X-ray spectral properties of the pulsar EXO during an outburst

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1 Mon. Not. R. Astron. Soc. 302, 700±706 (1999) X-ray spectral properties of the pulsar EXO during an outburst P. Reig*² and M. J. Coe Department of Physics and Astronomy, Southampton University, Southampton SO17 1BJ Accepted 1998 September 17. Received 1998 September 7; in original form 1998 February 26 1 INTRODUCTION EXO was discovered in 1985 May by the EXOSAT satellite when the source underwent a giant outburst (Parmar et al. 1989a). On the rst detection the X-ray luminosity was of the order of erg s 1, close to the Eddington luminosity for accretion on to a neutron star. By 1985 August the ux had declined below the EXOSAT detection threshold (L x, erg s 1 ). The source turned on again in 1985 October showing quasi-periodic are events every 3.96 h. The large variation in X-ray luminosity of the rst outburst allowed the dependence of the pulse period and pulse pro le on luminosity to be studied for the rst time for an individual pulsar (Parmar et al. 1989a; Parmar, White & Stella 1989b). EXO is a member of the massive X-ray binaries known as Be/X-ray binary systems. These systems contain a neutron star and a Be star. Be stars are non-supergiant B-type stars which show emission lines in their spectra and an infrared excess when they are compared with normal B stars of the same spectral type. These two characteristics can be explained assuming the existence of a photoionized, dense (,10 11 electron cm 3 ) circumstellar disc surrounding the equator of the Be star. The optical counterpart was identi ed by Coe, Hanson & Longmore (1985). The work by Motch & Janot-Pacheco (1987) and Coe et al. ABSTRACT We present the results of the spectral analysis carried out of the X-ray pulsar EXO The observations were made using the Rossi X-ray Timing Explorer (RXTE) satellite. Our study concentrates on the X-ray characteristics of this source at lower luminosities (L x < erg s 1 ) than have previously been observed and in the energy range 2.7±30 kev. The PCA X-ray continuum spectra can be represented by a model containing two components: a power law with an exponential cut-off at higher energies and a blackbody. The power-law component accounts for the high-energy part of the continuum whereas the blackbody component describes the part of the spectrum below,10 kev. The radius of the X-ray emission surface is found to be,1 km, which agrees with the size of the neutron star polar caps. Our results are compared with higher-luminosity studies and are seen to agree with trends observed in these previous ndings. We also show the rst hard X-ray spectrum (17±65 kev) of EXO A possible spectral feature at,36 kev is tentatively ascribed to a cyclotron absorption line. Key words: binaries: general ± stars: emission-line, Be ± pulsars: individual: EXO ± infrared: stars ± X-rays: stars. (1988) revealed a highly reddened [E B V ˆ3:80 6 0:15Š V < 20 star, showing Ha in emission and an infrared excess. The X-ray emission of EXO presents variability on all time-scales. It is characterized in the short term by 41.7-s pulsations and rapid pulse period variations; in the medium term by the periodic 46-d outbursts that occur during periastron passage of the neutron star; and in the long term by the decline of the optical and infrared emission over periods of several years (Reig et al. 1998) and by the shift in the onset of the periodic outburst. The timing properties of this system are discussed in Reig & Coe (1998). The continuous monitoring carried out by the Burst and Transient Source Experiment (BATSE) since its launch allowed the determination of the following orbital parameters (Stollberg 1997): P orb ˆ d, e ˆ 0: , a x sin i ˆ lightsecond, q ˆ 223:85 6 1:88 and T peri ˆ JD X-ray spectral analyses of EXO have been carried out by Reynolds, Parmar & White (1993, hereafter RPW) and Sun et al. (1994, hereafter SLWC) using EXOSAT data (1±20 kev) and by Mavromatakis (1994), who explored the soft end of the X-ray spectrum (0.1±2.4 kev) with ROSAT and the Low Energy Experiment on EXOSAT. In this paper we investigate the medium (2.7±30 kev) and hard (17±65 kev) parts of the spectrum at X-ray luminosities of the order of,10 36 erg s 1. *Present address: Astronomy Group, Physics Dept., University of Crete, PO Box 2208, Herakloon, Crete, Greece. ² pablo@physics.uch.gr 2 OBSERVATIONS AND DATA REDUCTION The observations were made using the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) on q 1999 RAS

