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Thomson Scattering and Collisional Ionization in the X-rays Grating Spectra of the Recurrent Nova U Scorpii PDF

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Preview Thomson Scattering and Collisional Ionization in the X-rays Grating Spectra of the Recurrent Nova U Scorpii

Mon.Not.R.Astron.Soc.000,1–13(2002) Printed4January2013 (MNLATEXstylefilev2.2) Thomson Scattering and Collisional Ionization in the X-rays Grating Spectra of the Recurrent Nova U Scorpii 3 M. Orio,1,2 E. Behar,3 J. Gallagher,2 A. Bianchini,4 E.Chiosi,1, G.J.M. Luna,5 1 T. Nelson,6 T. Rauch,7 B.E. Schaefer8 and B. Tofflemire2 0 2 1INAF–Osservatorio di Padova, vicolo dell’ Osservatorio 5, I-35122 Padova, Italy 2 Department of Astronomy, Universityof Wisconsin, 475 N. Charter Str., Madison WI 53704 n 3Department of Physics, Technion, Haifa, Israel a 4 Astronomy Department, Padova University,vicolo dell’ Osservatorio 3, I-35122 Padova, Italy J 5 Institutode Astronoma y Fisica del Espacio (IAFE/Conicet), CC67, Suc. 28 C1428ZAA CABA, Argentina 3 6 School of Physicsand Astronomy, University of Minnesota, 116 Church St SE, Minneapolis, MN 55455 7 Institute for Astronomy & Astrophysics, KeplerCenterfor Astro & Particle Physics, ] Eberhard Karls University,Sand 1, 72076 Tu¨bingen, Germany R 8 Physicsand Astronomy, Louisiana State University, Baton Rouge, LA 70803 S . h p Received: - o r t ABSTRACT s We present a Chandra observation of the recurrent nova U Scorpii, done with the a HRC-Sdetector andthe LETGgratingonday18 after the observedvisualmaximum [ of 2010, and compare it with XMM-Newton observations obtained in days 23 and 35 2 after maximum. The total absorbed flux was in the range 2.2-2.6 ×10−11 erg cm−2 v s−1, corresponding to unabsorbed luminosity 7-8.5 × 1036×(d/12 kpc)2 for N(H)=2- 6 2.7×1021 cm−2.Onday18,70%ofthesoftX-trayfluxwasinacontinuumtypicalof 2 a veryhot white dwarf (WD) atmosphere,which accountedfor about 80% of the flux 0 on days 23 and 35. In addition all spectra display very broad emission lines, due to 5 higher ionization stages at later times. With Chandra we observed apparent P Cygni . 2 profiles.Wefindthatthesepeculiarprofilesarenotduetoblueshiftedabsorptionand 1 redshiftedemissioninphotoionizedejecta,liketheopticalPCygofnovae,buttheyare 2 ratherasuperpositionofWDatmosphericabsorptionfeaturesreflectedbythealready 1 discoveredThomsonscatteringcorona,andemissionlines due tocollisionalionization : v in condensations in the ejecta. On days 23 and 35 the absorption components were i no longermeasurable,havinglostthe initiallargeblue shift thatdisplacedthem from X the core of the broad emission lines. We interpret this as indication that mass loss r a ceased between day 18 and day 23. On day 35, the emission lines spectrum became verycomplex,withseveraldifferentcomponents.Modelatmospheresindicatethatthe WD atmospheric temperature was about 730,000 K on day 18 and reached 900,000 K–one million K on day 35. This peak temperature is consistent with a WD mass of at least 1.3 M⊙. Key words: stars: novae, cataclysmic variables – stars: winds, outflows — stars: individual(UScorpii)–whitedwarfs–X-rays:binaries–X-rays:individual(UScorpii) 1 INTRODUCTION 2005 and references therein). The models predict that RN occureither with moderately high accretion rate m˙ on WD The recurrent nova (RN) U Scorpii is one of a handful of ofabout1M⊙,oratveryhighm˙ (M>10−8 M⊙ year−1)on such objects known in the Galaxy, although this class may WD with M> 1.2 M⊙. The velocity of the ejecta increases, be much more numerous than we have been able to assess and the duration of the outburst decreases, with the WD so far. “Recurrent” means that the thermonuclear flash is mass. About half of all RNe are symbiotic novae, that is repeated on a human timescale. The outburst supposedly systemswith agiantsecondary andahot WD,withorbital recurs also in classical novae, however the time scales can periodsofhundredofdays(asopposedtomostclassicalno- beaslongas severaltimes105 yearson WDat thelow end vae,whichhaveorbitalperiods ofhoursandmain sequence of the nova mass distribution (0.6-0.7 M⊙), accreting mat- companions). ter at a low rate of 6 10−11 M⊙ year−1 (see Yaron et al. 2 M. Orio et al. the event occurred, a strategy was in place to observe the 1 outburst at all wavelengths. InthispaperwepresentgratingX-rayspectra,theonly U Scorpii Swift/XRT means to probe the effective gravity, chemical composition [0.3-10keV] andeffectivetemperatureoftheunderlyingwhitedwarfonce 0 the photosphere shrinks back to the WD radius (e.g. Nel- sonetal.2008andreferencestherein).Oneobservationwas proposedbyus,17 daysafterthevisualmaximumwith the -1 Chandra LETG grating. We adopt here the epoch of the observed visual maximum 2010-01-28.1 (JD 2,455,225,605) as the initial time (although Schaefer, 2010, extrapolated thelightcurveconcludingthatthepeakhadoccurredafew -2 hours earlier). Two XMM-Newton observations were done on2010-02-19at15:41:09UT,andon2010-03-14at14:33:32 UR,22and35daysaftermaximum,respectively(notethat for the different XMM-Newton detectors the beginning of -3 theobservationsvariesslightly,andfordetailsseealsoOrio et al. 2011, Nesset al., 2011a and 2012). InadditiontopresentingandanalysingtheearlyChan- dra spectrum, in this paper we compare it with the XMM- -4 0 20 40 60 NewtonRGSgratingspectra.Theproposersofoneofthese days after 2010-01-29 observations, Ness et al. (2012) have analysed the X-rays and UV data and examine thetime variability in these two Figure1.SwiftXRTcountrateversustimemeasuredsincesince bandsindetail.TheyfindthatapartialeclipseoftheX-ray therecordedoutburston2010-01-29, inthe0.2-10keVrange. continuumcanbeexplainedonlyifthehotcentralsourceis