bins. It is evident that the BAT flux started to decay quasi-simultaneously with the increase in the optical flux, which started around December 2014, it became too faint for its detection right around the optical maximum. (c) Swift BAT softness ratio (15–25/25–100keV) with 100 days bins. This ratio steeply increased owing to the softening of the X-ray emission. (d) Swift XRT 0.3-10 keV count rate. (e) Swift UVOT UVM2-magnitude light curve. Stars indicate magnitudes determined from the CCD readout streak. Even during the rise to optical maximum, the UV flux has already increase dramatically. (f) Swift XRT hardness ratio (2-10/0.3-2 keV). Since the beginning of the optical brightening, the X-ray emission has gotten about 100-times softer than before.

smaller than in model A, with a value of 1.3×105 km2 (covering The light curves from the XMM-Newton observation, binned much less than 0.1% of the WD surface; model B in Table 1). at 300 s, show strong variability with fractional amplitudes of

3 G. J. M. Luna et al.: Dramatic changes in T CrB.

1 1 −1 Model A −1 Model B keV 0.1 keV 0.1 −1 −1

0.01 0.01

10 −3 10 −3 normalized counts s counts normalized normalized counts s counts normalized

2 2

2

2

∆χ ∆χ 0 0

−2 −2 0.51 2 5 0.51 2 5 Energy(keV) Energy(keV) Fig. 2. Left: XMM-Newton pn (black), MOS1 (red) and MOS2 (green) X-ray spectra of T CrB data with the best spectral model (solid line) and the contribution of each model’ component (dotted lines). The model consists of a blackbody plus an optically thin cooling-flow and a Gaussian profile centered at the Fe Kα energy of 6.4 keV. All components are absorbed by interstellar and local, partial covering absorption (see Table 1 for resulting parameters). Right: An alternative model, with a soft blackbody,a hard thermal plasma component, and a medium energy optically thin thermal component to take into account photons in the ∼0.6–3 keV region. The soft optically thick and medium energy, optically thin components are modified by mostly interstellar absorption, while the 2 hard, cooling flow is also affected by a partial covering absorber. Both bottom panels show the fit residuals in units of χν.

0.32 in the soft (0.3–0.7 keV) X-ray band and 0.47 in the hard 0.8

(0.7–10 keV) X-ray band, respectively (Figure 3). The XMM- 0.6

Newton X-ray light curves did not contain statistically signifi- 0.4 cant periodic modulations. We searched for periodicities in the X-ray light curves (with bins of 300 s) in the 0.3–0.7 and 0.7– 0.2 Rate (0.3-0.7 keV) [c/s] 0.0 10 keV energy ranges by computing the Fourier power spec- 0.3 trum. The log-log power spectrum in the frequency region f . 0.2 0.0033 Hz is dominated by red noise. To search for periods with amplitudes in excess of the red-noise, we modeled the log-log 0.1

