A&A 473, 185-189 (2007)
J. M. Bonnet-Bidaud1 - D. de Martino2 - M. Falanga1 - M. Mouchet3 - N. Masetti4
1 - Service d'Astrophysique, DSM/DAPNIA/SAp CE Saclay, 91191 Gif-sur-Yvette, France
2 - INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
3 - APC, UMR 7164, University Denis Diderot, 2 place Jussieu, 75005 and LUTH, Observatoire de Paris, 92195 Meudon Cedex, France
4 - INAF - Istituto di Astrofisica Spaziale di Bologna, via Gobetti 101, 40129 Bologna, Italy
Received 14 May 2007 / Accepted 16 June 2007
Context. Following an extensive survey of the galactic plane by the INTEGRAL satellite, new hard X-ray sources are discovered with a significant fraction of Cataclysmic Variables (CVs) among them.
Aims. We report here the identification of one of these hard X-ray sources, IGR J00234+6141, as an accreting magnetic white dwarf of intermediate polar type.
Methods. We analyse the high energy emission of the INTEGRAL source using all available data and provide complementary optical photometric and spectroscopic data obtained in August and October 2006, respectively.
Results. Based on a refined INTEGRAL position, we confirm the proposed optical identification. We clearly detect the presence of a 564 s periodic optical modulation that we identify as the rotation of the white dwarf. The analysis of the optical spectrum also demonstrates that the emission lines show a modulation in radial velocity with an orbital period of h.
Conclusions. The two periodicities indicate that IGR J00234+6141 is a magnetic CV of the intermediate polar type. This is one of the faintest and hardest sources of this type detected by INTEGRAL. This confirms earlier conclusions that intermediate polars contribute significantly to the population of galactic X-ray sources and represent a significant fraction of the high energy background.
Key words: stars: binaries: general - stars: individual: IGR J00234+6141 - stars: white dwarf - novae, cataclysmic variables - X-ray: binaries
IGR J00234+6141 was first discovered from the INTEGRAL deep survey in the Cas-A region (den Hartog et al. 2006). From a 1.6 Ms exposure, a weak (0.7 mCrab) source was detected in the (20-50 keV) band with significant flux detected up to 100 keV. The best source position was provided with an estimated 3 accuracy, marginally consistent with a ROSAT soft X-ray counterpart, 1RXS J002258.3+614111, suggesting an X-ray binary (den Hartog et al. 2006). Optical investigation of the much smaller (10 ) ROSAT error box led to the identification of a ( ) object showing strong emission lines of the Balmer series typical of cataclysmic variables (Halpern & Mirabal 2006; Bikmaev et al. 2006; Masetti et al. 2006). From a preliminary short 2 h photometric R-band observation, the presence of a possible 570 s periodic modulation was also reported suggesting a rotating white dwarf (Bikmaev et al. 2006). We present here the first detailed photometric and spectroscopic observations leading to the secure identification of the source as an intermediate polar.
We also note that contrary to what was reported by Bikmaev et al. (2006, see their Fig. 6) the position of the ROSAT source, 1RXS J002258.3+614111, is fully consistent with the optical counterpart with an error circle much smaller than shown by the authors and an X-optical separation less than 6.1 . The identification of IGR J00234+6141 with both the soft X-ray source and the optical star is therefore quite secure.
|Figure 1: New INTEGRAL position and uncertainty for the high energy source IGR J00234+6141, derived from our 3.6 Ms exposure (cross and outer 5.2 circle). The inner (11 radius) circle is the ROSAT position, fully consistent with the proposed optical counterpart (arrow).|
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|Figure 2: The hard energy spectrum of IGR J00234+6141 with the best keV bremsstrahlung fit superposed and corresponding residuals (bottom).|
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We find significant differences with the source parameters published from a smaller dataset by Barlow et al. (2006), with a factor 2 higher bremsstrahlung temperature and with a factor 1.3 lower flux. But our reduced is much more consistent than the one quoted by these authors ( ). For the energy band (20-40 keV), we also have a higher significance of ( for Barlow et al. 2006) that can be explained since we used only the pointings where the source was within 9, therefore increasing the S/N.
A total of 455 frames were acquired in 5.3 h with the log of the observations reported in Table 1. The photometric data were reduced using iraf package with standard procedures including bias, flat-field and sky subtraction. Aperture 3 photometry was obtained for the target and several comparison stars. The flux ratios were obtained by dividing the counts of the target by the best combination (weighted mean) of three reference stars. These ratios were converted to fractional intensities by dividing by the mean flux ratio and heliocentric correction was also applied. The resulting light curve, shown in Fig. 3 (top panel), displays clear short term periodic variations superposed on a longer term variability.
Table 1: Log of photometric and spectroscopic observations.
|Figure 3: Top: IGR J00234+6141 g-band light curves (points) overlaid with a simulated curve (line) including the best spin period (564 s) and the long term variations (see text for details). Middle: power spectrum (DFT) of the light curve, detrended from the low-frequency variations. Ordinates are power amplitudes in units of 10-3. The inserted panel shows the spectral window Bottom: DTF of the observed light curve showing a strong peak at 153 day-1. Some additional power is also present at lower frequencies (see text).|
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|Figure 4: The photometric g-band light curve folded with the best 563.5 s ephemeris. The modulation is very sinusoidal with a full amplitude of 0.10 mag (full line).|
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The DFT of the detrended light curve is shown
in the middle panel of Fig. 3.