observedafterhavingbeenThomsonscattered,aconclusion that we reached independently (Orio et al. 2011), and with U Sco has an orbital period of 1.23 days and belongs whichwefullyagree.Theseauthorsremarkaboutthelackof to the class of RNe with a near-main sequence or moder- absorption lines, expected and usually observed in novaein ately evolved secondary (Schaefer 1990, Schaefer & Ring- outburst in X-rays, and attribute it to possible “smearing” wald1995).Thesecondaryspectrumisnotdetectedinopti- byThomsonscattering.Theyalsodescribeandexaminethe cal spectra at quiescence, butat a late post-outburst phase spectra, proposing lines identifications and attributing the Anupama & Dewangan (2000) measured the spectral fea- emissionlinestoresonantscattering.Inthiswork,wefound turesofaK2subgiant.Inthepreviousoutburststheejecta that the nova development with time, including the XMM- were extremely depleted in hydrogen (Barlow et al. 1981), Newtonobservations,isfundamentaltofullyunderstandthe so the secondary must have lost a significant fraction of its physics of the X-ray grating spectra. Without the intent to hydrogenrich envelope.At quiescence,HeII linesare dom- replicate Ness et al.’s (2012) work, we analyse and attempt inant in optical spectra and the hydrogen lines are absent to model with detailed physics all the grating X-ray spec- orweak,afact that hasbeen interpretedasaccretion of He tra, thereby reaching different conclusions than the above rich material from a H-depleted secondary (e.g. Duerbeck authors on the origin of the emission lines observed with et al. 1993). Although Maxwell et al. (2011) cast doubtson XMM-Newton and on the reason for the lack of absorption thehighHeabundance,thepossibilityofveryenhancedhe- features at late stages. lium in U Sco is very interesting as it may result in a type In Section 2 we describe the observations, the data ex- Ia SN (SN Ia) without a hydrogen rich envelope, which is tractionmethod,andwhentheobservationsoccurredinthe consistent with the upper limits on hydrogen in the SNeIa context of the long term X-ray evolution. In Section 3 we spectra. show the short term light curve, which we find relevant to TheshortandluminousoutburstofUScowasobserved understand the physics of the spectra. In Section 4 we dis- 10 times (1863, 1906, 1917, 1936, 1945, 1969, 1979, 1987, cuss the apparent P Cyg profiles observed in the Chandra 1999, 2010), with peak magnitude V≃7.5, decay times by2 spectrum. After having analysed the origin and meaning of and 3 optical magnitudes, t2=1.2 and t3=2.6 days respec- thesefeatures,weperformedatmosphericfittingforallspec- tively, and a return to quiescence within 67-68 days. These tra,andevaluatedtemperatureevolutionandpeaktemper- parametersindicateaverymassiveWD,suchthatsufficient ature(Section4).InSection5wediscussthephysicalmech- pressureforathermonuclearrunawayisbuiltupatthebase anisms producingtheemission lines and howwe attempted oftheenvelopeevenwithasmallaccretedenvelope(e.g.few a global fit;finally conclusions are presented in Section 6. 610−7 M⊙, see Starrfield et al. 1988). We adopt 12±2 kpc as distance to the nova, measured thanks to its eclipse in theoptical (Schaefer et al. 2010). 2 THE OBSERVATIONS, AND THEIR Since U Sco has shown outbursts on an almost regular POSITION IN THE LONG TERM LIGHT timescaleof10±2years(assumingacoupleoferuptionswere CURVE missedbecausetheyoccurredwhenthenovawasbehindthe sun),Schaefer(2010)wasabletopredictthelasteruptionof TheSwiftsatellitemonitoredUScoalmostdailyonalldates Januaryof2010withanuncertaintyofonlyoneyear.When when it was technically possible. We extracted the Swift X-ray spectra of U Scorpii 3 threespectraarelabelled(foradditionallinesidentification, see Table 1). WeseeinFig. 2thatforat least threelines(theNeVI Heγ linewithrestwavelength23.77˚A,theNVIIhydrogen αline,withrestwavelength24.78˚A,andtheresonanceline oftheNVItripletat28.78˚A)theemissionportionappears red-shiftedandtheabsorptionwingblue-shifted.Thisisin- triguingly similar to the P Cyg profiles observed in optical spectra. We compared the Chandra spectrum with archival XMM-Newton grating spectra obtained on 2010-02-19 at 15:41:09 UT and on 2010-03-3 at 14:33:32 UT, (5 and 17 dayslater,respectivelydays23and35).TheXMM-Newton spectrawereextractedwithXMM-SASversion11.0.2.There was variability during both XMM-Newton observations, as wediscussinthenextSection,butinFebruary(day23after theoutburst)wemeasuredaveragecountrates0.881±0.004 cts s−1 and 0.824±0.004 cts s−1 with the RGS1 and RGS2 Figure 2. The U-Sco spectrum measured with the Chandra gratings respectively, while in March (day 35) the average LETGgratingon2010-02-14,day18postopticalmaximuminthe count rates were 1.037±0.005 cts s−1 and 0.951±0.004 cts Vband(+1and-1ordershavebeensummed).Theredsolidline s−1.Theunabsorbed fluxseems was rising moderately, and shows the best fit with two components: an atmospheric model wemeasured2.5±0.2×10−11and2.6±0.2×10−11inthesec- with a 740,000 K Teff and a collisionally ionized optically thin ond and third observation, respectively. All XMM-Newton plasma with tenfold He and N abundance abundances with re- specttosolar,andplasmatemperaturekT=93eV(seeTable2). instrumentswereobservingthenovasimultaneously,includ- ing the Optical Monitor (OM), and the broad band X-ray imagers EPIC-pn and MOS1 and MOS2. The RGS spectra data and present the XRT X-ray light curve in Fig. 1. Al- are shown in Figures 3 to 5. Figures 4 and 5 present the thoughthispaper’sfocusisonthelongerobservationsdone count rate spectrum (like Fig. 2 for Chandra). The fit with with Chandra and the comparison with the RGS of XMM- atmosphericandplasmaemissionmodels,alsoshowninthe Newtondata,theSwift-XRTlongtermlightcurveputsthe Figures, are discussed in detail in Sections 