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XMM-Newton and Chandra observations of the ultra-compact binary RX J1914+24 PDF

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Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed5February2008 (MNLATEXstylefilev1.4) XMM-Newton Chandra and observations of the ultra-compact binary RX J1914+24 Gavin Ramsay1, Mark Cropper1, Pasi Hakala2,3 1Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH56NT, UK 2Observatory,University of Helsinki, PO Box 14, FIN-00014 University of Helsinki, Finland 3Tuorla Observatory, V¨ais¨al¨antie 20, 21500 Piikki¨o, Finland 6 0 5February2008 0 2 n ABSTRACT a J ThenatureoftheX-raysourceRXJ1914+24hasbeenthesubjectofmuchdebate. 5 It shows a prominent period of 569 sec in X-rays and the optical/infra-red: in most modelsthis hasbeeninterpretedasthe binaryorbitalperiod.We presentouranalysis 1 of new XMM-Newton and Chandra data. We find a longer term trend in the XMM- v 4 Newton data and power at 556 and 585 sec in 5 sets of data. It is not clear if they 0 are produced as a result of a beat between a longer intrinsic period and the 569 1 sec modulation or if they are due to secular variations. We obtain a good fit to the 1 XMM-Newton spectrum with a low temperature thermal plasma model with an edge 0 at 0.83keV. This model implies an unabsorbed bolometric X-ray luminosity of 1× 6 1033 ergs s−1(for a distance of 1kpc) - this is 2 orders of magnitude lower than our 0 previous estimate (derived using a different model). If the distance is much less, as h/ the absorptionderivedfrom the X-rayfits suggest,then it is even lowerat ∼3×1031 p ergs s−1. - o Key words: Stars:individual:– RXJ1914+24– Stars:binaries– Stars:cataclysmic r variables – X-rays: stars t s a : v i X 1 INTRODUCTION the UI model (Marsh & Nelemans 2005). This conclusion is based on the X-ray luminosity determined using ROSAT r a The natureof theX-ray source RX J1914+24 has been the (Cropperetal1998)andXMM-Newtonobservations(Ram- subject of much debate in recent years. Only one modula- sayetal2005a). TheROSATdatawerewellfittedusingan tion period (569 sec) has been seen in both its X-ray and absorbed blackbody model while the higher signal to noise opticallightcurves(Ramsayetal2000).Forthis,andother data of XMM-Newton found a poor fit using this model. reasons, the period of 569 sec has been taken to be the bi- Ramsayetal(2005a)foundthebestfittothedatacouldbe nary orbital period, which would make it the second most achievedusinganabsorbedblackbodyplusGaussianmodel, compact binary system known. althoughtheydidnotexpectthatthismodelwasphysically realistic. Themodelswhichhavebeenputforwardtoexplainthe observational characteristics of RX J1914+24 fall into two broad categories: accretion and non-accretion driven mod- SincethosefirstXMM-Newtonobservationsweremade els. The fact that the optical spectrum of RX J1914+24 in Nov 2003, a further two sets of observations made in (also known as V407 Vul) shows no emission lines, puts a Sept/Oct 2004 have become publically available. These ob- question mark over the accretion driven models. The non- servationswerelongerthantheinitialXMM-Newtonobser- accreting model is the unipolar-inductor (UI) model of Wu vationsandtheparticle backgroundwaslower. Inaddition, etal(2002).Whilethismodelhasbeenabletoexplainmany afurthersetofX-rayobservationsofRXJ1914+24madeus- oftheobservedcharacteristicsofthissource,itappearsthat ingChandrahasbecomepublicallyavailable.Wehavethere- its X-ray luminosity implies an induction torque which is fore extracted these data from the archive and use them to much greater than the amount