We present low and intermediate resolution spectroscopy of the optical counterpart to the recently discovered pulsating X-ray source 1WGA J1958.2+3232. The presence of strong HI , HeI and HeII emission lines together with the absence of absorption features rules out the possibility that the object is a massive star, as had recently been suggested. The observed X-ray and optical characteristics are consistent with the object being an intermediate polar. The double-peaked structure of the emission lines indicates that an accretion disc is present

Clark, J. S.

Negueruela, I.

Reig, P.

English

arXiv

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe nature of 1WGA J1958.2+3232: a newintermediate polarJournal ItemHow to cite:Negueruela, I.; Reig, P. and Clark, J. S. (2000). The nature of 1WGA J1958.2+3232: a new intermediatepolar. Astronomy & Astrophysics, 354(1) L29-L32.For guidance on citations see FAQs.c© 2000 European Southern Observatory (ESO)Version: Version of RecordLink(s) to article on publisher’s website:http://aa.springer.de/bibs/0354001/2300l29/small.htmCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukAstron. Astrophys. 354, L29–L32 (2000) ASTRONOMYANDASTROPHYSICSLetter to the EditorThe nature of 1WGA J1958.2+3232: A new intermediate polarI. Negueruela1, P. Reig2,3, and J. S. Clark41 SAX SDC, ASI, c/o Nuova Telespazio, via Corcolle 19, 00131 Rome, Italy2 Foundation for Research and Technology-Hellas, 711 10, Heraklion, Crete, Greece3 University of Crete, Physics Department, 710 03, Heraklion, Crete, Greece4 University of Sussex, Astronomy Centre, CPES, Falmer, Brighton, BN1 9QH, U.K.Received 10 November 1999 / Accepted 20 December 1999Abstract. We present low and intermediate resolution spec-troscopy of the optical counterpart to the recently discoveredpulsating X-ray source 1WGA J1958.2+3232. The presence ofstrong H i, He i and He ii emission lines together with the ab-sence of absorption features rules out the possibility that theobject is a massive star, as had recently been suggested. The ob-served X-ray and optical characteristics are consistent with theobject being an intermediate polar. The double-peaked structureof the emission lines indicates that an accretion disc is present.Key words: stars: individual: 1WGA J1958.2+3232 – stars:binaries: close – stars: novae, cataclysmic variables – book re-views – X-rays: stars1. IntroductionThere are several types of X-ray sources which displaysignificant modulation in their X-ray lightcurves, amongwhich isolated neutron stars, anomalous X-ray pulsars and twotypes of well characterised binary systems: accreting X-raypulsars (accreting neutron stars with strong magnetic fieldsB > 1011 G) and magnetic cataclysmic variables (accretingwhite dwarfs with moderate magnetic fields B > 105 G).Recent systematic analysis of ROSAT observations has resultedin the detection of several new such sources. However, giventhe limited spectral information of the ROSAT data and theimpossibility of determining the intrinsic luminosity of thesources, the classification of these objects depends on theidentification of their optical counterpart. X-ray pulsators aregenerally part of a high mass X-ray binary (HMXRB) andtheir optical spectra are those of the massive companion,without any significant contribution from the vicinity of theneutron star. In cataclysmic variables (CVs), on the otherhand, the white dwarf is accreting from a late-type unevolvedstar and the optical spectrum is dominated by emission fromthe accretion disc (if present) or the accretion stream (whenCorrespondence to: ignacio@tocai.sdc.asi.itthe magnetic field is too strong to allow the formation of anaccretion disc). In polars (AM Her stars), the magnetic field(B > 5  106 G) is dominant: there is no accretion disc andthe orbit and spin periods are synchronised. In magnetic CVswith weakermagnetic fields (intermediate polars) rotation is notsynchronous and an accretion disc, an accretion stream or bothcan be present.Strong modulation (at an 80% level) was discov-ered in the X-ray signal from the ROSAT PSPC source1WGA J1958.2+3232 by Israel et al. (1998). The pulse periodwas poorly determined at 72114 s, though a later