4 and 5. data in the context of the outburst evolution. The arrows indicate the times at which the grating observations were done. The X-ray flux was still on the rise during both the 3 THE SHORT-TERM LIGHT CURVE: A 2010-02 observations (days18 and 23), while themaximum SIMULTANEOUS X-RAY AND OPTICAL X-ray flux was recorded around the date of the third and ECLIPSE final X-ray grating observation in March, on day 35. The X-rayspectrum remained remarkablysoft duringthewhole At optical wavelengths, the eclipse of the accretion disk by post-outburst phase, with more than 95% of thecountsbe- theKdwarfcompanionwasdiscoveredandmeasuredatqui- low 0.7 keV.A sharp decay followed theMarch observation escencebySchaefer&Ringwald(1995),anditwasobserved untilday67, when thenovawas almost back toX-raymin- again in the outburst by Schaefer et al. (2011), indicating imum. Further information on the UV and X-ray evolution thattheaccretiondiskhadalreadybeenreestablishedbythe observed with Swift can be found in Ness et al. (2012). third week after the optical maximum. The orbital period The first grating observation of U Sco was proposed of U Sco is 1.23054695 days (Schaefer & Ringwald 1995). by us and was done with the LETG/HRC-S instrument of For the ephemeris we assume in this paper the result of theChandraobservatoryon2010-02-14, at11:38:22UT.As the linear fit for the whole 1999-2010 inter-eruption inter- stated above,thiswas day18 afteroptical maximumin the val,withaHJDtimeoftheopticalminima2451234.5387 + V band (see Schaefer et al. 2010). The exposures lasted for N×1.23054695. Wecautionthatthemanyobservedeclipses 23 ks (6.38 hours). The observed count rate spectrum is duringthelast foureruptionsshowsignificant deviationsof showninFig.2,whileFig.3showsthefluxedspectrum,com- theminimumfromthisephemeris,withtheoffsetschanging pared with the spectra measured with the XMM-Newton throughout the eclipse. This means that the center of opti- RGSgratings on days23 and 35. Fig. 2also shows a model callightcurvechangesthroughouttheeruptions,andisnot fit, discussed in detail in Sections 4 and 5. In order to ex- exactly at thesame position duringthequiescent periods. tracttheLETGspectrum,weappliedthemostupdatedcal- WedidnotdetectsignificantvariabilityintheChandra ibration to the LETG pipeline processed level 2 event and zeroorderlight curve,butall theobservation occurred out- phaChandradatafiles.Responsematricesandancillary re- sidetheeclipseobservedinoptical.Theexposurestartedat sponse(effectivearea)fileswerecreatedusingthetoolsmk- orbital phase0.4463 and covered almost a quarter(22%) of grmf and mkgarf, part of the CIAO v4.2 package. Wemea- theperiod. suredasummed±1orderscountrateof0.477±0.008ctss−1 Both XMM-Newton observations covered about 60% in the 0.2-0.8 keVenergy range in which fluxis measurable of the orbital period and thus overlapped with the optical above the background (corresponding to approximately 15- eclipse time. The day 23 observation started at phase 0.84, 60˚Ainwavelengthrange).Wemeasuredanunabsorbedflux and theone of day 35 started at phase 0.55. 2.2±0.2 erg cm−2 s−1. In Fig. 3, the strongest lines of the In Fig. 6 we present the XMM-Newton EPIC pn light 4 M. Orio et al. Figure3.Measuredfluxversuswavelengthinthethreegratingspectraatdays18,23and35afterthediscovery,TheChandraspectrum isdisplayedinblack,theaveragedRGS1-RGS2spectrumof2010-02-14isplottedininredandthe2010-03-02RGS1-RGS2spectrumis inblue. Figure 4. The XMM-Newton-RGS-1 (left) and RGS-2 (right) spectra of U Sco on day 23 after the optical maximum. The blue line showsthefitwithaWDatmospherewithabsorptionlinesblue-shiftedby2000kms−1,andonlythefirstBVAPECcomponentinTable 2(above) oraddingthesecondBVAPECcomponents fortheRGS-2hardportionofthespectrum(smallpanelbelow). curves and compare them with the optical light curves of not regular) period of about 3 hours, extensively discussed Schaeferet al. (2010) measured in thesame twoweeksdur- byNessetal.(2012).Theseauthorsalsoextractedthespec- ingwhichthetwoX-raydatasetsweretaken.Alllightcurves trum in and out of the oscillation, showing no variation of arefolded overtheopticallymeasured orbitalphaseandre- the emission lines. The authors attribute the phenomenon peated twice. For theX-ray light curve,we plotted a quan- to large clumps, of absorbing material at a distance about tity that is 2.5 times the logarithm of the X-ray flux nor- 3.5 R⊙ from the WD, intersecting the line of sight in the malized to its minimum vale, for a direct comparison with direction of the accretion stream, during the process of re- optical magnitudes. The EPIC-pn exposures are 63185 and establishmentofthedisk.Wedonotwishtofurtherdiscuss 62318 seconds long respectively. theseoscillations,althoughwesuggestthatthissemi-regular On day 23 in the X-ray light curve the most evident period(evenifitisassociatedwithejectionofparcelsofma- oscillation is a pronounced one with a semi-regular (albeit X-ray spectra of U Scorpii 5 Figure 5.TheXMM-Newton-RGS-1(left)andRGS-2(right)spectraofUScoonday35after theopticalmaximum.Thefitisshown inred,theparametersaregiveninTable2. Table1.Rest,measuredwavelength,andfluxesinphotonscm−2 ks−1 fortheemissionlinesidentifiedand measuredinthethreegratingspectra.Seethetextforanevaluationoftheerrorsinthefluxestimate. Chandra02-14 XMM02-19 XMM03-05 line λ0 (˚A) λm (˚A) flux λm (˚A) flux λm (˚A) flux CVILyα 33.7342 33.78±0.02 0.39 33.78±0.05 0.43 33.75±0.05 NVIf 29.5346 29.50±0.02 0.93 29.50±0.03 1.23 29.60±0.05 0.35 NVIi 29.0819 29.16±0.02 0.91 29.20±0.02 1.65 29.22±0.05 0.42 NVIr 28.7800 28.93±0.02 2.20 28.82±0.05 2.10 28.80±0.05 0.31 CVILyγ 26.990 26.80±0.02 NVIHeβ 24.90 63.8 24.95±0.01 61.3 NVIILyα 24.78 24.94±0.03 1.71 24.92±0.03 5.64 24.80±0.02 2.51 NVIHeγ 23.771 23.81±0.05 