arXiv:astro-ph/9912494v1 23 Dec 1999 rto ic(fpeet rteaceinsra we the (when ac- stream the accretion the from or emission present) (if by disc the dominated cretion and is star unevolved spectrum late-type optical a white from the accreting hand, is other dwarf In the star. on neutron (CVs), the variables of cataclysmic vicinity signif- the any from without contribution companion, icant massive spec- the optical of their those and are tra (HMXRB) binary a X-ray of their mass part of high generally identification are classifica- pulsators the X-ray on the counterpart. depends optical sources, objects the these of of tion luminosity intrinsic the ing informa- new the spectral several of limited tion of the given detection However, the sources. such in resulted has observations 3 4 h rsneo togH strong recently of J1958.2+3232. the presence 1WGA to The source counterpart X-ray pulsating optical discovered the of spectroscopy Abstract. Accepted / Received msinlnsidctsta naceinds spresent. is disc the accretion an of that structure indicates lines double-peaked an emission being opti- The had object and polar. as the X-ray with intermediate star, consistent observed massive are characteristics The a cal is suggested. object been the out recently rules that features possibility absorption the of absence the with together 1 intermediate Negueruela I. new A J1958.2+3232: 1WGA polar of nature The orsodneto Correspondence magnetic moderate fields with dwarfs white (accreting variables ac- strong systems: with fields stars binary magnetic neutron characterised (accreting pulsars well pulsars X-ray X-ray of creting anomalous types two stars, display and among neutron which lightcurves, isolated sources X-ray their X-ray which in of modulation types significant several are There Introduction 1. dwarfs white stars – X-ray: binaries:close – – variables cataclysmic vae, words: Key 2 60.921G J1958.2+3232;08.02.1;08.14.2;08.23.01 1WGA 06(08.09.2 are: codes thesaurus later) Your hand by inserted be (will no. manuscript A&A hsc eatet nvriyo rt,700,Heraklion 03, 710 , of University Department, Physics srnm ete PS nvriyo usx amr Brig Falmer, Sussex, of University CPES, Centre, Astronomy A D,AI / uv eepzo i ocle1,I03 R I00131 19, Corcolle via Telespazio, Nuova c/o ASI, SDC, SAX onainfrRsac n ehooyHla,711,Her 10, 711 Technology-Hellas, and Research for Foundation B ∼ > ROSAT 10 epeetlwaditreit resolution intermediate and low present We 5 tr:niiul WAJ98233 no- – J1958.2+3232 1WGA stars:individual: ) eetsseai nlssof analysis systematic Recent G). 1 B .Reig P. , [email protected] : aaadteipsiiiyo determin- of impossibility the and data ∼ > 10 11 i 2 He , )admgei cataclysmic magnetic and G) , 3 n .S Clark S. J. and , i n He and ii msinlines emission 4 ROSAT rt, Crete, , kin rt,Greece Crete, aklion, tn N Q,U.K. 9QH, BN1 hton, inln betisd h 30 the inside object line sion pcrm sale l 19)casfidteojc saBe He strong a of as presence evident object the the of spite classified signal-to-noise in (1999) low star, al. a et on Israel Based spectrum, counterpart. optical the is lal dniyi sa nemdaeplr ahrta a than object rather the polar, of intermediate binary. properties an Be/X-ray this as optical In it the source. identify that clearly the us show of led we This spectra . paper resolution 1980) higher Klare been obtain & has (Neckel to direction small that be in to extinction measured the was that star given magnitude B0V likely apparent a an if have However, V should pc. it 800 reddened, of slightly distance B0Ve a reddened speculated slightly at a they star was lines, counterpart optical interstellar the (in as that features identified stars some some on they as Be Based shows that line). only classical photospheric it the present, in in all in-filling seen at if never binaries, Be/X-ray is which emission, N ul 98 eg&Rce19)o nitreit polar intermediate an or 1994). 1999) Patterson Roche (see & Negue- Reig (see 1998; was binary source ruela Be/X-ray the persistent whether Israel decide low-luminosity plane, to a Galactic unable the were (1998) to al. close et was source the that fact eidwspol eemnda 721 at pulse The determined (1998). later poorly al. was et Israel period by J1958.2+3232 1WGA n htnidxΓ=0 giv- = law power Γ simple index a photon by a fitted ing was spectrum 734 energy value accurate more the from signal X-ray the in ered present. be can both disc, or accretion stream an and accretion synchronous In an (intermediate not synchronised. fields is are rotation magnetic periods polars) weaker spin with and CVs orbit magnetic the and disc crto ic.I oas(MHrsas,temagnetic the stars), an of Her formation (AM ( the polars field allow In to disc). strong accretion too is field magnetic H m,Italy ome, ≈ ae sale l 19)lctdan located (1999) al. et Israel Later togmdlto a n8%lvl a discov- was level) 80% an (at modulation Strong = ahrthan rather 6 ASCA B ;13.25.5) 6 − +24 ∼ > 5 5  bevto loe h eiaino much a of derivation the allowed observation × × 10 10 20 6 ≈ cm )i oiat hr sn accretion no is there dominant: is G) 6 n eylrerdeigi un- is reddening large very a and 16, − 2 ie hs aaeesadthe and parameters these Given . ± Ire ta.19) The 1999). al. et (Israel s 1 . 8 − +1 ′′ 0 ASTROPHYSICS . . -a ro ice which circle, error X-ray 2 6 ASTRONOMY n oundensity column a and ROSAT ± AND V 4s huha though s, 14 SCsource PSPC 15 = ii . λ emis- 7 4686 A ˚ 2 Negueruela et al.: The intermediate polar 1WGA J1958.2+3232