2 X-ray properties of EXO in outburst 701 board RXTE. The PCA comprises ve co-aligned gas- lled proportional counter units (PCU), giving a total collecting area of,6500 cm 2. Each PCU has three layers of xenon- lled detectors with a front propane veto layer, which is primarily used for background rejection. Five collimator modules made of tin-coated beryllium± copper sheets are mounted on to each PCU, giving a eld of view of 1 ± (full width at half-maximum, FWHM). The energy and time resolution depend on the particular mode chosen for the observation. In this paper we have made use of the Standard 2 mode data, which provide 129 PHA channels covering the energy range 2±60 kev and have a time resolution of 16 s. For a more comprehensive description of the RXTE PCA see Jahoda et al. (1996). The HEXTE instrument consists of two independent clusters, A and B, each containing four NaI(Tl)/CsI(Na) phoswich scintillation detectors passively collimated to a 1 ± eld of view and co-aligned with the PCA. Each detector has an area of,225 cm 2 and covers the energy range 15±250 kev, with an intrinsic spectral resolution better than 9 kev at 60 kev. For more details on this instrument the reader is referred to Gruber et al. (1996). EXO was observed from 1996 July 1 (JD ; orbital phase 0.86) to 1996 July 10 (JD ; orbital phase 0.08). The observational data consisted of 25 pointings amounting to an on-source time of,1: s. However, owing to the low Earth orbit of the satellite (occultation of the source, South Atlantic Anomaly, etc.) the effective exposure time (good time intervals) represented about 60 per cent of this time. In order to study the X-ray spectral variability of the source throughout the outburst we obtained 16 PCA pulse phase-average spectra over the 10-d duration of the outburst. The count rate on the rst day was far too low to permit a statistically signi cant t to be obtained. Thus, data from that day were not analysed. For the spectral analysis of the PCA we used the latest version of the PCA response matrix (as released in version 4.1 of ftools). As a result of the large decrease in ux above 20 kev, the HEXTE data from various pointings had to be combined in order to have enough counts at higher energies. The HEXTE spectrum was obtained by combining the observations of the last six days (JD to ) of the outburst since they all share the same observation con guration mode, namely, E 2ms 256 DX0D. This mode gives 2-ms time resolution and detailed (256 channels) spectral information. The data reduction was carried out with the speci c packages for RXTE within ftools, whereas the data analysis was done with xspec (Arnaud 1996). 3 RESULTS The model that has been generally used to represent the X-ray spectra of accreting X-ray pulsars is a power-law continuum with low-energy absorption and a cut-off above 10±20 kev f E ˆAe N Hj MM E E a e E cut E =E fold : 1 In some systems, such as EXO , an iron emission line at, kev is required in order to have an acceptable t. Mathematically the line is represented by a Gaussian function of the form L E ˆK p 1 e 0:5 E E l j 2 : 2 j 2p In addition, SLWC reported the detection of a blackbody-like component with a temperature kt,1.16 kev in the high- ux spectra (L x >, erg s 1 ) of EXO obtained by EXOSAT during the major outburst in The model photon distribution has the form B E ˆN1: E 2 de e E=kT 1 : 3 This two-continuum X-ray component model was attempted in our data