0.43 23.81±0.05 0.60 23.82±0.02 2.70 OVIIf 22.097 22.108±0.050 22.13±0.05 0.30 OVIIi 21.801 21.807±0.020 60.24 21.80±0.02 0.49 OVIIr 21.602 21.603±0.050 0.35 21.61±0.02 0.39 OVIIILyα 18.969 19.05±0.03 0.74 19.00±0.02 3.21 OVIIHeβ 18.627 18.57±0.05 18.55±0.02 FeXVII 16.78 16.78±0.05 16.80±0.05 OVIILyβ 16.00 15.95±0.03 16.00±0.02 0.44 FeXVII 15.262 15.30±0.05 15.25±0.05 >0.10 FeXVII 15.01 15.15±0.02 15.03±0.05 0.12 FeXVIII 14.207 14.22±0.03 0.13 FeXIX 14.667 14.70±0.05 14.67±0.03 NeIXf 13.6984 NeIXi 13.5503 13.57±0.05 0.12 NeIXr 13.4474 13.45±0.02 13.48±0.05 0.15 NeXα 12.1321 12.14±0.05 0.15 NeIXHeβr 11.55 11.60±0.10 MgXIHer 9.09 9.20±0.03 9.20±0.03 terial), may represent the disk rotation period, which may O VII triplet). Extracting the spectrum over the short du- beslightly variable while the disk is still being formed. ration of the eclipse we find a variation of this line only at the2 σ level, so wesuggest that thisis not statistically sig- A clear orbital modulation in theday 35 spectrumwas nificant. In any case, the main purpose of Fig. 6 is to show noted by us and by Ness et al. (2012). These authors also thattheorbitalmodulationseemsalreadyclearlypresentin stress that almost all emission lines observed in the spec- theXMM-Newton EPIC-pnlight curveof day 23, although tra are not eclipsed, while the continuum is, and we agree it is not as pronounced as on day 35. On day 35, the aver- with their analysis. However, we cast some doubts regard- agecountrate duringorbitalphases 0to0.12 and0.88 to1 ing the dimming in eclipse of only one specific line, the O is 0.75 and 0.84 cts s−1 with RGS1 and RGS2 respectively, VIII Lyα (the above authors remark this is the only line while we measured 1.10 and 1.05 cts s−1 with RGS1 and that seems to vary,even there is clearly no variation of the 6 M. Orio et al. RGS2 respectively during the rest of the observation. We theOI 1s–2p transition at ∼23.476 ˚A,which doesoccurs in find that the variation in count rate in and out of eclipse, theISMalong theline of sight toU Sco. Thefact that this outsidethe≃3hoursoscillations,isbyatleast25%evenon absorptionfeaturewashardlymeasurableinNLMC2009,in day 23. one LETG spectrum that appears similar to this one (Orio Becausetheeclipsetakesoutonlyabout50%oftheflux et al. 2011, and 2012 article in preparation), confirms the in the X-ray continuum, on day 35 a large portion of this identificationbecauseNLMC2009isinadirectionofmuch continuum must originate in a much more extendedregion. lowerinterstellar absorption (N(H)≃4×1020 cm−2).Thus, We assume that this is a Thomson scattering corona, re- we realize that at least this particular P Cyg-like profile processes a fraction of the WD radiation in an achromatic is in fact only a sort of “pseudo P Cyg”, composed of an way (Ness et al. 2011a, 2012, Orio et al. 2011). The incli- absorption and an emission feature that do not have origin nation angle in U Sco is 82.7o±2.9o (Thoroughgood et al. in the same medium. 2001),soitisquiteconceivablethatwedidnotobservemost We examine now the P Cyg profiles of the other two of the WD flux directly. The accretion disk was placed by strong lines, shown in Fig. 7, to understand whether they Schaefer et al. (2011) at 3.4 R⊙ distance from the WD in may be due to the “real”, classic P Cyg phenomenon. As a light curve model for day 35. Because of the nearly edge- Fig. 7 shows in velocity space, the N VII emission appears on inclination, the short-duration and deep X-ray eclipse blue-shifted by 2200±200 km s−1 in absorption, and red- does not appear to be due to the disk. We assume that shifted by the same amount in emission. The N VI res- the source of the continuum is eclipsed by the secondary, onance line with rest wavelength 28.78 ˚A is instead blue- exactly like for the eclipse of the disk by the secondary ob- shifted by2700±300 km s−1,in absorption, andred-shifted served at quiescence at optical wavelength. Following the by 1500±300 km s−1 in emission, so the two components reasoning of Schaefer et al. (2011, see their Fig. 13 for the do not cross around zero, but rather towards the red. In optical eclipse) we conclude that most of the continuum is optical spectra of novae, crossing in the red is observed in observedasreflectedatadistanceequalorlargerthanthea spectra taken shortly before the P Cyg profile disappears. 3.4R⊙ distancefromtheWDatwhichSchaeferetal.place Typically,thePCygprofileisnolongerobservedinthefol- theouter rim of the disk. lowing weeks because the blue-shifted absorption becomes We intend to show that the Chandra spectrum with negligible compared to the emission. Fig. 8 shows thesame its absorption features is best explained assuming that this line profile in velocity space in the RGS spectra of days 23 scattering corona was already present, and at comparable and 35. distancefromtheWD,onday18,althoughnoX-rayobser- P Cyg profiles in X-ray spectra in general are rarely vations at eclipse minimum were done. observed, and the absorption wing, if measurable, is very shallow. This is because in X-rays we are likely to observe outflows and nebulae around compact objects, while in op- tical we observe them around stars with at least hundred 4 FITTING THE SPECTRUM, STEP BY STEP: of times larger radii. This includes the optical (as opposed THE WD toX-rays)spectraofclassical andrecurrentnovae,whereP Cyg profiles at optical wavelengths are observed when the 4.1 The absorption lines observed with Chandra photospherearoundtheWDhasnotrecededtoWDdimen- Two puzzles are immediately apparent when we examine sions yet, but is still of the order of at least a solar radius. Figures2to5.Firstofall,theChandraspectrumshowsdeep However, this typically occurs weeks or months before the absorption features of nitrogen, while