generated by gravitational make a more detailed investigation of its X-ray spectrum, radiation.Thereissomeevidencethattheobservedspin-up to determine if the 569 sec is continuing to spin-up at the rate of RX J1914+24 (Strohmayer 2002, 2004b, Ramsay et same rate and tosearch for evidencefor periods other than al 2005a) is therefore much smaller than that predicted by the569 sec period. (cid:13)c 0000RAS 2 XMM StartDate Duration MeanEPIC Orbit (ksec) pn(Ct/s) 0880 2004-09-28 17.0 1.08 0882 2004-10-02 19.0 1.16 Chandra StartDate Duration MeanChandra ObsId (ksec) Ct/s 4519 2004-04-05 15.4 0.44 4520 2004-06-13 14.7 0.37 4521 2004-09-05 15.2 0.34 Table 1. The log of the observations of RX J1914+24 reported here. Each observation was continuous with no data gaps. Top panel: The XMM-Newton observations of RX J1914+24 made in 2004. The earlier observations are detailed in Ramsay et al (2005a). Bottom panel: The Chandra made in 2004. The earlier Figure 1. The phase of the sharp rise to maximum X-ray flux observations aredetailedinStrohmayer (2004b). determinedfromROSAT,ChandraandXMM-Newtondata(see Ramsay et al 2005a for details of these observations). The more recentpointreferstothenewXMM-Newtondatapresentedhere, with the preceding point refers to Chandra data taken in April 2 OBSERVATIONS AND DATA REDUCTION 2004. 2.1 XMM-Newton observations RX J1914+24 was observed twice using XMM-Newton in P˙ =−3.31±0.09×10−12 s/s(theerrorbeingdeterminedus- Nov 2003. The details of these observations has been re- ingabootstrap method),whichisconsistentwith thevalue ported by Ramsay et al (2005a). It was observed again late we determined previously. in2004andweshowinTable1thestarttime,durationand mean X-ray count rate of the second epoch XMM-Newton observations.Inthispaperwediscussthedatacollected us- 4 SEARCHING FOR PERIODS OTHER THAN ingtheEPICX-raydetectors(pnandMOS)whicharesen- THE 569 SEC PERIOD sitive over the energy band 0.15-10keV. We extracted the data from the public archive and processed the data using Strohmayer (2004b) detected sideband structure in the theXMM-NewtonSASv6.5inasimilarmannertothatde- power spectrum of RX J1914+24 in observations made us- scribed in Ramsay et al (2005a). Data were barycentrically ingChandrabutconcludedthatthiswaslikelytohavebeen corrected and in unitsof Terrestrial Time (TT). caused by the dithering pattern of those observations. On the other hand, longer term variations have been seen in X-raydata (Ramsay et al 2000, Strohmayer2004b). 2.2 Chandra observations We have searched for evidence of periods other than the 569 sec period in these new X-ray data. We performed Observations of RX J1914+24 made using Chandra on 5 a Discrete Fourier Transform (DFT) on the combined data occasions were reported byStrohmayer(2004b). Sincethen takeninXMM-Newtonorbits0718and0721andalsoorbits another3observationshaveenteredthearchive.Allofthese 0880 and 0882 as well as each of theChandra observations. observationsweremadeincontinuousclockingmode:inthis Asexpected,the569secperiodanditsharmonicswereseen mode a one-dimensional image of the sky is produced. We stronglyinalltheamplitudespectra.Topre-whitenthedata haveextractedalloftheavailabledatafromthearchiveand we used two methods: we pre-whitened in the usual man- corrected and analysed them in a similar manner to that ner using a sinusoid at predicted period at the observation carried out byStrohmayer (2004b). epoch (based on the initial period of Strohmayer (2004b) and the spin-up rate reported here). We also pre-whitened the data by subtracting the mean X-ray pulse profile ap- propriatetoeachindividualobservation.Thelatteris