and proved to give good ts. In the above equations E is the energy in kev, A and K the normalization constants in units of photon kev 1 cm 2 s 1 and total photon cm 2 s 1 in the line respectively, a the photon index, N H the equivalent hydrogen column density in units of cm 2, E cut the cut-off energy in kev, E fold the folding energy in kev, j MM the photoelectric absorption cross-section (Morrison & McCammon 1983), j the linewidth in kev and E l the line energy in kev. The normalization constant of the blackbody component is given by N ˆ R 2 =D 2 10, where R is the radius of the emission surface and D 10 is the distance to the source in units of 10 kpc. The hydrogen column density N H gives the amount of matter intervening in the line of sight between the observer and the X-ray source. j MM accounts for the photoelectric absorption that affects low-energy photons (< 2±3 kev) by heavy elements in the interstellar medium. E fold has been associated by some authors (e.g. Unger et al. 1992) with the plasma temperature in the emission region. The iron line at,6.4 kev corresponds to Ka emission, which is produced by transitions from the energy levels L 3 (6.404 kev) and L 2 (6.391 kev) to the K level. The excitation energy is provided by the X-ray photons ( uorescence) emitted from the source. Therefore the detection of an iron line implies the presence of relatively cold matter in the vicinity of the X-ray source and give information of the ionization degree of iron in the circumstellar matter. However, E cut has no clear physical meaning, although it might be related to the strength of the magnetic eld (Makishima et al. 1990). The journal of the X-ray observations is presented in Table 1. Columns 5 and 6 give an indication of the quality of the t for the simple model, that is, power law and high-energy cut-off (PO+CO) and for the two-continuum X-ray component model (PO+CO+BB), which also includes the blackbody component. The reduced x 2 and Table 1. Journal of the X-ray observations and the model ts to the PCA data. See text for further explanation. x 2 r /dof Obs No MJD orbital L a x PL+CO PL+CO+BB ( ) phase / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /52 a In units of erg s 1, in the energy range 2.7±30 kev and at an assumed distance of 5 kpc.

3 702 P. Reig and M. J. Coe Figure 1. Typical PCA spectrum of EXO The lower panel displays the residuals from a line-free continuum model. The presence of an iron line at,6.4 kev is clearly seen. the number of degrees of freedom are given. Fig. 1 shows a typical PCA spectrum. The lower panel displays the residuals. i.e. data model, from a line-free continuum model. The presence of an iron line,6.4 kev is clearly seen. The ux is mostly emitted in the energy range 2±20 kev, with a rapid fall-off at energies above 20 kev. The inclusion of the iron line improved the quality of the t in all the observations. We let the width and energy of the line be free parameters of the t (Table 2). The average values found were E l ˆ 6: kev and j ˆ 0: kev, where the errors are the standard deviation of the N measurements s N P x 2 P 2 x : N N 1 Thus a relatively broad linewas required in most cases. This result is in contrast to the observations of other pulsars (Inoue 1985; Nagase 1989) for which the linewidths seem to be narrow but, does con rm the result found by Parmar et al. (1989a). We, however, xed the value of the hydrogen column density to cm 2. This value was chosen because it agrees with the value cm 2 predicted from optical measurements (Coe et al. 1988). In addition, it lies in the middle of the values resulting from previous X-ray studies: (Parmar et al. 1989a),, (SLWC) and,2± cm 2 (Mavromatakis 1994). In order to check the validity of this assumption we xed the iron line parameters to the average values and let the hydrogen column density be a free parameter. The measurements distributed around an average value of N H ˆ 2: cm 2, where the error is the standard deviation of the measurements. To