the XMM-Newton supersoftX-rayphase,andweexpectitonlyinthefirstfew ones do not display any features in absorption. Although days, or even in the first hours, in a very fast nova like U model atmospheres show that the absorption becomes less Sco.PCygprofilesatopticalwavelengthsareduemainlyto pronounced at high temperatures (Rauch et al. 2010), the resonancetransitions.Theblue-shiftedabsorptionisobserv- models predict deep absorption features of nitrogen, al- ableonlywherethematerial movesin ourdirection,that is ways detected in the previous X-ray gratings’ observations along the line of sight of the photo-ionizing central object. of other novae (e.g. Rauch et al. 2010, Ness et al. 2011b). The optical P Cyg absorption features occur, like theemis- The other surprising element, as we already noted, are the sion,inthe“wind”ornebulaaroundaphoto-ionizingcentral apparent P Cyg-like profiles in the Chandra spectrum. P- object, but they are observed in a cylinder with the radius Cyg profiles are typical of the optical spectrum of novae in of the central object, and only as long as the radius of the X-rays, but they have never been observed in their X-ray cylinder is not negligible compared to the rest of the shell, gratingspectra,except(possibly)inthefirstXMM-Newton from which we observe theemission lines. spectrum of another fast recurrent nova, LMC 2009 (Orio Luminous supersoft X-raycontinuum - observed in the 2012). Chandra spectrum as well as in the XMM-Newton ones - Emissionlinesattributedtotheejectawereobservedin in novae arises when the central object has shrunk and re- theX-raysspectraofseveralothernovae(Drakeetal.2003, gainedcompactdimensions,witharadiusoftheorderofat Nessetal.2003, Nelsonet al.2008, Rauchetal. 2010). Are most 109 cm, or else the X-ray luminosity would be largely theabsorptionwingsofthese“PCygprofiles”inthisUSco super-Eddington. Shortly after the outburst, velocities up spectrum produced in the nova shell, instead of being due to 5000 km s−1 were measured by Yamanaka et al. (2010), to the WD? At least one profile cannot be a P Cyg in the and on day 35 (the day of the last XMM-Newton observa- conventional sense, and it is the one of the N VI He γ line. tion), Mason et al. (2012) estimated an expansion velocity The absorption in this case is not the same line, but it is of 4000 km s−1 from thefullwidth at half maximum of the mainly of another overlapping line at zero velocity, due to opticallines.Assumingthisvaluefortheexpansionvelocity, X-ray spectra of U Scorpii 7 0 0 Magnitude difference 0.5 Magnitude difference 0.5 1 1 1.5 1.5 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Orbital phase Orbital phase 1.5 1.5 1 1 2.5*Log(EPIC-pn count rate/9.21) 0. 50 2.5*Log(EPIC-pn count rate/11.0) 0. 50 -0.5 -0.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Orbital phase Orbital phase Figure6.TheaverageopticalVmagnitudelightcurveofUScoduringdays21-26(lefttoppanel)anddays32-41aftervisualmaximum (right top panel; Schaefer et al. 2011), compared with the observed X-ray lightcurve measured with the EPIC-pn instrument aboard XMM-Newton on days 23 (left bottom panel) and on day 35 (right bottom panel). The logarithm of the count rate normalized to the minimum value of the measurements and multiplied by 2.5 is plotted as a function of phase, to match the magnitude scale. The light curvewasbinnedintimewith50sbinsbeforeconversiontophase,onday23(right)and35(left)aftertheoutburst. the nebular ejecta around U Sco at the time of the Chan- et al. 2008). Since the emission lines were strong in a very dra observation (day 18) had reached a distance from the different range, therewas no overlap with WD lines and no WD representing almost 300,000 times the WD radius of a apparent P-Cyg profile. We conclude that the P Cyg pro- very massive WD (1.3 M⊙). The region in which emission files in the Chandra spectrum were not “real P Cyg”, but occurred was overwhelmingly large in comparison with the theywereratherduetosuperimposedabsorptionandemis- cylinder around the line of sight towards the WD. We con- sion arising in different regions and different mechanisms. clude that the absorption features observed with Chandra Theabsorptionfeatureswereproducedintheoutmostlayer cannothavebeenproducedintheejecta.Byday18,anyab- of the WD atmosphere, which was still loosing mass, and sorption features oftheejecta would already behiddenand reflected at a distance of a few solar radii by the Thom- embeddedin thebroad emission lines. There isno evidence son scattering corona of the WD. The corona’s “reflection” ofnebularX-raysabsorptionlinesintheejecta,butinother allowed the lines to be detectable, despite the high inclina- novae absorption lines were always observed, and they are tionangleoftheWDinUSco.Sincethiscoronamusthave usually attributed to the WD atmosphere. There is only a already been present on day 23, we assume that it existed recent case in which the blue-shifted lines were attributed alreadyonday18(evenifwecouldnotmeasurealightcurve insteadtocollisional ionization inathinoutershell,thatof to proveit). V2491Cyg(Pintoetal.2012),buttheabsorptionlinesofU An intrinsic blue-shift that lasted while the WD was Sco are so broad compared to this model, that we can rule still loosing material in a wind, explains why the features out this origin. werestillobservableonday18,eveniftheemissionfeatures are so broadened in U Sco that they should hide the atmo- We concluded that the velocity of the absorption fea- sphericabsorption.OncethewindfromtheWDceased,the turesalsoforUScowasduetoadifferentphenomenon,one absorption lines velocity became close to zero, so they were causinganintrinsicvelocitythathadanalmostabruptend, embedded in the center of the emission lines and were not so that the