- Email: [email protected]
First high-resolution Chandra LETGS spectrum of the transient supersoft X-ray source RX J0513.9-6951
Advances in Space Research 40 (2007) 1294–1298 www.elsevier.com/locate/asr
First high-resolution Chandra LETGS spectrum of the transient supersoft X-ray source RX J0513.9-6951 V. Burwitz
a,*
, K. Reinsch b, J. Greiner a, T. Rauch c, V. Suleimanov R.E. Mennickent f, P. Predehl a
c,d
, F.W. Walter e,
a Max-Planck-Institut fu¨r extraterrestrische Physik, Giessenbachstr, D-85748 Garching, Germany Institut fu¨r Astrophysik, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany c Institut fu¨r Astronomie und Astrophysik, Sand 1, D-72076, Tu¨bingen, Germany d Kazan State University, Kremlevskaya Street 18, 420008 Kazan, Russia Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, NY 11794-3800, USA f Departamento de Fisica, Universidad de Concepcio´n, Casilla 160-C, Concepcio´n, Chile b
e
Received 21 November 2006; received in revised form 23 November 2006; accepted 23 November 2006
Abstract The transient luminous soft X-ray source RX J0513.9-6951 in the large magellanic cloud is a key object within the class of accreting binary supersoft X-ray sources. In this system, mass-transfer is thought to occur close to the Eddington limit of a solar mass white dwarf. The source switches quasi-periodically between two physically distinct states with anti-correlated X-ray and optical luminosities. We have obtained the first high-resolution X-ray spectrum of RX J0513.9-6951 on December 24, 2003 during an X-ray bright state as a 48 ks target of opportunity observation with the low energy transmission grating spectrograph (LETGS) on board Chandra. The X-ray observations were triggered using optical monitoring data obtained with ANDICAM on the 1.3-m telescope at Cerro Tololo, Chile of the SMARTS consortium. The X-ray spectrum deviates strongly from a smooth continuum and reveals complex structures which are probably a mixture of absorption and emission line patterns. Such features can be understood as a superposition of X-ray emission from a hot high-gravity stellar atmosphere and from an optically thin corona-like plasma enshrouding the system. Here, we present first results of our spectral analysis of the LETGS data using white dwarf atmosphere codes. 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Binaries: close; Stars: individual: RX J0513.9-6951; Magellanic clouds; X-rays: stars
1. Introduction Luminous supersoft stellar X-ray sources (SSS), first recognized by the Einstein observatory (Long et al., 1981), have been firmly established as a distinct class of objects by ROSAT (Tru¨mper et al., 1991). They are observationally distinguished by their very soft X-ray spectra with blackbody temperatures from 10 to 80 eV and luminosities of 1036–1038 erg/s (Kahabka and van den Heuvel, 1997). Currently, about 100 SSS are known, most of them
*
Corresponding author. E-mail address: [email protected] (V. Burwitz).
in the Magellanic Clouds and other nearby galaxies (Greiner, 2000; see also Kahabka, 2006). Several SSS have been identified as accreting close binaries with orbital periods of 1 day or less. They are interpreted as white dwarfs which accrete matter from a more _ acc just suffimassive main-sequence secondary at a rate M cient to permit (quasi-)stable nuclear burning near its surface (van den Heuvel et al., 1992). This implies that the luminosity must be close to the Eddington limit of a solar mass object. Stable burning stops below 107 Mx/yr, where shell flashes begin. The conventional model predicts _ acc > 4 107 M =yr, a red-giant envelope develthat at M ops and X-ray emission is temporarily quenched (van den Heuvel et al., 1992). Hachisu et al. (1996), however, have
0273-1177/$30 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.11.036
V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
shown that no static envelope solution on the white dwarf _ acc > 106 M =yr. Instead, excess matter should exists for M be expelled by a strong wind, providing a potential channel whereby the white dwarf mass can grow to near the Chandrasekhar limit. The transient luminous soft X-ray source RX J0513. 9-6951 (hereafter RX J0513) in the LMC (Schaeidt et al., 1993; Cowley et al., 1993; Pakull et al., 1993; Crampton et al., 1996) is a key object within the class of accreting binary supersoft X-ray sources. It undergoes optical low states accompanied by X-ray outbursts which last 30–50 days and repeat about every 140–180 days (Fig. 1). The rather sudden changes of the soft X-ray flux have been explained as the direct response of a white dwarf accreting close to the Eddington critical limit to slight changes of the mass-transfer rate (Pakull et al., 1993; Reinsch et al., 1996; MeyerHofmeister et al., 1997). While the hot white dwarf is most of the time enshrouded by a large envelope, a small drop of the mass-flow rate leads to the collapse of the envelope. A decrease of the photospheric radius by, e.g., a factor of 4 implies that the effective temperature rises by a factor of 2 and the Wien tail of the EUV/soft X-ray emission of the source becomes detectable in the soft X-ray range. This model of a contracting and expanding envelope of the hot central star at almost constant bolometric luminosity, and the associated variations in the irradiation of the accretion disk can quantitatively explain the observed X-ray and optical variability (Reinsch et al., 1996). Based on an extensive X-ray monitoring of RX J0513 with the ROSAT HRI, Reinsch et al. (2000) have shown that the radius changes of the white dwarf envelope occur on