ASCA ob-servation allowed the derivation of a much more accurate value7341 s (Israel et al. 1999). The energy spectrum was fitted bya simple power law giving a photon index Γ = 0.8+1.2−0.6 and acolumn density NH =(6+24−5  1020 cm−2. Given these pa-rameters and the fact that the source was close to the Galacticplane, Israel et al. (1998) were unable to decide whether thesource was a low-luminosity persistent Be/X-ray binary (seeNegueruela 1998; Reig & Roche 1999) or an intermediate polar(see Patterson 1994).Later Israel et al. (1999) located an V = 15.7 emission lineobject inside the 3000 X-ray error circle, which is the opticalcounterpart. Based on a low signal-to-noise spectrum, Israel etal. (1999) classified the object as a Be star, in spite of the evidentpresence of strong He iiλ4686A˚ emission, which is never seenin classical Be stars (in Be/X-ray binaries, if at all present, itonly shows as some in-filling in the photospheric line). Basedon some features that they identified as interstellar lines, theyspeculated that the optical counterpart was a slightly reddenedB0Ve star at a distance of 800 pc. However, if a B0V star wasslightly reddened, it should have an apparent magnitude V  6rather than  16, and a very large reddening is unlikely giventhat the extinction in that directionhas beenmeasured to be small(Neckel & Klare 1980). This led us to obtain higher resolutionspectra of the source. In this paper we show that the opticalproperties of the object clearly identify it as an intermediatepolar, rather than a Be/X-ray binary.LETTERL30 I. Negueruela et al.: The intermediate polar 1WGA J1958.2+3232Fig. 1. Blue spectrum of the optical counterpart to 1WGA J1958.2+3232, taken on July 12, 1999, with the WHT and ISIS. Emission lines aremarked.2. Observations2.1. Optical spectroscopyWe observed the optical counterpart to 1WGA J1958.2+3232on July 12, 1999, using the Intermediate Dispersion Spectro-scopic and Imaging System (ISIS) on the 4.2-m William Her-schel Telescope (WHT), located at the Observatorio del Roquede losMuchachos, La Palma, Spain. The blue armwas equippedwith the R300B grating and the EEV#10 CCD, which gives anominal dispersion of  0.9 A˚/pixel over  3500 A˚. The redarm was equipped with the R1200R grating and the Tek4 CCD,which gives a nominal dispersion of  0.4 A˚/pixel at Hα. Theexposure time was 1500 s. The data were processed using theStarlink packagesccdpack (Draper 1998) andfigaro (Short-ridge et al. 1997) The extracted spectra are displayed in Figs. 1and 2.We obtained lower resolution spectroscopy using the 1.3-mTelescope at the Skinakas Observatory (Crete, Greece) on July26, 1999. The telescope is an f/7.7 Ritchey-Cretien and wasequipped with a 2000  800 ISA SITe chip CCD. This camerahas 15µm pixels and reaches maximum efficiency ( 90%) inthe red, at around Hα. The spectrum (a 1800-s exposure) wastaken with a 1300 line mm−1 grating and a 320 µm width slit(600.7) which gave a dispersion of 1 A˚ pixel−1. The spectrum,which is displayed in Fig. 3, was reduced using figaro.2.2. Optical photometryWe obtained Stro¨mgren photometry of the field using the 1.3-m Telescope at Skinakas Observatory on August 16, 1999 (JD2,451,407). The telescope was equipped with a 1024  1024pixel SITeCH360CCD. The size of the pixels was 24µm, repre-senting approximately 000.5 on the sky. The sourcewas observedFig. 2. Hα andHe i λ6678A˚emission lines in the optical counterpartto 1WGA J1958.2+3232. Spectrum taken with the WHT and ISIS onJuly 12, 1999.through standard u, v, b, y filters with exposure times of 1200,900, 600 and 300 seconds, respectively. A sufficient number ofstandards were observed in order to compute the atmosphericextinction coefficients and allow the transformation to the stan-dard system.The results are displayed in Table 1. We have also obtainedmeasurements for the only other star of similar brightnesswhichwas inside both the ASCA and ROSAT error circles – dubbed“candidate A” by Israel et al. (1999). As can be seen, the valuesof y for the proposed optical counterpart and candidate A areLETTERI. Negueruela et al.: The intermediate polar 1WGA J1958.2+3232 L31Table 1. Observational details of the optical photometry.y b v u1WGA