features were still significantly blue shifted on measurable any longer. day 18, but not any more a few days later. This process must be mass loss from the nova. Absorption lines in X- Whatever absorption features may have actually been ray grating spectra of other novae in the early phases were producedintheejectabyphoto-ionization,iftherewassuch significantly blue-shifted, with comparable blue shift to the aphenomenon,wereembeddedintheemissionfeatureslong apparent“absorption wing”ofthe“PCyg”intheChandra beforeday18,duetothesmalldimensionsoftheabsorption spectrum of U Sco (e.g. Ness et al. 2003 and Rauch et al. regioncomparedtothefastejecta.Itisalsoconceivablethat 2010forV4743Sgr,Nessetal.2011bforV2491Cyg).InRS the Thomson scattering corona changed in geometry and Ophmanyofthesameabsorptionlinesasintheformertwo the WD absorption lines may have become more smeared novae were observed without measurable blue-shift (Nelson out between day 18 and 23, and perhaps better “hidden”. 8 M. Orio et al. 0.2 Although the Chandra spectrum absorption features were eroded by the blue wing of the partially overlap- ping emission lines, we note that they were still remark- ably close in depth and broadening to those observed in 0.15 other novae without X-ray eclipses, such as V4743 Sgr med counts (WReauacdhopettedal,th2e01a0tm) oasnpdheRricS mOopdhels(Ndeelsvoenlopeetdabl.y2c0o0a8u)-. m 0.1 thor Thomas Rauch, publicly available on the web site ETG +-1 su hvettrpy:/h/oatstwroh.iutnei-dtuweabrifns,geen.g.d.eR/∼SrOaupchh (Nanedlsondeevteloapl.ed200f8o)r L and V4743 Sgr (Rauch et al. 2010). We fitted the spectra 0.05 with XSPEC, v12.6, assuming that the absorption features wereblue-shifted by2000 km s−1 (as in Rauchet al.2010). To date, Rauch’s models represent the best approximation 0 to the atmosphere of a hydrogen burning white dwarf, al- -10000 -5000 0 5000 10000 though residual outflows that produce blue-shifted absorp- velocity in km/s 0.18 tion are not accounted for in these static models. Models including elements with atomic number up to the one of 0.16 oxygen are available for a grid of values of effective grav- ity, with a log(g) grid from 5 to 9 with 1 step increments. 0.14 The most recent grid of models, including all elements up mmed counts 0 0.1.12 twhooittmhcaotgnenmtienpsuieuurmamt,uowrfeastshacebaUolcvuSelca5ot0es0dp,0ef0co0trratKh.eSainhndoctetiseasbstuovwiteha5bit0lee0,df0ow0r0atrKhfse,, G +-1 su 0.08 tchomeWe lDarmgeulystsbuepveer-rEydcdominpgtaocnt.wanitdhtlhoeg(lugm)<in9o,sairteywpuobullidshbeed- LET 0.06 only for log(g)=9. 0.04 Model 201 in Rauch’s web page was developed specif- ically with similar abundances to the ejecta of U Sco mea- 0.02 sured using the optical spectra in the previous outbursts: 0 depletedhydrogen([He]=0.489oraheliummassfractionof -10000 -5000 0 5000 10000 70%, and almost 50 times enhanced nitrogen with respect velocity in km/s tothesolarvalue([N]=1.668).Wefittedmodelsofthisgrid Figure 7. The P Cyg profiles of the Ne VII Hydrogen α line with XSPEC, after blue-shifting the absorption features by (above),andNVIresonanceline(below),intheChandraLETG spectrum (±1 orders summed) plotted in velocity space. The fit 2000kms−1.Wedonotobtainthebestfitwithmodel201, withanatmosphericmodel(seeTable2)withabsorptionfeatures butratherwithmodel003,with[He]=0.382and[N]=1.803. blue-shiftedby2000kms−1isshownbythedottedline.Thetwo Thisisnot surprising,becausetheabundancesintheburn- weaker lines of the triplet appear on the right of the resonance ing layer can be quite different from those of the ejecta, line of N VI in emission. There is a contribution of N VI He β where mixing with unburnedmaterial has occurred. lineinemissionat24.889˚A(seeTable1). The final product in our fit, shown in Fig. 2 for the Chandra LETG, is a composite model of WD atmosphere andsuperimposedemission lines,butfortheChandraspec- In any case the transition giving rise to absorption lines in trumwe started byperforming an experiment,ignoring the theatmosphere, evenwhen not observableand measurable, channels containing the strong emission lines (24.8–25.6 ˚A must haveoccurred until theWD turned off. for N VII Ly α, and 28.7–30.0 ˚A for the N VI triplet), so thatthemodelfitwasbetterconstrained bythecontinuum and bytheabsorption features. For thecomposite fitsplot- 4.2 Atmospheric models: the hottest WD ted in blue in Figures 2, 4 and 5 we binned the data with Becauseoftheconclusionsabove,inordertofittheChandra 30 to 50 counts per bin, but for the initial experiment, in spectrum,weassumedthattheobservedabsorptioncompo- order to retain the high spectral resolution of the LETG, nents were not produced by photo-ionization of the ejecta we binned the data only by a factor of 4 resulting in wave- likeintheopticalPCygprofilesofnovae,butthattheyorigi- lengthbinsof0.01˚A.Withalowresultingnumberofcounts natedinsidetheWDatmosphere,andwerereflectedatlarge ineachbin,weusedtheC-statistic(Cash,1979)ratherthan distance(>3R⊙)bytheThomsonscattering.Wefittedonly themore commonly used χ2 statistic. Wefound that the C the continuum of the XMM-Newton spectrum. We assume parameter is smallest for the smallest C/N ratio (typical thatonday23and25theatmosphericabsorptionlineswere of CNO ashes), and steadily increases as the C/N ratio in- simply unobservable, hidden by the unusually broad emis- creases.Thedecreaseofthe“C”parameterwithC/Nratiois sionlinesofthisnovaoncethemassoutflow,andwithitthe about20%frommodel011tomodel003ofRauch’sgrid.All blue shift, ceased. Of course in the Chandra spectrum the models with solar abundances yield poor fits to the Chan- observed absorption may have been more pronounced if it dra LETG spectrum. The temperature for the “truncated- was not super-imposed on emission, so we can only obtain emission” spectrum in the 2σ confidence range (± 2σ) is lower limits on theabundances and effective gravity. 