Fig. 1. Blue spectrum of the optical counterpart to 1WGA J1958.2+3232, taken on July 12, 1999, with the WHT and ISIS. Emission lines are marked.

2. Observations 2.1. Optical spectroscopy We observed the optical counterpart to 1WGA J1958.2+3232 on July 12, 1999, using the Interme- diate Dispersion Spectroscopic and Imaging System (ISIS) on the 4.2-m William Herschel Telescope (WHT), located at the Observatorio del Roque de los Muchachos, La Palma, Spain. The blue arm was equipped with the R300B grating and the EEV#10 CCD, which gives a nominal dispersion of ∼ 0.9 A/pixel˚ over ∼ 3500 A.˚ The red arm was equipped with the R1200R grating and the Tek4 CCD, which gives a nominal dispersion of ∼ 0.4 A/pixel˚ at Hα. The exposure time was 1500 s. The data were processed using the Starlink packages ccdpack (Draper 1998) and figaro (Shortridge et al. 1997) The extracted spectra are displayed in Figures 1 and 2. We obtained lower resolution spectroscopy using the 1.3-m Telescope at the Skinakas Observatory (Crete, Greece) on July 26, 1999. The telescope is an f/7.7 Fig. 2. Hα and He i λ6678A˚ emission lines in the opti- Ritchey-Cretien and was equipped with a 2000 × 800 ISA cal counterpart to 1WGA J1958.2+3232. Spectrum taken SITe chip CCD. This camera has 15µm pixels and reaches with the WHT and ISIS on July 12, 1999. maximum efficiency (∼ 90%) in the red, at around Hα. The spectrum (a 1800-s exposure) was taken with a 1300 line mm−1 grating and a 320 µm width slit (6′′.7) which gave a dispersion of 1 A˚ pixel−1. The spectrum, which is a 1024 × 1024 pixel SITe CH360 CCD. The size of the displayed in Figure 3, was reduced using figaro. pixels was 24µm, representing approximately 0′′.5 on the sky. The source was observed through standard u, v, b, y filters with exposure times of 1200, 900, 600 and 300 sec- 2.2. Optical photometry onds, respectively. A sufficient number of standards were We obtained Str¨omgren photometry of the field using the observed in order to compute the atmospheric extinction 1.3-m Telescope at Skinakas Observatory on August 16, coefficients and allow the transformation to the standard 1999 (JD 2,451,407). The telescope was equipped with system. Negueruela et al.: The intermediate polar 1WGA J1958.2+3232 3