t the HEXTE spectrum we used a model photon distribution of the form C E ˆIE a f E, where f E can be any of the following exponentials (or both): f 1 E ˆe E=E c, f 2 E ˆ e jw2 E=E 0 2 = E E 0 2 W 2. The latter is related to the cross-section of cyclotron resonant scattering (Makishima et al. 1990). E 0, W and j are the energy, width and depth of the resonance, respectively. Acceptable ts were obtained in all three cases, as summarized in Table 3. The power law plus cut-off gave a ˆ 0:99 0:41 0:34, E c ˆ :0 4:1. When using the cyclotron resonant scattering exponential the best t is achieved for E 0 < 36 kev, W < 1 kev, j < 1:4, E c < 16.3 kev and a< 0.5. An equally good t (x 2 r ˆ 1:09) is obtained by removing the exponential cut-off. In this case, E 0,36 kev also but the width of the cyclotron line is unacceptably large (,40 kev), and what the t is re ecting is probably the known fact that the cyclotron model can describe not only a linelike feature at E 0 but also a power-law break at,e 0 /2 (Makishima et al. 1990). In fact, a t using only the power law and cut-off componets gives a cut-off energy at 20 kev,e 0 =2 (see discussion below). The observed average hard X-ray spectrum of EXO in the energy range 17±65 kev is shown in Fig 2. The residuals shown correspond to a model photon distribution that includes a power law, cut-off and cyclotron resonant scattering. Table 2. Results of the spectral ts to the PCA spectra. The values of Obs No 1 are not well constrained as a result of the poor quality of the t. Obs L x E cut Spectral Temperature Radius E l Fe j(fe) EW(Fe) No (10 36 erg s 1 ) (kev) index (kt) (km) (kev) (kev) (ev) ,8.7 -,1.3,1.2,6.5, :55 0: :30 0: :06 0: :19 0: :15 0: : :39 0: :13 0: :03 0: :12 0: :09 0: : :37 0: :07 0: :04 0: :13 0: :09 0: : :35 0: :14 0: :05 0: :19 0: :12 0: : :41 0: :08 0: :03 0: :18 0: :09 0: : :34 0: :13 0: :04 0: :17 0: :11 0: : :31 0: :06 0: :06 0: :15 0: :09 0: : :53 0: :08 0: :05 0: :17 0: :10 0: : :33 0: :06 0: :05 0: :13 0: :08 0: : :32 0: :07 0: :03 0: :08 0: :08 0: : :31 0: :06 0: :06 0: :16 0: :08 0: : :39 0: :14 0: :03 0: :10 0: :11 0: : :68 0: :12 0: :06 0: :22 0: :12 0: : :36 0: :14 0: :02 0: :12 0: :10 0: : :73 0: :30 0: :03 0: :07 0: :18 0: :49 0: : : : : : : : : : : : : : :38 97

4 X-ray properties of EXO in outburst 703 Table 3. Results from the spectral tting of the HEXTE spectra. See text for the functional form of the energy distribution. x 2 r dof f E ˆf 1 E f E ˆf 2 E f E ˆf 1 E f 2 E In order to test the signi cance of the cyclotron scattering resonance feature we performed pulse phase-resolved spectroscopy. Four segments corresponding to pulse phase intervals of width 0.25 each were obtained (see Fig. 3). No obvious cyclotron line was found. We tried xing the cyclotron energy to the value obtained from the pulse-averaged spectrum but the value of the line strength, once the errors were determined, was consistent with zero. The main problem with this analysis was that we simply had too few counts in each phase bin to search sensibly for the line feature. 4 DISCUSSION 4.1 PCA observations The results presented in this paper correspond to a variation in X-ray luminosity of a factor of 5 from, to, erg s 1 and hence complement the work done by RPW. These authors studied the evolution of the X-ray spectral parameters during the major outburst in 1985, covering a luminosity variation of roughly two orders of magnitudes, from to erg s 1.In the present work we study the behaviour of EXO at lower Figure 2. Averaged HEXTE spectrum of EXO in the energy range 17±65 kev and residuals. Only data from Cluster A were used to produce this spectrum. The continuous line represents the model photon distribution given by a power law, exponential cut-off and cyclotron scattering. Note the absorption feature at,36 kev. Figure 3. Pulse pro les of EXO at two different energy ranges obtained with HEXTE. The four segments used for phase-resolved spectroscopy are indicated by the letters a; b; c and d. luminosities (RXTE data) and also compare our results with those obtained by RPW and SLWC. Figs 4(a)±(d) show the evolution of the cut-off energy, photon index, radius of the blackbody emission region and blackbody temperature as a function of luminosity throughout the outburst. Note, however, that the t corresponding to the RXTE observation with the lowest X-ray luminosity presented an unacceptable reduced x 2 of 2.24, and hence the resulting parameters of this t are not shown in the gures. The rst and most obvious trend visible in these gures is that, as the luminosity increased, the X-ray spectrum became softer with the photon index increasing from 0.20 to 1.02 (Fig. 4b). This result con rms the correlation found by RPW of increasing hardness as the 1985 outburst declined. In Fig. 5 the data from RPW ( lled triangles) have been combined with our data. In order to allow direct comparison between our results and those obtained by RPW, the same model has been used, i.e. a power-law continuum with low-energy absorption and a cut-off at higher energy. Where we have comparable data, the agreement is consistent within the errors. There is no clear change of the cut-off energy throughout the 1996 outburst (Fig. 4a) but the values distribute around an average value of E cut ˆ kev, which agrees with the lower end trend found from the EXOSAT data (Fig. 5a). RPW reported a decrease of the cut-off energy as the X-ray luminosity declined, from,19 kev when L x,10 38 erg s 1 to 9.2 kev when L x,3: erg s 1. When the RXTE and EXOSAT data are combined (Fig. 5a) the cut-off energy seems to converge gradually to a value of,6 kev. SLWC did not need to include a high-energy cut-off because they analysed data at very high X-ray luminosity (L x > erg-s 1 ) in the energy range 1±20 kev, that is, below the cut-off.

5 704 P. Reig and M. J. Coe Figure 4. The evolution of the spectral parameters with increasing luminosity for EXO during the 1996 July outburst. The X-ray luminosity corresponds to the energy range 2.7±30 kev. The point represented by a lled circle corresponds to a are event (see text). All errors are at the 90 per cent con dence level. The point in Fig. 4 represented by a lled circle requires further discussion. This point corresponds to the are event that occurred at the beginning of the outburst (Reig & Coe 1998). By xing N H to cm 2 we obtained a cut-off of 6.5 kev for this observation, that is,,7j from the average value previously given. However, by letting N H be a free parameter and xing a we obtained E cut ˆ 8:7, which agrees with the average value within 1j, and N H ˆ 3: cm 2, which is within 2j of the average value. In this case we think that a variation in N H of,2j is more likely to be real than a variation of,7j in E cut. The increase in the amount of circumstellar material during the ares might be the result of inhomogenities in the accretion ow. Some parts of the accretion ow, at the beginning of the outburst, may have had higher density, (blobs) which would have contributed to produce a higher N H and increase the accretion rate and hence the X-ray luminosity. With regard to the iron line parameters, neither the linewidth, the line centre nor the equivalent width show any clear evolution in our data, which implies that at the X-ray uxes discussed here the amount of reprocessed radiation is not important. The average values are j ˆ 0:57 6 0:20 kev, E l ˆ 6: kev and EW Fe ˆ ev, where the error represents the standard deviation of the observations. These numbers agree well with the average values of kev, kev and ev respectively, reported by RPW. Fig. 6 shows the energy and width of the line as a function of the X-ray luminosity. With regard to the spectral parameters of the blackbody Figure 5. Photon index and cut-off energy as a function of the X-ray luminosity. Our results (circles) represent an extension of those of RPW ( lled triangles) for low values of the X-ray luminosity. The X-ray luminosity corresponds to the energy range 1-20 kev.