650,000-750,000 K. The best fit yielded a temperature of X-ray spectra of U Scorpii 9 0.8 We note that after the previous outburst in 1999, Ka- habkaetal.(1999),fittedtheBeppoSAXlowresolution,low 0.7 S/N LECS spectrum with a WD atmospheric model. They derived aluminosity in therange 2.7 to 18 × 1037 ×(d2/12 0.6 kpc) 19-20 days after the optical maximum, and an effec- tive temperature of 75 eV (870,000 K). The count rate was 0.5 however only 0.06 cts s−1, a value that appears too low by S cts/s 0.4 a factor of about 2 if translated into both the Chandra or RG XMM Newton count rates with PIMMS for a blackbody at 0.3 75 eV and N(H)=2.5 ×1021 cm−2. The BeppoSAX obser- vation lasted for about 14 hours, so the low count rate is 0.2 not explained with orbital variability. We cannot reconcile it with thelower luminosity ofthepresent WDcomponent. 0.1 Probably the atmospheric model had significant differences from themost recent anddetailed models ofRauch,butwe 0 -10000 -5000 0 5000 10000 alsocannotruleoutthatthefitwascorrect,andtheThom- velocity in km/s 0.25 son scattering corona may have been very different, or not present,in theprevious outburst. 0.2 5 THE ORIGIN OF THE EMISSION LINES: SHOCKS, CONDENSATIONS AND AN GS2 counts/s 0.15 In thEeVCOhLaVndIrNaGspeVctIrOuLmE2N0%ToMf tEhDeIflUuxMwas in the emis- R 0.1 sion features, and in the RGS spectra about 30 %, while theremainingfluxwasduetothecontinuum.Despitesmall uncertainties in therelative calibration of theChandra and 0.05 XMM-Newton instruments, as we see in Fig. 3, the spec- traareremarkablysimilarondays18and23.However,two 0 striking changes happened: theabsorption features seem to -10000 -5000 0 5000 10000 havedisappearedexceptforOIat23.476˚A,whichweiden- velocity in km/s tifiedasinterstellarinorigin,andoxygenemissionlinessud- Figure8.TheNVIILymannαline(above)andNVIresonance denlyappeared.TheOVIIIline,whichisnotmeasurablein line(below)measuredwiththeRGS2gratinginFebruary(dots) andMarch(crosses),respectively,plottedinthevelocityspace. absorption in the WD atmosphere on day 18 but is clearly detected in emission with XMM-Newton on day23, was al- mostsymmetricaroundzerovelocitysinceitappeared,con- 730,000 K, N(H)=2.5 × 1021 cm−2, and an absorbed flux sistentlywithnoerosionbyanearbyblue-shiftedabsorption 1.64×10−11 ergcm−2 s−1,correspondingtoanunabsorbed line. This does not imply that the line did not exist at all flux4.2×10−10 ergcm−2 s−1.Thebolometricluminosityof intheWDatmosphere,becausetheleveloftheWDcontin- theWD in this model is 7.2 × 1036×(d/12 kpc)2. uum in that energy range is too low to measure absorption We fitted also the spectra observed later with XMM- (see Rauch et al., 2010) Newton,althoughtherearenoabsorptionfeaturestobetter Inemission,thefullwidthathalfmaximumofthisline constrainthetemperaturerange.Thefitreturnsalmost the isapproximately4000kms−1,consistentlywiththevelocity same temperature for day 18 and 23. In the spectrum of measured in the optical spectra of Mason et al. (2012). As day 35 the fit yields instead a higher temperature, in the can be seen comparing Fig. 7 with Fig. 8, the emission fea- range900,000 to1,100,000 K,andis unvariedif weadd the turesin thethirdspectrum appear symmetrical around the emission component, discussed in Section 5. The WD tem- rest wavelength of the line, and have an almost triangular peratureatmaximumX-rayluminosityistightlycorrelated shape,indicatingthatlinesareproducedinanopticallythin withWDmass.Weknowfromthemodelsthattheeffective expandingmedium.Incontrast, theprofiles oftheemission temperature exceeds 900,000 K only for WD mass around linesintheopticalspectraofnovaeafterthePCygprofiles 1.3 M⊙ (Yaron et al. 2006). havedisappearedusuallyappearflat-topped,indicatingthat The models yield an absorbed WD luminosity around theexpandingmedium is optically thick. 7×1036,whichat12kpcdistanceisafactorof5-10smaller We measured the emission line fluxes by fitting the thantheluminositysofarmeasuredinotherWDwitheven strongest lines with Gaussian profiles, and we attempted lower peak temperature (e.g. Balman et al. 1998, Orio et a fit to a few weak lines with triangles. The photon flux is al. 2003a, Nelson et al. 2008). For theothernovae,theWD given for each line in the three spectra in Table 1. We esti- luminosity at peak was several times 1037 erg s−1. We ex- matethatourmeasurementsareaccuratetowithin10%for plain also this puzzle, liketheone posed bytheapparent P isolatedlinesandfluxaboveapproximately0.4×10−4 pho- Cygprofiles,ifweassumeUScowedidnotobservetheWD tons cm−2 ks−1 (which were fitted with a Gaussian). The directly.ThereprocessingfactorfortheWDradiationmust accuracy is probably not more than ≃25% below thisvalue havebeen of theorder of 10%. (where we fitted a few lines with a triangle, especially for 10 M. Orio et al. theday35 spectrum). Wedid notattempt tomeasure line with the Table 2 models for days 18 and 23. However, the fluxes below 0.12 photons cm−2 ks−1, because the uncer- BVAPEC collisional ionization model in XSPEC assumes a tainty seems very large below this value, but in some cases low density limit and does not predict that the forbidden we can still identify lines with probably almost an order of lineisquenchedcomparedtotheintercombination line. We magnitude lower flux (e.g. Mg XI in the last spectrum). seeinFigures2and4thattheNVIλ29.5446forbiddenline Theemission lines, muchlessstronginthiswavelength iitheNVItripletondays18and23,andtheOVIIλ22.097 range in other novae, in the U Sco spectra are very pro- line at day 23 are less strong than in the models shown in nouncedandbroad(comparedwiththermalandinstrumen- theFigures. tal broadening, see e.g. Brinkman