more 3 DETERMINING THE RATE OF SPIN-UP appropriate since the X-ray pulse profile is non-sinusoidal. Ramsay et al (2005a) folded all available X-ray data As expected most of the peaks are removed after this pro- (ROSAT, ASCA, Chandra and XMM-Newton) on the best cedure. However, in the second epoch set of XMM-Newton fit initial period of Strohmayer (2004b) (ie without the pe- data(0880 and0882), side-bandstructureisclearly present riod derivative) and found that the sharp rise of the X-ray at 584.1 sec and 552.8 sec in the pre-whitened spectrum flux arrives earlier in phase over time. A best fit to the (Figure 2). They are located in thewings of the main peak data showed that RX J1914+24 is spinning up at a rate intheoriginalamplitudespectrumandhaveanamplitudeof of 3.2×10−12 s/s, which was similar to that reported by ∼1/10 of the main peak. In the earlier XMM-Newton data Strohmayer(2002,2004b).Wedidthesameanalysisasdone (0718 and 0721) there is only weak evidence of these peri- in Ramsay et al (2005a) and show the phase of the rise in ods. In the Chandra dataset of 2004 June 13 we also find X-rayfluxfortheChandradatatakeninApril2004andthe strongevidenceforperiodsverysimilartothoseseeninthe new XMM-Newton data in Figure 1. We find a best fit of XMM-Newton data (Figure 2).In two others, (2004 Jan 31 (cid:13)c 0000RAS,MNRAS000,000–000 3 584.1" 552.8" XMM-Newton 2004-09-28+ e 2004-10-02 d u plit m A 584.1" 55 2.8" Chandra 2004-01-31 e d u plit m A Chandra 584.1" 552.8" Figure 3. The XMM-Newton EPIC pn spectra from the data 2004-06-13 takenin2004.Thesolidlineisthebestfittothemodelconsisting de ofanabsorbedthermalplasmamodelwithanedge. u plit m A 5 SPECTRA 584.1" 552.8" Chandra Ramsay et al (2005a) showed the X-ray spectrum of RX 2004-09-05 J1914+24 was poorly modelled using an absorbed black- de body model. Their best fit was obtained using an absorbed u plit blackbody plus a broad Gaussian line near 0.6keV. This fit m A was still poor (χ2ν=1.63) and the model was unlikely to be physically realistic. WiththebetterqualitydataaffordedbythenewXMM- 0 0.002 0.004 Newton data we have re-visited the X-ray spectrum of RX Freq (1/sec) J1914+24. WefittedtheEPIC-pndatafromXMM-Newton orbits0880and0882separately.Giventheunphysicalnature Figure 2. The pre-whitened amplitude spectra of 5 sets of X- ofthepreviousfitsandworkingfromthesatisfactory fitsto raydata. Theyhavebeenpre-whitenedbysubtractingthemean our X-ray observations of other compact binaries (the AM X-rayprofileappropriatetoeachepoch. CVnstars,Ramsayetal2005b) wetried fittingthespectra with an absorbed thermal plasma model with variable ele- mentalabundance(vmekalinXSPEC).Thismodelstillgave apoorfittothedata(χ2ν=4.6 forthe0880 data).However, and 2004 Sept 5) there are peaks in the amplitude spectra when we added an edge to the model we obtained a good which correspond to these periods, although they are only fit to the 0880 data (χ2ν=0.97, 122 dof; Figure 3). The fit marginally significant compared tootherpeaks in thespec- to the 0882 data using the same model was slightly poorer tra. There is no evidence for these periods in the longest (χ2ν=1.21, 141 dof). Elements which were required in the Chandra dataset which lasted 35 ksec. We also determined fit include N, O, Ne, Ca, Fe for the 0880 data, with some howtheX-raypulseprofilechanged overtheindividualob- evidence for Na in the 0882 data. The edge is located at servation duration.We findthat themean profile brightens 0.83keVwhichisconsistentwithitbeingduetoOVIII.The over the duration of both the 0880 and the 0882 observa- absorption derived from the fits is ∼ 1.2×1021 cm−2. We