power spectrum with a simple power-law using a Least-squared Rate (0.7-10 keV) [c/s] 0.0 fit (Vaughan 2005). The detection threshold was determined by 3 simply assuming that the model is a good description of the un- 2 derlying power spectrum and thus their ratio will be distributed 2 like χ . We estimated the probability (at a 95 and 99.74% con- 1 Hardness ratio fidence levels) that at a given frequency, a large peak would be 0 present in the periodogram by comparing the ratio of modeled 0. 10000. 20000. 30000. 40000. 50000. 60000. Time [s] over observed power spectrum to the χ2 probability distribution. No peaks in the power spectrum excedeed the detection thresh- Fig. 3. Background-subtracted XMM-Newton EPIC light curves old. None of the power spectra, in any of the selected energy with bins of 300 s. Top: 0.3–0.7 keV. Middle: 0.7–10 keV. ranges, show periods with a significance greater than 3σ. Our Bottom: Hardness ratio (0.7–10 keV/0.3–0.7 keV) observations were sensitive to pulsed fraction (following Israel & Stella 1996) greater than 15% for f . 0.0033 Hz in the 0.3- 0.7 keV range and greater than 22% for f . 0.0033 Hz in the M˙ increased and the boundary layer became predominantly opti- 0.7-10 keV range. cally thick. The non-detection of periodicities in the light curves of both the hard and soft X-rays suggests that the emission is not powered by magnetically-channeled accretion. The stochas- 4. Discussion. tic variability in the hard band is, however, expected if the emis- 4.1. The boundary layer and unstable disk sion arise in the accretion disk boundary layer. Furthermore, the strong variability observed in the soft band and the small area of From the spectral models fit to the XMM-Newton EPIC spec- the blackbody emitting region derived from the spectral model tra, we consider model A to be the most compelling. While the A indicates that this emission is not powered by thermonuclear size of the blackbody-component in models A’ and B are un- burning on the WD surface. +33 likely small to arise from the WD surface or the accretion disk At a distance of 806−30 pc (as recently determined with boundary layer, in model A it might well corresponds to the size GAIA Bailer-Jones et al. 2018), the unabsorbed luminosity of 4 35 of the boundary layer. Changes in strength and character of UV the blackbody-emitting region is Lbb=AσT =6.68×10 (d/806 and X-rays, along with optical changes reported by Munari et al. pc)2 ergs s−1, with A being the surface area of a sphere with ra- 8 (2016); Zamanov et al. (2016); Iłkiewicz et al. (2016), demon- dius Rbb=1.8×10 cm. The UVM2 fluxes indicate that LUV > strate that after early 2014,the rate of mass transferonto the WD, 2×1034 (d/806 pc)2 ergs s−1. Assuming that half of the ac-