et al. 2000, Ness et al. We attribute this phenomenon to line formation in a 2003). We attribute the line broadening to the expansion zone of such high density that the collisional de-excitation velocityofthenovashell,likethebroadeningoftenobserved rates are close to the radiative decay rates. When the two intheopticalspectra.TherecurrentnovaeLMC2009(Orio rates are comparable, the radiative decay fraction from et al. 2011) and T Pyx (Tofflemire et al. 2012) have been an excited level are reduced and so the emission becomes the only other ones with broad emission features. We also weaker. The electron density ne at which the N VII forbid- note that the carbon lines are much weaker than those of den line is de-excited is a few times 109 cm−3 (Ness et al. nitrogen.OnlytheCVILyαlineinemissionat33.90±0.12 2001, Chen et al. 2004). This density is not unheard of for ˚A (rest wavelength 33.7342 ˚A) is clearly detected. novae shells in the first few weeks after the outburst. ne of order of 1012 cm−3 was derived by Neff et al. (1978) from 5.1 A comprehensive spectral fit: days 18 and 23 theanalysis of the optical spectra of V1500 Cyg. Thishighdensityishigherthanthevaluederivedfrom In Fig. 2, 4 and 5 we plotted in red the fit to the lines and continuumobtainedwithphysicalmodels.Thesemodelscan the emission measure in the fit in Table 2, ne = a few 107 with the assumptions of the model, if the ejected shell vol- beregardedasasortoffirstorderapproximationtothecor- ume is uniformly filled. Since estimates of the ejecta mass rect model, to explore the relevant physical mechanisms in thesespectra.Wehavenotreachedaperfectfit,asthevalue of U Sco converge vary from a few 10−6 M⊙ to a few 10−7 of the reduced χ2 in these fits varies from 2.7 (Fig. 2) to M⊙ (seeSchaefer2011,Starrfieldetal.1988),assuminguni- 4 (Fig. 5). Rather than obtaining a statistically significant form filling of the shell, we expect that ne does not exceed a few times 105 at day 18, and it should be decreasing as fit, we focused on a good qualitative agreement and on re- theejectaexpand.Thisdiscrepancyimpliesanemittingvol- producing the emission line ratios as well as possible. We ume of the order of only 10−4 the volume of the shell. The think that, as novae observations are continued in the next explanation seems to be that line emission occurs in dense few years, we should be able to gather many more details condensations. We note that the large clumps that explain of the nova physics, that will allow to fine tune these mod- the light curve ≃3 hours period in the Ness et al. (2012) els of the very complex phenomena occurring in novae WD model occupy a much smaller fraction of the shell volume atmospheres and in their ejected shells. corresponding to only a few 10−9. Clumpiness is not com- Ondays18and23,therelativelinestrength,combined pletelyunexpected,andithasoftenbeeninvokedtoexplain with the lack of a broad radiative recombination RRC fea- ture of N VI at 22.5 ˚A, which should be clearly present novae optical spectra, starting with the pioneering work of Gallagher&Anderson(1976).Interestingly,alreadyin1992 in the case of photoionization (e.g. X-ray spectra of plan- Lloyd et al. discussed how the X-ray emission from a nova etary nebulae or some AGN, see Fig. 5 of Kinkhabwala et shell, albeit observed only with a broad band instrument al.,2002),indicatethatthetransitionsareduetocollisional (ROSAT),impliescompactclumpsintheejectaasemitting ionization. We used the XSPEC package, v12.6, adding to regions. theatmosphericmodelofthepreviousSectionandacompo- nent of collisional ionization, namely the BVAPEC model, Inourspectralfits,whoseparametersaregiveninTable thatallowstoincludevelocitybroadening (1σ value,which 2,theappearanceofoxygenlineson day23dependson the correspondstothehalfwidthathalfmaximumofthelines). interplay of plasma temperature and emission measure. In- Table 2gives relevant parameters of thespectral fitsshown creasing theplasma temperaturebyonly ≃30%,or increas- intheFigures.Weassumedthatheliumintheplasmacom- ing the emitting volume by a factor of a few (assuming for ponent is enhanced 10 times with respect to solar values. instance that additional mass is being ejected) the oxygen In order to improve the fit to the emission lines, we need a lines are predicted, but then it becomes difficult to repro- large overabundance of nitrogen not only in the also in the duce the other lines, correctly modeled only with a plasma BVAPEC component,at least 10times thesolar value,and temperatureinthe100-120eVrange.ThestrengthoftheO even up to 70 times like in thefit we show in Fig. 2 for the VIII Lyα feature is much stronger than all the lines of the Chandraspectrum(thisisthebestfitwithonlytwocompo- O VII He-like triplet, indicating an overlapping component nents, although we obtained sightly worse, but similar fits at higher temperature. In conclusion, in order to reproduce with anenhancementbyafactor of10andaslightly differ- both the soft and hard portion of the spectrum, we need ent set of parameters). We note that the WD unabsorbed atleasttwocomponentsatdifferenttemperatures.Anaddi- luminosityresultingfromthesefitsisreducedbyupto30% tionalcomponentat180eVreproducestherelativestrength with respect tothevaluescontained inthe“experiment”in oftheoxygenfeatures.Thecomponentthatexplainstheni- Section 4.1, in which we fitted only the continuum, cutting trogen lines, at 120-130 eV, fails instead to reproduce the theprominent emission features. “harder”lines.Itisthuslikelythatonday23thecondensa- Figures2and4showthatwecanreproducetherelative tionsinwhichemission linesoriginatedwerenotatuniform strengthofmostlinesbyassumingcollisionalionizationlines temperature.

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