1295
the Kelvin–Helmholtz time scale and that the duration of the X-ray on and off states is compatible with the viscous time scales of the inner and outer accretion disk, respectively. In this scenario, the mass-flow rate at the surface of the white dwarf varies while the mass-transfer rate from the secondary star remains constant. This model predicts that at the beginning of the X-ray on state, the effective radius of the white dwarf reaches a minimum and, hence, its effective temperature reaches a maximum. In the course of the X-ray outburst, the accretion rate on the white dwarf is expected to rise and its effective temperature will start to decrease while a new envelope forms. As an alternative model, Hachisu and Kato (2003a,b) have suggested a self-sustained mechanism based on an optically thick wind model of accreting white dwarfs to explain the long-term lightcurve variations of RX J0513. Their ‘‘accretion wind evolution’’ model predicts just the opposite evolution of the white dwarf photosphere radius and the effective temperature during an X-ray on state (from 0.09 Rx and 280,000 K at the start to 0.04 Rx and 430,000 K after about 20–25 days). 2. Observations To observe RX J0513 during its X-ray bright state it is necessary to monitor the optical lightcurve on a quasi-daily basis in order to detect the exact time when it enters its optical faint state. An optical monitoring program was performed using telescopes and instruments run by the SMARTS Consortium. The monitoring program started
Fig. 1. R-filter SMARTS 1.3-m + ANDICAM lightcurve of RX J0513.9-6951 during the SMARTS 03B semester (August 2003/January 2004). The left vertical line corresponds to the optical trigger date and the right vertical line indicates the time of the Chandra LETGS observation.
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V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
in mid August 2003 and continued until the end of January 2004, using the ANDICAM at the 1.3 m CTIO Telescope. The R-filter images where taken every 1–2 days. The optical data was processed in the standard way, the images were bias- and dark-subtracted, flat-fielded, and relative aperture photometry was performed for RX J0513 to obtain the lightcurve. Based on this photometry a target of opportunity observation with the Chandra LETGS was triggered on December 15, 2003, when the optical lightcurve indicated that RX J0513 was becoming fainter and going into its optical low state. The Chandra LETGS observation (ObsID 3503) was then performed starting on December, 24, 2003 at 19:03 UT and lasting a total of 47.69 ks. The X-ray data was reprocessed and the X-ray spectra were extracted using the Chandra X-ray Center CIAO data analysis package. 3. Spectral analysis As a starting point, we have fitted the X-ray spectrum of RX J0513 with a simple black body spectrum. The fit parameters are shown in Table 1. The best black body fit
to the count-rate spectrum is shown in Fig. 2. The NH value obtained is very close to that found by Ga¨nsicke et al. (1998) from HST spectra. Therefore, we have frozen this parameter at a value of NH = 7 · 1020 cm2 for the remaining fits. It is obvious that the observed spectrum deviates strongly from a blackbody and other smooth continuum models. Instead, it shows a forest of emission and/or ˚ , similar to those absorption structures between 25 and 50 A found in CAL 83 (Paerels et al., 2001; Lanz et al., 2005). The latter were interpreted as parts of a continuum between strong absorption blends. Therefore, in the next step we have fitted the observed spectrum with our hot WD model-atmosphere spectra. The model atmospheres were constructed following standard temperature correction methods in order to satisfy the hydrostatic and radiative equilibrium constraints by using a modified version of Kurucz’s code ATLAS5 (Kurucz, 1970). The 15 most abundant elements from H through Ni are included with bound-free opacities computed from the photoionization cross-sections of Verner and Yakovlev (1995). Line blanketing is included using about 26000 spectral lines from the CHIANTI, Version 3.0, atomic database (Dere
Table 1 Fitting parameters of the RX J0513 spectrum for different models and derived physical parameters of the source Model
T (105 K)
NH (1020 cm2)
R (109 cm)
LBol (1037 erg cm2 s1)
Black body One T WD atm. Two T WD atm.