4 G. J. M. Luna et al.: Dramatic changes in T CrB.

Table 1. X-ray spectral fitting results. Unabsorbed X-ray flux ing the IUE spectra during the low and high states, Stanishev −13 −1 −2 31 −9 2 −1 and luminosity, in units of 10 ergs s cm and 10 ergs et al. (2004) found M˙ low=1.53×10 (d/806 pc) M⊙ yr and −1 −8 2 −1 s , respectively, are calculated in the 0.3-50 keV energy band M˙ high=1.1×10 (d/806 pc) M⊙ yr . Selvelli et al. (1992) an- for the optically thin thermal components and in the 1 eV to alyzed IUE data and suggests that during the decade of 1980, −9 2 10 keV for the optically thick thermal component. Elemental the accretion rate was on average M˙ low=9.6×10 (d/806 pc) −1 abundances are quoted in units of abundances from Wilms et al. M⊙ yr . The optical depth of the boundary layer previous to (2000). Luminosity, size of the black-body emitting region, and Swift launch is unknown. Only short pointed X-ray observation M˙ are determined assuming a distance of 806 pc. ofTCrB(≈2 ks) with Einstein obtained on 1979-02-26 were re- ported by Cordova et al. (1981), with a 0.1–4.5 keV luminosity 30 2 Parameter Model A1 Model B2 of 7.2×10 (d/806 pc) erg/s. In this energy range, T CrB was a 22 −2 ± +12 even fainter than the current X-ray state. It is unknown, however, NH [10 cm ] 0.049 0.002 63−34 b − if a hard X-ray component existed in the past. N [1022 cm 2] 68±2 &39 −8 2 H The current brightness state, with M˙ &6.6×10 (d/806 pc) b ± −1 CF 0.997 0.001 &0.6 M⊙yr being the highest on record, dramatically changed the kTmax [keV] 12.9±0.5 13±5 structure of the boundary layer, making it mostly thick to its ± +0.8 own radiation. NuSTAR observationsin Luna et al. (in prep.) sup- Z/Z⊙ 1.5 0.3 1.5−0.6 −9 −1 port this interpretation. Theory predicts that the transition from M˙ [10 M⊙ yr ] 0.16±0.01 0.17±0.1 c 22 −2 mostly optically thin to mostly optically thick should occuratac- N [10 cm ] ... .0.09 −9 −1 H cretion rates of about 10 M⊙yr fora1M⊙ WD (Popham & +25 kTmid [keV] ... 4.5−2 Narayan 1995) or even lower values as proposed by Suleimanov d 22 −2 ± NH [10 cm ] ... 0.05 0.01 et al. (2014). The accretion rate before the optical brightening −9 −1 kTbb [keV] 0.035±0.001 0.035±0.002 was therefore likely smaller than a few 10 M⊙yr . Phenomenologically,the simultaneous quenching of the hard Rbb [km] 1830±30 103±10 X-rays, with the remarkable increase in UV flux is exactly what F 61.0±0.2 65±2 cf is observed in well-known dwarf novae in outburst. Also, as it is ± Fmid ... 0.15 0.3 observed in some dwarf novae (Wheatley et al. 2003), in T CrB ± ± Fbb 86000 1000 285 5 the maximum temperature of the cooling flow dropped from Lcf 47 50.5 kTmax=57±10 keV as observed with Suzaku in September 2006 ± Lmid ... 0.12 (Luna et al. 2008) to kTmax=12.9 0.5 keV during the XMM- L 66800 221 Newton observation reported here. The origin of the residual bb hard X-ray emission during outburst and the decrease of kT , χ2/dof 0.92/226 0.95/222 max ν are still a matter of debate and could be related to an accretion (1) a b TBabs ×(partcov×TBabs) ×(bbodyrad + mkcflow + disk coronaeor the presence of an additional cooling mechanism gaussian) (see Mukai 2017, for a detailed discussion). (2) (TBabsa×(partcov×TBabs)b×(mkcflow + gaussian)+TBabsc ×apec+TBabsd ×bbodyrad) The enhancement of accretion rate in the accretion disk must be due to an increased mass transfer rate from the donor, a