The results are displayed in Table 1. We have also flux is strongly pulsed and an accretion disc is present, obtained measurements for the only other star of similar the object must be an intermediate polar. Therefore the brightness which was inside both the ASCA and ROSAT observed X-ray variation should represent the spin period error circles – dubbed “candidate A” by Israel et al. of the cataclysmic variable or the beat period between (1999). As can be seen, the values of y for the proposed the spin and orbital periods, since it should be an asyn- optical counterpart and candidate A are compatible with chronous system. The sharpness of the peaks indicates the V values obtained by Israel et al. (1999) – 15.7±0.2 that the 25-min exposure does not represent a significa- and 15.4±0.2 respectively. tive portion of the orbit (otherwise the peaks would be blurred). This is consistent with expected orbital 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 orbital phase. Even though the resolution is rather lower than in the WHT spectrum, a peak sep- aration similar to that measured in the first spectrum (v ≈ 375 kms−1) should have been resolved. The inter- stellar Na i lines are not detectable above the noise level. Due to their weakness and the irregularity of the con- tinuum, no diffuse interstellar bands (DIB) can be mea- Fig. 3. Low-resolution spectrum of the optical counter- sured even in the higher resolution spectra. We set up- part to 1WGA J1958.2+3232,taken on July 26, 1999, with per limits for the Equivalent Width (EW) of the DIBs the 1.3-m telescope at Skinakas. at λ4430A˚ and λ6613A˚ as EW< 400 mA˚ and < 50 mA,˚ both of which are consistent with E(B −V ) < 0.2 (Herbig 1975). This is in accordance with the measurements of in- terstellar absorption in this direction (l = 69deg,b =1.7) 3. Discussion by Neckel & Klare (1980), who find AV < 0.5 mag and The blue spectrum of the optical counterpart to 1WGA AV < 1.0 mag at 1 kpc for the two fields between which J1958.2+3232 is displayed in Figure 1. The spectrum is 1WGA J1958.2+3232 approximately lies. typical of a cataclysmic variable with no obvious absorp- In the WHT observations, we set the slit in such a way tion stellar features and strong emission in all Balmer lines as to also observe the nearby star dubbed “Candidate A” ′′ (down to H12). The absence of photospheric features rules by Israel et al. (1999), which is about 40 away from the out the possibility that 1WGA J1958.2+3232 is a Be/X- optical counterpart to 1WGA J1958.2+3232, and there- ray binary – see, for example, Steele et al. (1999), where fore could provide some information on the reddening in it is shown that even for the Be stars with the strongest that direction. Even though Israel et al. (1999) claim that emission veiling, the photospheric features allow spectral this object is an early-type star, comparison with the spec- classification to the spectral subtype. tra of several stars taken from the electronic database of In the spectrum of 1WGA J1958.2+3232, on the other Leitherer et al. (1996) shows that its spectral type is F8V hand, as is typical in intermediate polars, He ii λ4686A˚ (see Fig. 4). We cannot see the λ4430A˚ DIB down to the and the Bowen complex are strongly in emission. Many level of the many weak features in the spectrum, which other He i and He ii transitions are also in emission. The gives an upper limit of EW ≈ 300mA.˚ From the measured Balmer lines are all double-peaked and asymmetric with (b − y)=0.59 ± 0.07 and the intrinsic (b − y)0 = 0.350 a stronger blue peak (note that the profile of Hǫ is modi- for an F8V star (Popper 1980) we obtain the interstellar fied by the interstellar Ca ii λ3968A˚ line). The asymmetry reddening E(b − y)=0.24. Using the relation of Craw- is still stronger in the He ii lines and can be seen in the ford & Mandwewala (1976) E(B − V )=1.35E(b − y), weaker He i lines. The centroids of emission lines (deter- this implies E(B − V )=0.32, significantly higher than mined by fitting a single Gaussian to the profile) show no the upper limit that could be derived from the interstellar displacement from the rest wavelength within the resolu- λ4430A˚ DIB, which implies E(B − V ) < 0.13, according tion achieved. The blue peaks of the H i and He ii lines are to the relation by Herbig (1975). Assuming MV = +4.2 displaced by ∼ 250 kms−1. for a main-sequence F8 star (Deutschman et al. 1976) and Figure 2 displays Hα and He i λ6678A˚ at higher res- the standard reddening R =3.1, this star is situated at a olution. The double-peaked shape can be seen in greater distance d ≈ 0.9 kpc. detail in the Hα line. This is evidence for the presence of Given its brightness, 1WGA J1958.2+3232 should be an accretion disc surrounding the . The exact located at a distance 1 ∼< d ∼< 1.5 kpc (see Israel et al. shape of the lines must depend on the orbital phase at 1998), i.e., farther away than the F8V star and there- which the observation was taken. Given that the X-ray fore would have a higher reddening. If the reddening is 4 Negueruela et al.: The intermediate polar 1WGA J1958.2+3232