þ0:25 5:040:77
7:11þ2:67 0:63
4.65 3.85 6.11
7.00 7.00 7.00
2.53 1.32 2
29.4 5.75 7.5
f (%)
96.5 3.5
v2/dof 3.57/180 6.49/181 4.8/179
R, source radius; f, fraction of the emitting surface for given model components.
Fig. 2. Top panel: Chandra LETG count-rate spectrum of RX J0513 in which each spectral bin contains at least 100 counts. A blackbody model with kTbb = 45 eV and NH = 6.5 · 1020 cm2 is overplotted. The bottom panel shows clearly the deviations from a smooth blackbody spectrum.
V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
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Fig. 3. Observed spectrum (black) and absorbed LTE white dwarf atmosphere model (grey) (log g = 8.0, T = 420 kK), and NH = 6.5 · 1020 cm2. Both ˚ bins and smoothed with a boxcar of 10. spectrum and model are rebinned into 0.05 A
et al., 1997). The grid covers Teff = 200–700 kK and log g = 7.5, 8.0, 8.5, and 9.0 with solar and LMC abundances (Fig. 4). A previous version of these models with less spectral lines (about 1200) had successfully been used to fit the spectra of SSS (Swartz et al., 2002; Ibragimov et al., 2003) and to derive fundamental system parameters of SSS (Suleimanov and Ibragimov, 2003).
The best WD model spectral fit is shown in Fig. 3. For this model log g = 8.5 and LMC chemical composition was taken. Similar fits (withP comparable reduced v2 values) were obtained for other log g and solar chemical compositions. It is obvious that this fit is worse than the black body fit. But we found that the v2 vs. WD model effective temperature dependence has a secondary minimum at higher
Fig. 4. Comparison of 3 LTE white dwarf atmosphere models at different temperatures (from top to bottom 680, 560, and 440 kK) at log g = 8.5. Spectra ˚ bins and smoothed with a boxcar of 10. are rebinned into 0.05 A
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V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
temperatures. Therefore, we have fitted the observed spectrum with a two component spectrum with different effective temperatures. The high temperature (Teff = 611 kK) ˚ ) part of model spectrum fits the short-wavelength (630 A the observed spectrum and the low temperature (Teff = 385 kK) model spectrum fits the long wavelength part of the spectrum. Both models have log g = 8.5 and LMC chemical abundances. Similar fits were obtained for some other values of log g and chemical composition, but they had slightly larger v2 values. In summary, our fits have shown that the observed spectrum of RX J0513 appears to be more complicated than a single-temperature model spectrum of a hot white dwarf. A possible reason for this may be fast rotation of the white dwarf in RXJ0513, which leads to an ellipsoidal form of the white dwarf envelope. Hydrogen burns at the bottom of this envelope and the fast rotation can reveal hot polar caps and a relatively cool equatorial belt in the envelope. The relation of the obtained normalization constants (R/d)2 suggests that the surface-area of the cool component is about 25 times larger than that of the hot component. Alternatively, the envelope could be structured, leading to a partial covering of the hot component by cooler material. 4. Summary and conclusions In this pilot study with the Chandra LETGS, we have obtained the first high-resolution X-ray spectrum of RX J0513. Our observations provide an instantaneous diagnostic of the accreting object, but no information on the temporal evolution of the photospheric parameters. Our spectral analysis with line-blanketed LTE models ˚ , of the suggests that the short-wavelength part, k < 40 A spectrum may be strongly modified by the white dwarf’s radiation-pressure driven wind, and by radiation from the accretion-disc corona. This, in turn, suggests that per˚ ) of the spechaps only the long-wavelength part (k > 40 A trum should be used to obtain reliable values of the effective temperature and radius of RX J0513. On the whole, our model fits are still far from perfect and do not include the possible contribution of emission lines to the spectrum. The observed features could be understood as a superposition of X-ray emissions from a hot, high-gravity stellar atmosphere and an optically thin, corona-like plasma enshrouding the system. References Cowley, A., Schmidtke, P.C., Hutchings, J.B., et al. Detection, identification, and observed properties of large magellanic cloud supersoft X-ray sources. ApJ 418, L63–L66, 1993.