6 X-ray properties of EXO in outburst 705 Figure 6. Width and energy of the iron line as a function of the X-ray luminosity (2.7±30 kev). All errors are at the 90 per cent con dence level. component, Figs 4(c) and (d) show the radius of the emission region and temperature of the blackbody component. The temperature tends to increase weakly as the X-ray luminosity increases. This would not be surprising since a higher luminosity implies a higher accretion rate and hence more matter available to produce highenergy radiation. As mentioned above, the normalization constant of the blackbody component is directly related with the source radius. Therefore, the results from the spectral tting may provide important clues about the size of the area where the X-rays are being produced, which in turn allows to test the validity of accretion theory. The evolution of the emission surface radius with luminosity, assuming a distance to the source of 5 kpc, is shown in Fig. 4(c) and Fig. 7, where data from SLWC have also been used ( lled triangles). No variation throughout the 1996 outburst is seen, the mean value of the source radius being R p = km. Here again the error is the standard deviation of all the measurements. The same analysis carried out by SLWC at very high X-ray luminosities revealed a decrease of the blackbody emission area from 6.1 0:9 0:7 km at L x ˆ erg-s 1 to 3.6 1:1 0:9 km at erg s 1. X-ray pulsations in EXO are detected even at low X-ray luminosities. Pulsations are the result of the strong magnetic eld of the neutron star. When the gas particles penetrate into the magnetosphere they can move only along the magnetic eld lines. The accretion ow is then channelled towards the magnetic poles. This means that matter is not being accreted all over the neutron star surface but only on to a small part of it, namely the polar caps. The Figure 7. Emission source radius as a function of X-ray luminosity. Data from SLWC are represented by lled triangles. At low luminosity the radius agrees with the typical size of the polar cap. The X-ray luminosity corresponds to the energy range 1±20 kev. magnetic eld lines near the star, above the magnetic poles, adopt then the con guration of a roughly column-shaped surface or funnel, which is referred to as the accretion column, the polar cap being its base. SLWC interpreted the radius of the emission area of the blackbody in terms of the radius of the funnel. If the accretion luminosity is very high ± close to the Eddington luminosity ± the gas particles will be stopped at a certain height above the surface of the neutron star owing to the radiation pressure provided by the X-ray photons. As the X-ray luminosity decreases, the particles will fall down until the accretion density and photon ux are highenough to set up a new balance because of the narrower funnel there. In the case of low-luminosity sources the radiation pressure is not capable of decelerating the accretion ow at a certain height above the neutron star surface (Wang & Frank 1981). Instead, the plasma travels all the way down to the neutron star surface where it is slowed down by Coulomb scattering with thermal electrons, and by nuclear collisions with atmospheric protons (Harding 1994). Thus, as the X-ray luminosity decreases, the radius of the emission surface would decresase. How small this radius can become will be determined by the size of the polar caps. A typical radius for the polar caps is in the range 0.1±0.01 R x (MeÂszaÂros 1992), where R x is the radius of the neutron star. Assuming a neutron star radius of R x,10 6 cm, the polar cap radius is R p,0.1±1 km. In Fig. 7, we have plotted together our values and those obtained by SLWC. The results seem to imply a limit for the size of the emission surface of,1.5 km, in good agreement with the theoretical size of the polar caps. 4.2 HEXTE observations Care should be taken in interpreting the absorption line at 36 kevas a cyclotron scattering resonance feature since it does not show up in the pulse-resolved spectra. However, the low count rate in each of the pulse phase spectra, especially at E > 35 kev, would make its detection dif cult. Also, the line does not seem to show up when data from Cluster B are used. This instrument, however, suffered the loss of one of its detectors, hence the statistics are not as good as those of Cluster A. In support of the cyclotron line interpretation, however, there is the observational relationship found by Makishima et al. (1990) between the cyclotron and cut-off energies, E 0 < 1:4 1:8 E cut. From our data we obtain E 0 < 1:8E cut for EXO With the discovery of new cyclotron features this range has now widened to 1.2±2.2 (White, Nagase & Parmar 1995). If the line feature is interpreted as the rst harmonic cyclotron absorption, then the derived magnetic eld of the neutron star is, G.