disk instability, or both. Although irradiation is cited as a potential cretion luminosity is radiated in the boundary layer, and that cause of enhanced mass transfer in some interacting binaries, it 8 MWD=1.2 M⊙and RWD=5×10 cm, we found that the accretion is unlikely to explain the super high state of T CrB. First, the rate feeding the optically thick portion of the boundary layer irradiating flux is low: (Munari et al. 2016) used the infrared −8 2 −1 was M˙ bb ≈6.6×10 (d/806 pc) M⊙ yr . The luminosity of the light curves to infer a temperature increase of the heated side of boundarylayer is the sum of the luminosity of the optically thick 80 K, or about 2% increase compared to the unirradiated side, ∼ and optically thin cooling flow components, LBL=Lbb+Lc f . The implying an irradiating flux of 8% of the intrinsic stellar flux. luminosity of the optically thin portion of the boundary layer, Second, B¨uning & Ritter (2004) concluded that irradiation of from the cooling flow spectral model in the 0.3-50 keV, was a giant donor cannot result to mass transfer cycles. Finally, the 32 2 −1 Lc f ≈4.7×10 (d/806 pc) ergs s . The accretion rate feed- time scale on which the envelope of the giant responds to chang- ing the optically thin portion of the boundary layer was thus ing irradiation is many thousands of years according to the same −10 2 −1 M˙ c f =1.6×10 (d/806 pc) M⊙ yr . These accretion rates are authors. consistent with the expected theoretical values where the bound- In a steady-state disk, the effective temperature as a function 1/4 ˙ 1/4 1/4 −3/4 ary layer is optically thick/thin to its own radiation (Patterson & of disk radius is given by T(R) = (3G/8σπ) M MWDR Raymond 1985; Narayan & Popham 1993). (eq. 5.43 in Frank et al. 2002, with R >> RWD) and has to be The current high optical brightness state appears to be as- greater than 104 K, enough to keep H ionized. In T CrB, with a −8 sociated with the highest accretion rate on record. The long WD mass of 1.2 M⊙ and a mass accretion rate M˙ ∼6.6×10 −1 term optical light curve of T CrB shows a history of differ- M⊙ yr (determined from the XMM-Newton observation), the ent levels of activity (see Stanishev et al. 2004; Munari et al. steady-state disk would extend out to about R≈0.75 R⊙. Given 2016; Iłkiewicz et al. 2016) and with the exception of the the size of circularisation radius in T CrB, it is highly unlikely nova eruptions, all of them had lower intensity than the cur- that the entire disk can remain in a high state (with T&104 K). rent level. We can rescale, at d=806 pc, the reported accretion If the region in the vecinity of the circularisation radius is cold, rates and compare then with the current level. The U-band light and unstable with mostly neutral H and low viscosity, then the curve from May 1979 until August 2002 presented by Stanishev accreted matter would accumulate near that region until a disk et al. (2004) shows that T CrB was in a low brightness state instability develops, allowing the material to flow inwards. A from JD 2447300 – JD 2450000 (1987 August through 1995 similar inside-stable, outside-unstable, hybrid accretion disk has September), while from JD2444300 to JD2447300 (1980 March been discussed for the recurrent novae RS Oph by Wynn (2008) through May 1988) was in an optical high state. By model- in an attempt to explain how mass is transferred during qui-