Crampton, D., Hutchings, J.B., Cowley, A.P., et al. The large magellanic cloud supersoft X-ray binary RX J0513.9-6951. ApJ 456, 320–328, 1996. Dere, K.P., Landi, E., Mason, H., et al. CHIANTI – an atomic database for emission lines. A&AS 125, 149S–173S, 1997. Ga¨nsicke, B.T., van Teeseling, A., Beuermann, K., et al. Hubble space telescope ultraviolet spectroscopy of the supersoft X-ray binaries CAL83 and RXJ0513.9-6951. A&A 333, 163–171, 1998. Greiner, J. Catalog of supersoft X-ray sources. New Astr. 5, 137–141, 2000. Hachisu, I., Kato, M., Nomoto, K. A new model for progenitor systems of type IA supernovae. ApJ 470, L97–L100, 1996. Hachisu, I., Kato, M. A new clue to the transition mechanism between optical high and low states of the supersoft X-ray source RX J0513.96951, implied by the recurrent nova CI Aquilae 2000 outburst model. ApJ 588, 1003–1008, 2003a. Hachisu, I., Kato, M. RX J0513.9-6951: the first example of accretion wind evolution, a key evolutionary process to type IA supernovae. ApJ 590, 445–459, 2003b. Ibragimov, A., Suleimanov, V., Vikhlinin, A., et al. Supersoft X-ray sources. Parameters of stellar atmospheres. Astr. Rep. 47, 186–196, 2003. Kahabka, P. Supersoft X-ray sources. Adv. Space Res. 38, 2836–2839, 2006. Kahabka, P., van den Heuvel, E.P.J. Luminous supersoft X-ray sources. ARA&A 35, 69–100, 1997. Kurucz, R.L. Atlas: a computer program for calculating model stellar atmospheres. SAO Special Rep. 309, 1–291, 1970. Lanz, T., Telis, G.A., Audard, M., et al. Non-LTE model atmosphere analysis of the Large Magellanic Cloud supersoft X-ray source CAL 83. ApJ 619, 517–526, 2005. Long, K.S., Helfand, D.J., Grabelsky, D.A. A soft X-ray study of the large magellanic cloud. ApJ 248, 925–944, 1981. Meyer-Hofmeister, E., Schandl, S., Meyer, F. The structure of the accretion disk rim in supersoft X-ray sources. A&A 321, 245–253, 1997. Paerels, F., Rasmussen, A.P., Hartmann, H.W., et al. A high resolution spectroscopic observation of CAL 83 with XMM-Newton/RGS. A&A 365, L308–L311, 2001. Pakull, M.W., Motch, C., Bianchi, L., et al. Optical/UV counterpart of the supersoft transient X-ray source RX J0513.9-6951 in the Large Magellanic Cloud. A&A 278, L39–L42, 1993. Reinsch, K., van Teeseling, A., Beuermann, K., et al. Optical low states of the supersoft X-ray source RX J0513.9-6951. A&A 309, L11–L14, 1996. Reinsch, K., van Teeseling, A., King, A.R., et al. A limit-cycle model for the binary supersoft X-ray source RX J0513.9-6951. A&A 354, L37– L40, 2000. Schaeidt, S., Hasinger, G., Tru¨mper, J. Discovery of a variable supersoft X-ray source in the Large Magellanic Cloud during the ROSAT AllSky Survey. A&A 270, L9–L12, 1993. Suleimanov, V., Ibragimov, A. Supersoft X-ray sources: basic parameters. Astr. Rep. 47, 197–205, 2003. Swartz, D., Ghosh, K., Suleimanov, V., et al. Chandra discovery of luminous supersoft X-ray sources in M81. ApJ 574, 382–397, 2002. Tru¨mper, J., Hasinger, G., Aschenbach, B., et al. X-ray survey of the Large Magellanic Cloud by ROSAT. Nature 349, 579–583, 1991. van den Heuvel, E.P.J., Bhattacharya, B., Nomoto, N., et al. Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC. A&A 262, 97–105, 1992. Verner, D., Yakovlev, D.G. Analytic FITS for partial photoionization cross sections. A&AS 109, 125–133, 1995.