tions. note we cannot obtain a good fit with this model when we We believe that 553 and 584 sec periods could result fixtheabsorptionat8.3×1021 cm−2,thelargervaluedeter- when a modulation signal (ie the 569 sec period) is mixed minedfrom opticalinterstellarabsorption features(Steeghs withanotherlongertermvariation(eitheraperiodicornon- et al 2006). periodic modulation). Ifit was a periodic modulation, then The observed flux in the 0.1-10keV energy band itsperiodwouldbethedifferencebetweenthe569secperiod (1.30×10−12 ergs s−1 cm−2for the orbit 0880 data and and the 553 and 584 sec periods: just over 6 hrs. Alterna- 1.48×10−12 ergs s−1 cm−2for the orbit 0882 data) is very tively if a modulation signal is mixed with either a 553 sec similartothatfoundusingtheblackbodyplusGaussianline or584secperiodthenthiswouldgiverisetothelongerterm ofRamsayetal(2005a).However,theunabsorbedbolomet- trend in the data. The question remains if this longer term ricfluxinthefitpresentedhere(1.14×10−11 ergss−1 cm−2; trend is periodic or due to the secular variability that have 0880 and 1.24×10−11 ergs s−1 cm−2; 0882) is substantially alreadybeenobservedinthissystem:wediscussthisfurther reducedcomparedtotheearlierresults(3.2×10−10 ergss−1 in §6.1. cm−2) where a blackbody spectrum was used. (cid:13)c 0000RAS,MNRAS000,000–000 4 6 DISCUSSION 33..00 6.1 The periods in RX J1914+24 Chandra Until now there has been no convincing evidence for any 22..55 XMM-Newton otherperiodsinRXJ1914+24apartfromthe569secperiod. In the case of 5 X-ray observations we have found evidence for power at 552 and 584 sec. This changes our perception V) 22..00 WXP ke XP ofRXJ1914+24. Thedataareclear-theX-rayfluxclearly 0 increasesintheXMM-Newtondatatakeninbothorbit0880 4-1. 11..55 WXPXP 0. WXP andinorbit 0882. Thiscouldbeconsideredtobetheresult x ( u XP of an underlying ∼6 hour period, or it could be the result Fl 11..00 WXP ofsecularvariabilityknownalreadyinthissystematlonger timescales (Ramsay et al 2000, Strohmayer 2004b). Both 00..55 could producetheobserved amplitudespectrum. To explore this question, we have obtained the mean 00..00 X-ray flux in each of the Chandra and XMM-Newton ob- 22660000 22770000 22880000 22990000 33000000 33110000 33220000 33330000 servations. Since the Chandra ACIS-I detector has a low HJD - 2450000 effective area at energies less than ∼0.4keV, we determined Figure4.Theobservedfluxinthe0.4-1keVenergyband(unitsof the observed mean flux in the 0.4-1.0keV energy band. We 10−12ergss−1cm−2)determinedinChandraandXMM-Newton showhowthefluxinthisbandvariesovertimeinFigure4. observations. ‘XP’ denotes the presence of periods at 552 and Wealsoshowthoseepochsatwhichwedetectedthe552sec 583 sec: in the case of the most recent Chandra observation the and 584 sec periods which could be signatures of a longer significanceismarginal-‘WXP’weakX-rayperiods. periodtrendintheobservation.Althoughnotconclusive,it appears that at the higher flux levels there is no (or weak) evidence of these other periods, so that shorter (<1 day) timescale variations may take place only when the system the new model is an optically thin plasma, a correction ef- is emitting at intermediate or low levels. The detection of fect for projection is now no longer applicable. Moreover, power at 552 and 584 sec does not appear to be related absorption derived from the X-ray fits is ∼15% of that de- to theobservation length since thelongest observation (the rived from the optical data of Steeghs et al (2006) based Chandra observation of Feb 2003 which lasted 35 ksec) did on a G9 spectral type. If the G star is not the secondary, not show obviouspower at