5 G. J. M. Luna et al.: Dramatic changes in T CrB. escence periods, given the low accretion rate determined from of 6.8 × 1023 cm−2. This admittedly crude estimate suggests that X-ray observations in quiescence (Nelson et al. 2011). In this the accretion disk wind is a plausible origin of the local absorber scenario, disk instability outbursts in the outer parts of the disk in T CrB in high state. are infrequent, with recurrence time of the order of hundreds However, our Model A suggests a partial covering absorber of years. Recently, Bollimpalli et al. (2018) studied a similar with a covering fraction of 99.7%, and the remaining ∼0.3% ex- scenario, where the amount of material necessary to trigger a periences hardly any absorption at all (4.5 × 1020 cm−2, or less thermonuclear outburst in RS Oph every ∼20 years is delivered than a thousandth of the column of the partial-coveringabsorber, to the WD in a series of disk-instability outbursts, with optical and this may well be dominated by the ISM). While the 3D sim- brightening amplitudes of about 1 magnitude. A similar scenario ulation of Dyda & Proga (2018) does show a clumpy wind, the could be at work in T CrB. density contrast between the clumps and inter-clump regions is only of order 10. In general (i.e., regardless of whether the ab- sorber is the accretion disk wind or something else), we do have 4.2. X-ray absorption not identified a natural explanation for a near 100% covering X-ray observationsfrom the past decade indicate that the absorb- fraction and a factor of 1000 density contrast. ing column was very high during that time (Luna et al. 2008; The scattering model for CH Cyg of Wheatley & Kallman Kennea et al. 2009; Iłkiewicz et al. 2016), completely covering (2006) may provide a possible alternative to our best-fit spec- the source (Luna et al., in prep.) and absorbing all X-rays with tral model with its extraordinarily high absorption covering frac- energies lower than ∼2-3 keV. This might lead us to suspect that tion. This model requires a low density matter that gets highly the blackbody-like spectral component could have been perma- ionized, and a clear line of sight to that region. Such a region nently emitting at the current level but was only detected now could exist in the polar cavity of the wind. For this region to re- that there was a favorable sight. However, optical spectra dur- main highly ionized, it cannot be optically thick to X-rays from ing the past decade (see Munari et al. 2016) indicate that, with the central source, which also limits its scattering efficiency. the exception of the current super-active state, the intensity of Combined with geometrical factors, it appears plausible for this highly-ionized emission lines, such as HeIIλ4686 has been low region to scatter ∼0.3% of the total luminosity into our line of and the optical continuum weak, in contrast to what is observed sight. now in the super-active state, simultaneous with the appearance In a normal state there are problems for the accretion disk of the blackbody-like component. If blackbody-like X-ray emis- wind as the origin of X-ray absorbers in all δ-type symbiotic sion had been present but absorbed, we would have expected stars. One is that we would expect a strong inclination angle de- very strong optical line and continuum emission from the ab- pendence, with little absorption when one is seen pole-on. The sorbing material. other is the accretion rate dependenceof the wind mass loss rate. Partial-covering absorber is sometimes inferred for δ-type The numbers appear to work for T CrB in the high state. In a symbiotics. However, a covering fraction of 99.7% is extreme. normal state, the accretion rate may only be a few times 10−9 −1 While we cannot exclude a geometrical explanation for this cov- M⊙ yr , hence the wind mass loss rate is expected to be of or- −10 −1 23 −2 ering fraction, we have also explored a possible alternative. In der 10 M⊙ yr at most. Yet NH in excess of 10 cm is high accretion rate, non-magnetic CVs, accretion disk wind fea- seen in T CrB in a normal state. If all X-ray absorbers in δ-type tures are routinely observed in the UV spectra, and perhaps also symbiotics are due to accretion disk wind, we would expect a in the optical spectra (Matthews et al. 2015). It therefore makes stronger dependence on the accretion rate. Similarly, this model sense to ask if the accretion disk wind could be responsible for may have difficulty explaining the high NH values seen in lower the partial covering absorption we observe in T CrB in high- X-ray luminosity δ-type systems. M˙ state in particular, and in δ-type symbiotic stars in general. At the time of submission, the current optically bright state −9 Matthews and others consider a wind mass loss rate of 10 M⊙ continues. Our finding that the accretion-disk boundary layer −1 −8 yr to be a reasonable estimate for an accretion rate of 10 M⊙ around the > 1.2 M⊙WD in T CrB transitioned from primarily yr−1; the former is a non-linear function of the latter, such that optically thin to primarily optically thick when the accretion rate −9 −1 −8 −1 the fraction of mass lost in the wind is lower for lower accre- rose from ∼10 M⊙yr to ∼10 M⊙yr (assuming d=806 pc) tion rate. Their model (see a schematic in Figure 3 of Matthews challenges theoretical boundary-layer models, which must also et al.) has a biconical geometry with an X-shaped cross sec- explain BL transitions at much lower accretion rates for some tion, with wind being launched from 4–12 Rwd. The launch radii DNe in outburst. may be particularly uncertain as wind is launched from the in- nermost regions of the disk in other simulations (e.g., Dyda & Acknowledgements. Based on observations obtained with XMM-Newton an Proga 2018). We express the mass loss rate in the units of 10−9 ESA science mission with instruments and contributions directly funded by −1 24 ESA Member States and NASA. We thanks the entire Swift team for accepting M⊙ yr as M˙ w,−9; for M˙ w,−9=1.0, the mass loss rate is 2.0 × 10 −1 × 16 −1 and planning our multiple Target-of-Opportunity requests. We thanks Norbert gyr =6.3 10 g s . We further express the characteristic ra- Schartel for approving the XMM-Newton DDT request. We acknowledge with dius of the wind where the line-of-sight to the X-ray emitting thanks the observations from the AAVSO International Database 9 region crosses it in the units of 10 cm as rw,9, and the wind contributed by observers worldwide and used in this research. We thanks Ulisse −1 Munari for the helpful discussions about the current super-active state. GJML velocity at the same point in the units of 1000 kms as vw,8. and NEN are members of the CIC-CONICET (Argentina) and acknowledge Dividing mass loss rate by 4πrv , for rw,9=1.0 and vw,8=1.0, we / −2 support from grant ANPCYT-PICT 0478 14. GJML also acknowledges sup- obtain a mass column density expected from a wind of 5.0×10 port from grants PIP-CONICET/2011 #D4598. JLS acknowledge support from −2 wind ∼ × 22 −2 gcm , or NH 5 10 cm . NASA grants NNX15AF19G and NNX17AC45G. Given our estimate for the accretion rate in T CrB in the high −8 −1 state of 6.6 × 10 M⊙ yr , M˙ w,−9=3.0 would be a plausible as- sumption. According to Matthews et al. (2015), it takes the wind References of order 100 Rwd to obtain its final velocity, and it has poloidal −1 Bailer-Jones, C. A. L., Rybizki, J., Fouesneau, M., Mantelet, G., & Andrae, R. velocities of less than 100 km s within a few Rwd,sowecantry 2018, ArXiv e-prints, arXiv:1804.10121 vw,8=0.1. Then rw,9=2.2 would result in the observed NH value Belczynski, K. & Mikolajewska, J. 1998, MNRAS, 296, 77

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