J1958.2+3232 15.77±0.04 15.94±0.04 16.12±0.05 16.48±0.07Candidate A 15.55±0.05 16.15±0.05 16.56±0.07 17.60±0.16Fig. 3. Low-resolution spectrum of the optical counterpart to 1WGAJ1958.2+3232, taken on July 26, 1999, with the 1.3-m telescope atSkinakas.compatible with the V values obtained by Israel et al. (1999) –15.70.2 and 15.40.2 respectively.3. DiscussionThe blue spectrum of the optical counterpart to 1WGAJ1958.2+3232 is displayed in Fig. 1. The spectrum is typicalof a cataclysmic variable with no obvious absorption stellarfeatures and strong emission in all Balmer lines (down to H12).The absence of photospheric features rules out the possibilitythat 1WGA J1958.2+3232 is a Be/X-ray binary – see, for ex-ample, Steele et al. (1999), where it is shown that even for theBe stars with the strongest emission veiling, the photosphericfeatures allow spectral classification to the spectral subtype.In the spectrum of 1WGA J1958.2+3232, on the other hand,as is typical in intermediate polars, He iiλ4686A˚ and theBowencomplex are strongly in emission. Many other He i and He iitransitions are also in emission. The Balmer lines are all double-peaked and asymmetric with a stronger blue peak (note that theprofile of H is modified by the interstellar Ca ii λ3968A˚ line).The asymmetry is still stronger in the He ii lines and can beseen in the weaker He i lines. The centroids of emission lines(determined by fitting a single Gaussian to the profile) showno displacement from the rest wavelength within the resolutionachieved. The blue peaks of the H i and He ii lines are displacedby  250 km s−1.Fig. 2 displays Hα and He i λ6678A˚ at higher resolution.The double-peaked shape can be seen in greater detail in theHα line. This is evidence for the presence of an accretion discsurrounding the white dwarf. The exact shape of the lines mustdepend on the orbital phase at which the observation was taken.Given that theX-ray flux is strongly pulsed and an accretion discis present, the object must be an intermediate polar. Thereforethe observed X-ray variation should represent the spin periodof the cataclysmic variable or the beat period between the spinand orbital periods, since it should be an asynchronous system.The sharpness of the peaks indicates that the 25-min exposuredoes not represent a significative portion of the orbit (otherwisethe peaks would be blurred). This is consistent with expectedorbital periods of a few hours.In the lower resolution spectrum taken two weeks later(Fig. 3), Hα and the He i are single-peaked and red-dominated,indicating that the source was observed at a different orbitalphase. Even though the resolution is rather lower than in theWHT spectrum, a peak separation similar to that measured inthe first spectrum (v  375km s−1) should have been resolved.The interstellar Na i lines are not detectable above the noiselevel. Due to their weakness and the irregularity of the con-tinuum, no diffuse interstellar bands (DIB) can be measuredeven in the higher resolution spectra. We set upper limits for theEquivalent Width (EW) of the DIBs at λ4430A˚ and λ6613A˚as EW< 400 mA˚ and < 50 mA˚, both of which are consistentwith E(B − V ) < 0.2 (Herbig 1975). This is in accordancewith the measurements of interstellar absorption in this direc-tion (l = 69deg, b = 1.7) by Neckel & Klare (1980), who findAV < 0.5 mag and AV < 1.0 mag at 1 kpc for the two fieldsbetween which 1WGA J1958.2+3232 approximately lies.In the WHT observations, we set the slit in such a way asto also observe the nearby star dubbed “Candidate A” by Is-rael et al. (1999), which is about 4000 away from the opticalcounterpart to 1WGA J1958.2+3232, and therefore could pro-vide some information on the reddening in that direction. Eventhough Israel et al. (1999) claim that this object is an early-typestar, comparison with the spectra of several stars taken from theelectronic database of Leitherer et al. (1996) shows that its spec-tral type is F8V (see Fig. 4). We cannot see the λ4430A˚ DIBdown to the level of the many weak features in the spectrum,which gives an upper limit of EW  300 mA˚. From the mea-sured (b− y) = 0.59 0.07 and the intrinsic (b− y)0 = 0.350for an F8V star (Popper 1980) we