First high-resolution Chandra LETGS spectrum of the transient supersoft X-ray source RX J0513.9-6951 V. Burwitz
a,*
, K. Reinsch b, J. Greiner a, T. Rauch c, V. Suleimanov R.E. Mennickent f, P. Predehl a
c,d
, F.W. Walter e,
a Max-Planck-Institut fu¨r extraterrestrische Physik, Giessenbachstr, D-85748 Garching, Germany Institut fu¨r Astrophysik, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany c Institut fu¨r Astronomie und Astrophysik, Sand 1, D-72076, Tu¨bingen, Germany d Kazan State University, Kremlevskaya Street 18, 420008 Kazan, Russia Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, NY 11794-3800, USA f Departamento de Fisica, Universidad de Concepcio´n, Casilla 160-C, Concepcio´n, Chile b
e
Received 21 November 2006; received in revised form 23 November 2006; accepted 23 November 2006
Abstract The transient luminous soft X-ray source RX J0513.9-6951 in the large magellanic cloud is a key object within the class of accreting binary supersoft X-ray sources. In this system, mass-transfer is thought to occur close to the Eddington limit of a solar mass white dwarf. The source switches quasi-periodically between two physically distinct states with anti-correlated X-ray and optical luminosities. We have obtained the first high-resolution X-ray spectrum of RX J0513.9-6951 on December 24, 2003 during an X-ray bright state as a 48 ks target of opportunity observation with the low energy transmission grating spectrograph (LETGS) on board Chandra. The X-ray observations were triggered using optical monitoring data obtained with ANDICAM on the 1.3-m telescope at Cerro Tololo, Chile of the SMARTS consortium. The X-ray spectrum deviates strongly from a smooth continuum and reveals complex structures which are probably a mixture of absorption and emission line patterns. Such features can be understood as a superposition of X-ray emission from a hot high-gravity stellar atmosphere and from an optically thin corona-like plasma enshrouding the system. Here, we present first results of our spectral analysis of the LETGS data using white dwarf atmosphere codes. 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Binaries: close; Stars: individual: RX J0513.9-6951; Magellanic clouds; X-rays: stars
1. Introduction Luminous supersoft stellar X-ray sources (SSS), first recognized by the Einstein observatory (Long et al., 1981), have been firmly established as a distinct class of objects by ROSAT (Tru¨mper et al., 1991). They are observationally distinguished by their very soft X-ray spectra with blackbody temperatures from 10 to 80 eV and luminosities of 1036–1038 erg/s (Kahabka and van den Heuvel, 1997). Currently, about 100 SSS are known, most of them
*
Corresponding author. E-mail address: [email protected] (V. Burwitz).