From a sinusoidal fit to the detrended curve, the main peak is found at
s modulation is then clearly and safely detected
with an ephemeris as:
Standard reduction was performed in the ESO-MIDAS package, including cosmic rays removal, bias subtraction, flat-field correction and wavelength calibration. The wavelength calibration was checked on sky lines which were found within 0.6 Å from their expected wavelengths. All radial velocity measurements have been corrected from the Earth motion and from the small instrumental shifts measured on the OI 5577 Å line and times have been converted in the heliocentric system. Flux calibration has been performed using the standards BD+28 4211 and EG 247.
The mean optical spectrum of IGR J00234+6141 is typical of magnetic CVs with strong emission lines of the Balmer series, HeII (4686 Å), HeI (4471, 5875, 6678 and 7065 Å) and CIII-NIII (4655 Å), superimposed on a relatively blue continuum.
The mean V flux is estimated at
erg cm-2 s-1Å-1, which corresponds to a V magnitude of 16.7, consistent to that reported by Bikmaev et al. (2006) and Masetti et al. (2006).
|Figure 5: Power spectrum of the H radial velocities. The best value is found at 5.9 day-1 (4.1 h). Secondary maxima are 1-day and 4-day harmonics.|
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Table 2: The parameters of the orbital modulation of the emission lines radial velocities.
|Figure 6: Radial velocities of the H line folded with the orbital period 4.033 h. The best sine fit is also shown (line).|
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The radial velocities of the H line, folded at the above best orbital ephemeris, are shown in Fig. 6 and the parameters of the corresponding best sine fits for the strongest emission lines are given in Table 2. It should be noted that a significant velocity scatter is present around 0.25, at maximum red velocity. We carefully checked by using different sky lines that the effect is not due to a faulty wavelength calibration. In fact, the difference in velocity appears in the two consecutive cycles obtained in the same night (Oct. 25) when negligible wavelength shifts were observed. Inspection of the line profiles shows some distortions but an attempt to fit with two Gaussians failed to give significant results. The Balmer lines show a 50-60 range in velocity amplitudes while the HeII lines display a slightly higher value with no significant phase shift.
Without any knowledge of the absorption, no attempt was made to fit jointly the ROSAT and INTEGRAL data. From the reported ROSAT hardness ratios, and (Voges et al. 1999), and using the distributions for CVs (see Motch et al. 1998, Figs. 4 and 5), a very rough estimate of the absorption can be obtained as , significantly less than the total line-of-sight interstellar value ( ). With this value, the extrapolation of the best INTEGRAL fit yields a (0.1-2.4 keV) flux of (with nearly a factor 2 uncertainty). Using the count conversion given in Voges et al. (1999), the estimated (0.1-2.4 keV) ROSAT flux is significantly lower at erg cm-2s1 which may indicate that a more significant absorption is present.
IGR J00234+6141 is up to now the faintest CV detected by INTEGRAL. There is no good indication of the distance, although a useful lower limit can be set using the infrared magnitudes from the 2MASS survey and the CV donors sequence recently computed by Knigge (2006). For an orbital period of 4.03 h, the donor is expected to be an M3 star with an absolute magnitude of . If attributed solely to the secondary, the observed K magnitude ( ) (Skrutskie et al. 2006) would then yield a minimum distance of 520 pc. To compare with the other high energy IPs detected by INTEGRAL, a distance independent colour-colour diagram was built by comparing the flux ratios in the ROSAT (L=0.1-2.4 keV), and the low (M=20-40 keV) and high (H=40-100 keV) INTEGRAL energy ranges (see Fig. 7). Flux ratios are derived from the ROSAT (Voges et al. 1999) and INTEGRAL (Bird et al. 2007) catalogues for uniformity reasons though fluxes derived from the actual spectral fits may slightly differ for IGR J00234+6141 (this analysis), V709 Cas (Falanga et al. 2005) and IGR J15479-4529 (Bonnet-Bidaud & Falanga 2007) for which a more detailed analysis exists. The sources shown in Fig. 7 are the confirmed IPs with known spin and orbital periods. With ) and ), IGR J00234+6141 has characteristics comparable to the hardest sources.
|Figure 7: High-energy colour-colour diagram of the confirmed IPs detected by INTEGRAL. The energy bands are indicated in the labels. Sources for which only upper values exist are marked by crosses. IGR J00234+6141 (filled square) is among the hardest sources. Note that FO Aqr with M-L= 59.8, due to significant absorption, is kept out of the scale.|
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The high observed temperature ( keV) is a lower limit to the maximum shock temperature and therefore indicative of a white dwarf (WD) mass higher than 0.75 (see Aizu 1973). Assuming that the Balmer lines trace the WD orbital motion, the radial velocity amplitude gives a system mass function of . Together with the WD minimum mass, this yields an estimate of the system inclination - for a companion mass in the range (0.3-0.7) comptatible with the orbit and a maximum inclination of 37 for a 1.4 WD mass. If a distance of 500 pc is assumed, the unabsorbed luminosity will be Lx(0.1-100 keV) (d/500 pc)2 , corresponding to a rather low accretion rate of for a 0.75 WD. IGR J00234+6141, with a ratio , is also in a relatively high degree of asynchronism. For the above WD mass and accretion rate, the condition for accretion to take place, corresponding to a magnetospheric radius lower than the corotation one, will imply a magnetic moment lower than G cm3, corresponding to a surface magnetic field of 2.4 G. Unless the white dwarf is significantly less massive than indicated by the shock temperature, IGR J00234+6141 could therefore be a particular case of a low accreting IP with a low magnetic field.
We thank Maria Magri (Obs. Napoli) for her help in collecting photometric data. D.D.M. and N.M. acknowledge the ASI and INAF financial support via grant No. 1/023/05/0.