obtain the interstellar red-dening E(b − y) = 0.24. Using the relation of Crawford &Mandwewala (1976) E(B − V ) = 1.35E(b − y), this impliesE(B − V ) = 0.32, significantly higher than the upper limitthat could be derived from the interstellar λ4430A˚ DIB, whichimplies E(B − V ) < 0.13, according to the relation by Herbig(1975). Assuming MV = +4.2 for a main-sequence F8 star(Deutschman et al. 1976) and the standard reddening R = 3.1,this star is situated at a distance d  0.9 kpc.Given its brightness, 1WGA J1958.2+3232 should be lo-cated at a distance 1 < d < 1.5 kpc (see Israel et al. 1998), i.e.,farther away than the F8V star and thereforewould have a higherreddening. If the reddening is E(B − V ) > 0.3, the soft X-rayflux could be absorbed, which would explain the relatively lowLx/Lopt of the source when compared to less distant interme-LETTERL32 I. Negueruela et al.: The intermediate polar 1WGA J1958.2+3232Fig. 4. Spectrum of the star called Candidate A in Israel et al. (1999),which is only ∼ 40′′ from 1WGA J1958.2+3232. The comparisonspectra correspond to HD 5015 (top, F8V) and HD 22879 (bottom,F9V) and are taken from the database of Leitherer et al. (1996).diate polars (see Israel et al. 1998). We note that the interstellarlines indicate a lower reddening, but in the F8V star this esti-mate is also rather lower than the photometric determination ofthe reddening.With a pulse period of 734 s, this system falls in betweenthe two groups of short and long period intermediate polarsdefined by Norton et al. (1999), and characterised by differentX-ray pulse shapes. Clearly further X-ray observations of thesource are needed and either RXTE or Chandra could providemore detailed timing observations. Also, future time-resolvedphotometric and spectroscopic observations are needed in orderto determine the orbital period and whether the observed X-raypulsations correspond to the spin period.4. ConclusionsBased on intermediate-resolution spectroscopy, we concludethat 1WGA J1958.2+3232 is an intermediate polar, rather thana Be/X-ray binary. From the magnitudes measured for the ob-ject and a very nearby F8V star we can estimate that 1WGAJ1958.2+3232 is situated at a distance of 1 – 1.5 kpc and mod-erately reddened with E(B − V ) < 0.3.Acknowledgements. The WHT is operated on the island of La Palmaby the Royal Greenwich Observatory in the Spanish Observatorio delRoque de Los Muchachos of the Instituto de Astrofı´sica de Canarias.The observations were taken as part of the ING service observing pro-gramme. Skinakas Observatory is a collaborative project of the Uni-versity of Crete, the Foundation for Research and Technology-Hellasand the Max-Planck-Institut fu¨r Extraterrestrische Physik. The au-thors would like to thank Dr. GianLuca Israel for his help with thiswork and Drs D. di Martino and A. J. Norton for their helpful com-ments on the draft. We are also grateful to Drs E. V. Paleologou andI. E. Papadakis for helping with the spectroscopic and photometricobservations at Skinakas Observatory, respectively. IN is supportedby an ESA external fellowship. PR acknowledges partial support viathe European Union Training and Mobility of Researchers NetworkGrant ERBFMRX/CT98/0195. JSC is supported by a PPARC researchassistantship.ReferencesCrawford D.L. & Mandwewala N., 1976, PASP 88, 917Deutschman W.A., Davis R.J. & Schild R.E., 1976, ApJ, 30, 97Draper P.W., 1998, Starlink User Note 139.7, R.A.L.Herbig G.H., 1975, ApJ 196, 129Israel G.L., Angelini L., Campana S., et al., 1998, MNRAS 298, 502Israel G.L., Covino S., Polcaro V.F., Stella L., 1999, A&A 345, L1Leitherer C., AlloinD., Fritz-V.AlvenslebenU., et al., 1996, PASP 108,996Neckel Th., Klare G., 1980, A&AS 42, 251Negueruela I., 1998, A&A 338, 505Norton A.J., Beardmore A.P., Allan A., Hellier C., 1999, A&A 347,203Patterson J., 1994, PASP 106, 209Popper D.M., 1980, ARA&A, 18, 115Reig P., Roche P., 1999, MNRAS 306, 100Shortridge K., Meyerdicks H., Currie M., et al., 1997, Starlink UserNote 86.15, R.A.LSteele I.A., Negueruela I., Clark J.S., 1999, A&AS 137, 147LETTER

The nature of 1WGA J1958.2+3232: a new intermediate polar

Abstract

Similar works

Full text

Available Versions

Open Research Online