7 706 P. Reig and M. J. Coe 5 CONCLUSION We have performed a study of the spectral properties of the X-ray pulsar EXO in the X-ray band during a predicted outburst. The X-ray continuum is represented by a power-law and blackbody components. In addition, a cut-off at 8.6 kevand an iron line at 6.4 kevare needed to obtain acceptable ts. The results have been shown to agree with those from previous studies when an extrapolation at low luminosities is realised. While the cut-off energy and the width of the iron line did not change throughout the outburst, the photon index and temperature of the blackbody component show a weak correlation with luminosity. The radius of the emission surface is of the same order as the typical size of the polar caps. Owing to the low luminosity there is no stand-off shock that stops the accretion material at a certain height above the neutron star surface. The particles in the accretion ow are able to travel all the way down to the polar caps, where they are stopped and their potential energy converted into electromagnetic radiation. Although we cannot disregard the fact that even at lower luminosity the radius of the emission region may be smaller, the low-luminosity X-ray radiation detected by RXTE in 1996 July comes from very deep in the accretion column, most likely from the polar caps themselves. We are unable to make a de nitive statement on the presence of a 36 kev absorption line in the phase-averaged spectrum, but we are encouraged by the increasing number of X-ray pulsars with detected cyclotron features with energies 20±40 kev, discovered in recent years. ACKNOWLEDGMENTS We are grateful to Dr Mark Finger and Dr Arvind Parmar for helpful discussions. We also thank the referee of this paper, Dr Robin Corbet, for his suggestions and comments. The data reduction was carried out using the Southampton University node, which is funded by the PPARC. REFERENCES Arnaud K. A., 1996, in Jacoby G., Barnes J., eds, ASP Conf. Ser. Vol. 101, Astronomical Data Analysis Software and Systems V. Astron. Soc. Pac., San Francisco, p. 17 Coe M. J., Hanson C. G., Longmore A. J., 1985, IAU Circ. No.4096 Coe M. J., Longmore A. J., Payne B. J., Hanson C. G., 1988, MNRAS, 232, 865 Gruber D. E., Blanco P. R., Heindl W. A., Pelling M. R., Rothschild R. E., Hink P. L., 1996, A&AS, 120, C641 Harding A. K., 1994, in Holt S. S., Day C. S. eds, The Evolution of X-ray binaries. AIP Press, New York, p. 429 Inoue H., 1985, Space Sci. Rev., 40, 317 Jahoda K., Swank J. H., Stark M. J., Strohmayer T., Zhang W., Morgan E. H., 1996, in Siegmund O. H. W., Gummin M. A., eds, SPIE 2808, EUV, X- ray and Gamma-ray Instrumentation for Space Astronomy VII. SPIE, Bellingham, WA, p. 59 Makishima K. et al., 1990, ApJ, 365, L59 Mavromatakis F., 1994, A&A, 285, 209 MeÂszaÂros P., 1992, in High-Energy Radiation from Magnetized Neutron Stars. Univ. Chicago Press, Chicago, p. 21 Morrison R., McCammon D., 1983, ApJ, 270, 119 Motch C., Janot-Pacheco E., 1987, A&A, 182, L55 Nagase F., 1989, PASJ, 41 1 Parmar A. N., White N. E., Stella L., Izzo C., Ferri P., 1989a, ApJ, 338, 359 Parmar A. N., White N. E., Stella L., 1989b, ApJ, 338, 373 (RPW) Reig P., Coe M. J., 1998, MNRAS, 294, 118 Reig P., Stevens J. B., Coe M. J., Fabregat J., 1998, MNRAS, 301, 42 Reynolds A. P., Parmar A. N., White N. E., 1993, ApJ, 414, 302 (RPW) Stollberg M. T., 1997, PhD thesis, Univ. Alabama Sun X. J., Li T. P., Wu M., Cheng L. X., 1994, A&A, 289, 127 (SLWC) Unger S. J., Norton A. J., Coe M. J., Letho H. J., 1992, MNRAS, 256, 725 Wang Y.-M., Frank J., 1981, A&A, 93, 255 White N. E., Nagase F., Parmar A. N., 1995, in Lewin W. H. G., van Paradijs J., van den Heuvel E. P. J., eds, X-ray Binaries. Cambridge Univ. Press, Cambridge, p. 34 This paper has been typeset from a T E X=L A T E X le prepared by the author.