in the Magellanic Clouds and other nearby galaxies (Greiner, 2000; see also Kahabka, 2006). Several SSS have been identified as accreting close binaries with orbital periods of 1 day or less. They are interpreted as white dwarfs which accrete matter from a more _ acc just suffimassive main-sequence secondary at a rate M cient to permit (quasi-)stable nuclear burning near its surface (van den Heuvel et al., 1992). This implies that the luminosity must be close to the Eddington limit of a solar mass object. Stable burning stops below 107 Mx/yr, where shell flashes begin. The conventional model predicts _ acc > 4 107 M =yr, a red-giant envelope develthat at M ops and X-ray emission is temporarily quenched (van den Heuvel et al., 1992). Hachisu et al. (1996), however, have
0273-1177/$30 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.11.036
V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
shown that no static envelope solution on the white dwarf _ acc > 106 M =yr. Instead, excess matter should exists for M be expelled by a strong wind, providing a potential channel whereby the white dwarf mass can grow to near the Chandrasekhar limit. The transient luminous soft X-ray source RX J0513. 9-6951 (hereafter RX J0513) in the LMC (Schaeidt et al., 1993; Cowley et al., 1993; Pakull et al., 1993; Crampton et al., 1996) is a key object within the class of accreting binary supersoft X-ray sources. It undergoes optical low states accompanied by X-ray outbursts which last 30–50 days and repeat about every 140–180 days (Fig. 1). The rather sudden changes of the soft X-ray flux have been explained as the direct response of a white dwarf accreting close to the Eddington critical limit to slight changes of the mass-transfer rate (Pakull et al., 1993; Reinsch et al., 1996; MeyerHofmeister et al., 1997). While the hot white dwarf is most of the time enshrouded by a large envelope, a small drop of the mass-flow rate leads to the collapse of the envelope. A decrease of the photospheric radius by, e.g., a factor of 4 implies that the effective temperature rises by a factor of 2 and the Wien tail of the EUV/soft X-ray emission of the source becomes detectable in the soft X-ray range. This model of a contracting and expanding envelope of the hot central star at almost constant bolometric luminosity, and the associated variations in the irradiation of the accretion disk can quantitatively explain the observed X-ray and optical variability (Reinsch et al., 1996). Based on an extensive X-ray monitoring of RX J0513 with the ROSAT HRI, Reinsch et al. (2000) have shown that the radius changes of the white dwarf envelope occur on
1295
the Kelvin–Helmholtz time scale and that the duration of the X-ray on and off states is compatible with the viscous time scales of the inner and outer accretion disk, respectively. In this scenario, the mass-flow rate at the surface of the white dwarf varies while the mass-transfer rate from the secondary star remains constant. This model predicts that at the beginning of the X-ray on state, the effective radius of the white dwarf reaches a minimum and, hence, its effective temperature reaches a maximum. In the course of the X-ray outburst, the accretion rate on the white dwarf is expected to rise and its effective temperature will start to decrease while a new envelope forms. As an alternative model, Hachisu and Kato (2003a,b) have suggested a self-sustained mechanism based on an optically thick wind model of accreting white dwarfs to explain the long-term lightcurve variations of RX J0513. Their ‘‘accretion wind evolution’’ model predicts just the opposite evolution of the white dwarf photosphere radius and the effective temperature during an X-ray on state (from 0.09 Rx and 280,000 K at the start to 0.04 Rx and 430,000 K after about 20–25 days). 2. Observations To observe RX J0513 during its X-ray bright state it is necessary to monitor the optical lightcurve on a quasi-daily basis in order to detect the exact time when it enters its optical faint state. An optical monitoring program was performed using telescopes and instruments run by the SMARTS Consortium. The monitoring program started
Fig. 1. R-filter SMARTS 1.3-m + ANDICAM lightcurve of RX J0513.9-6951 during the SMARTS 03B semester (August 2003/January 2004). The left vertical line corresponds to the optical trigger date and the right vertical line indicates the time of the Chandra LETGS observation.
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V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
in mid August 2003 and continued until the end of January 2004, using the ANDICAM at the 1.3 m CTIO Telescope. The R-filter images where taken every 1–2 days. The optical data was processed in the standard way, the images were bias- and dark-subtracted, flat-fielded, and relative aperture photometry was performed for RX J0513 to obtain the lightcurve. Based on this photometry a target of opportunity observation with the Chandra LETGS was triggered on December 15, 2003, when the optical lightcurve indicated that RX J0513 was becoming fainter and going into its optical low state. The Chandra LETGS observation (ObsID 3503) was then performed starting on December, 24, 2003 at 19:03 UT and lasting a total of 47.69 ks. The X-ray data was reprocessed and the X-ray spectra were extracted using the Chandra X-ray Center CIAO data analysis package. 3. Spectral analysis As a starting point, we have fitted the X-ray spectrum of RX J0513 with a simple black body spectrum. The fit parameters are shown in Table 1. The best black body fit
to the count-rate spectrum is shown in Fig. 2. The NH value obtained is very close to that found by Ga¨nsicke et al. (1998) from HST spectra. Therefore, we have frozen this parameter at a value of NH = 7 · 1020 cm2 for the remaining fits. It is obvious that the observed spectrum deviates strongly from a blackbody and other smooth continuum models. Instead, it shows a forest of emission and/or ˚ , similar to those absorption structures between 25 and 50 A found in CAL 83 (Paerels et al., 2001; Lanz et al., 2005). The latter were interpreted as parts of a continuum between strong absorption blends. Therefore, in the next step we have fitted the observed spectrum with our hot WD model-atmosphere spectra. The model atmospheres were constructed following standard temperature correction methods in order to satisfy the hydrostatic and radiative equilibrium constraints by using a modified version of Kurucz’s code ATLAS5 (Kurucz, 1970). The 15 most abundant elements from H through Ni are included with bound-free opacities computed from the photoionization cross-sections of Verner and Yakovlev (1995). Line blanketing is included using about 26000 spectral lines from the CHIANTI, Version 3.0, atomic database (Dere
Table 1 Fitting parameters of the RX J0513 spectrum for different models and derived physical parameters of the source Model
T (105 K)
NH (1020 cm2)
R (109 cm)
LBol (1037 erg cm2 s1)
Black body One T WD atm. Two T WD atm.