theseperiods. thenwecan reducethedistancebyasimilar factor and the Atthisstage, wecannot distinguish betweentheX-ray luminosityistherefore 3×1031 ergss−1.Inthiscase, theX- longterm behaviourbeingduetorandom changesin either ray luminosity of RX J1914+24 is more than ∼3 orders of the accretion rate (in the case of the accreting models) or magnitudeless than that found by Ramsay et al (2005a). theelectricalcurrentdissipation(inthecaseoftheunipolar- In the unipolar-inductor model of Wu et al (2002) the inductormodel).Weurgethesearchforradialvelocityvari- amount of power which is emitted is determined by vari- ations on a timescale of less than 1 day,or dedicated X-ray ous factors, with the degree of asynchronisity being one of and/oropticalobservationsofthissystemtosearchforvari- the key determinants (along with component masses and ations on this period. magnetic field strength). However, to produce theobserved IfthelongertermvariationintheX-rayfluxisperiodic, luminosity,theasynchronisitywouldhavetobe0.001orless thenweshowedearlier thatitsperiod wouldapproximately (Wuetal2002).ThisreducestheobjectionstotheUImodel be 6 hrs. Smith & Dhillon (1998) show that for a binary onenergeticgroundsraisedbyMarsh&Nelemans(2005)to systemwiththisorbitalperiodthenitsspectraltypewould a significant degree. beexpectedtobeamidK-type,althoughtherangeisupto5 The abundances are clearly non-solar, which is consis- sub-classes.WenotewithinterestthediscoveryofSteeghset tent for an evolved secondary as is seen in the fits to spec- al(2006)offeaturesintheopticalspectrumofRXJ1914+24 tra of AM CVn systems (eg Marsh, Horne & Rosen 1991, whichappeartoresembleaG9/K0spectraltype.Therefore, Strohmayer 2004a, Ramsay et al 2005b). If this model re- an orbital period of 6 hrs would be marginally consistent flects the properties of the material leaving the secondary with this. star then it rules out the G9/K0 star seen in the optical spectrum being thesecondary star. 6.2 The X-ray spectrum of RX J1914+24 WehaveshownthatthederivedluminosityofRXJ1914+24 is highly model dependent. Further, the distance to RX J1914+24 is still rather uncertain, but Steeghs et al (2006) 7 ACKNOWLEDGMENTS suggest a distance of 1kpc based on its optical spectrum. Foradistanceof1kpcthemeanbolometricluminosityfrom This is work based on observations obtained with XMM- the XMM-Newton orbits 0880 and 0882 data is therefore Newton, an ESA science mission with instruments and 1.4×1033ergss−1-anorderofmagnitudelowerthanthepre- contributions directly funded by ESA Member States and viousluminosityof3.8×1034 ergss−1.Ramsayetal(2005a) the USA (NASA). The XMM-Newton data were extracted wentfurtherandappliedacorrection toaccountforprojec- from the archive at Vilspa and the Chandra data from the tionaffectsgivingaluminosityof∼1×1035 ergss−1.Since HEASARCarchiveat Goddard Space Flight Center. (cid:13)c 0000RAS,MNRAS000,000–000 5 REFERENCES Cropper, M., Harrop-Allin, M. K., Mason, K. O., Mittaz, J. P. D.,Potter,S.B.,Ramsay,G.,1998,MNRAS,293,L57 Marsh,T.R.,Nelemans,G.,2005,MNRAS,363,581 Ramsay,G.,Cropper,M.,Wu,K.,Mason,K.O.,Hakala,P.,2000, MNRAS,311,75. Ramsay, G., Hakala, P., Wu, K., Cropper, M., Mason, K. O., Co´rdova,F.A.,Priedhorsky,W.,2005a, MNRAS,357,49 Ramsay, G., Hakala, P., Marsh, T., Nelemans, G., Steeghs, D., Cropper,M.,2005b,A&A,440,675 Smith,D.A.,Dhillon,V.S.,1998, MNRAS,301,767 Steeghs, D.,etal,inprep Strohmayer,T.E.,2002,ApJ,581,577 Strohmayer,T.E.,2004a,ApJ,608,L53 Strohmayer,T.E.,2004b,ApJ,610,416 Wu,K.,Cropper,M.,Ramsay,G.,Sekiguchi,K.,2002,MNRAS, 331,221 (cid:13)c 0000RAS,MNRAS000,000–000

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