Table 1. Observational details of the optical photometry.

y b v u

1WGA J1958.2+3232 15.77±0.04 15.94±0.04 16.12±0.05 16.48±0.07 Candidate A 15.55±0.05 16.15±0.05 16.56±0.07 17.60±0.16

Acknowledgements The WHT is operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observa- torio del Roque de Los Muchachos of the Instituto de Astrof´ısica de Canarias. The observations were taken as part of the ING service observing programme. Skinakas Observatory is a collaborative project of the , the Foundation for Research and Technology- Hellas and the Max Planck Institut f¨ur Extraterrestrische Physik. The authors would like to thank Dr. GianLuca Fig. 4. Spectrum of the star called Candidate A in Is- Israel for his help with this work and Drs D. di Mar- rael et al. (1999), which is only ∼ 40′′ from 1WGA tino and A. J. Norton for their helpful comments on J1958.2+3232. The comparison spectra correspond to HD the draft. We are also grateful to Drs E. V. Paleolo- 5015 (top, F8V) and HD 22879 (bottom, F9V) and are gou and I. E. Papadakis for helping with the spectro- taken from the database of Leitherer et al. (1996). scopic and photometric observations at Skinakas Obser- vatory, respectively. IN is supported by an ESA external fellowship. PR acknowledges partial support via the Eu- ropean Union Training and Mobility of Researchers Net- work Grant ERBFMRX/CT98/0195. JSC is supported by − E(B V ) > 0.3, the soft X-ray flux could be absorbed, a PPARC research assistantship. which would explain the relatively low Lx/Lopt of the source when compared to less distant intermediate po- lars (see Israel et al. 1998). We note that the interstellar References lines indicate a lower reddening, but in the F8V star this Crawford D.L. & Mandwewala N., 1976, PASP 88, 917 estimate is also rather lower than the photometric deter- Deutschman W.A., Davis R.J. & Schild R.E., 1976, ApJ, 30, mination of the reddening. 97 With a pulse period of 734 s, this system falls in be- Draper P.W., 1998, Starlink User Note 139.7, R.A.L. tween the two groups of short and long period interme- Herbig G.H., 1975, ApJ 196, 129 Israel G.L., Angelini L., Campana S., et al., 1998, MNRAS diate polars defined by Norton et al. (1999), and charac- 298, 502 terised by different X-ray pulse shapes. Clearly further X- Israel G.L., Covino S., Polcaro V.F., Stella L., 1999, A&A 345, ray observations of the source are needed and either RXTE L1 or Chandra could provide more detailed timing observa- Leitherer C., Alloin D., Fritz-V.Alvensleben U., et al., 1996, tions. Also, future time-resolved photometric and spectro- PASP 108, 996 scopic observations are needed in order to determine the Neckel Th., Klare G., 1980, A&AS 42, 251 orbital period and whether the observed X-ray pulsations Negueruela I., 1998, A&A 338, 505 correspond to the spin period. Norton A.J., Beardmore A.P., Allan A., Hellier C., 1999, A&A 347, 203 Patterson J., 1994, PASP 106, 209 Popper D.M., 1980, ARA&A, 18, 115 4. Conclusions Reig P., Roche P., 1999, MNRAS 306, 100 Shortridge K., Meyerdicks H., Currie M., et al., 1997, Starlink Based on intermediate-resolution spectroscopy, we con- User Note 86.15, R.A.L clude that 1WGA J1958.2+3232 is an intermediate po- Steele I.A., Negueruela I., Clark J.S., 1999, A&AS 137, 147 lar, rather than a Be/X-ray binary. From the magnitudes measured for the object and a very nearby F8V star we can estimate that 1WGA J1958.2+3232 is situated at a distance of 1–1.5 kpc and moderately reddened with E(B − V ) ∼< 0.3.