þ0:25 5:040:77
7:11þ2:67 0:63
4.65 3.85 6.11
7.00 7.00 7.00
2.53 1.32 2
29.4 5.75 7.5
f (%)
96.5 3.5
v2/dof 3.57/180 6.49/181 4.8/179
R, source radius; f, fraction of the emitting surface for given model components.
Fig. 2. Top panel: Chandra LETG count-rate spectrum of RX J0513 in which each spectral bin contains at least 100 counts. A blackbody model with kTbb = 45 eV and NH = 6.5 · 1020 cm2 is overplotted. The bottom panel shows clearly the deviations from a smooth blackbody spectrum.
V. Burwitz et al. / Advances in Space Research 40 (2007) 1294–1298
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Fig. 3. Observed spectrum (black) and absorbed LTE white dwarf atmosphere model (grey) (log g = 8.0, T = 420 kK), and NH = 6.5 · 1020 cm2. Both ˚ bins and smoothed with a boxcar of 10. spectrum and model are rebinned into 0.05 A
et al., 1997). The grid covers Teff = 200–700 kK and log g = 7.5, 8.0, 8.5, and 9.0 with solar and LMC abundances (Fig. 4). A previous version of these models with less spectral lines (about 1200) had successfully been used to fit the spectra of SSS (Swartz et al., 2002; Ibragimov et al., 2003) and to derive fundamental system parameters of SSS (Suleimanov and Ibragimov, 2003).
The best WD model spectral fit is shown in Fig. 3. For this model log g = 8.5 and LMC chemical composition was taken. Similar fits (withP comparable reduced v2 values) were obtained for other log g and solar chemical compositions. It is obvious that this fit is worse than the black body fit. But we found that the v2 vs. WD model effective temperature dependence has a secondary minimum at higher
Fig. 4. Comparison of 3 LTE white dwarf atmosphere models at different temperatures (from top to bottom 680, 560, and 440 kK) at log g = 8.5. Spectra ˚ bins and smoothed with a boxcar of 10. are rebinned into 0.05 A
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temperatures. Therefore, we have fitted the observed spectrum with a two component spectrum with different effective temperatures. The high temperature (Teff = 611 kK) ˚ ) part of model spectrum fits the short-wavelength (630 A the observed spectrum and the low temperature (Teff = 385 kK) model spectrum fits the long wavelength part of the spectrum. Both models have log g = 8.5 and LMC chemical abundances. Similar fits were obtained for some other values of log g and chemical composition, but they had slightly larger v2 values. In summary, our fits have shown that the observed spectrum of RX J0513 appears to be more complicated than a single-temperature model spectrum of a hot white dwarf. A possible reason for this may be fast rotation of the white dwarf in RXJ0513, which leads to an ellipsoidal form of the white dwarf envelope. Hydrogen burns at the bottom of this envelope and the fast rotation can reveal hot polar caps and a relatively cool equatorial belt in the envelope. The relation of the obtained normalization constants (R/d)2 suggests that the surface-area of the cool component is about 25 times larger than that of the hot component. Alternatively, the envelope could be structured, leading to a partial covering of the hot component by cooler material. 4. Summary and conclusions In this pilot study with the Chandra LETGS, we have obtained the first high-resolution X-ray spectrum of RX J0513. Our observations provide an instantaneous diagnostic of the accreting object, but no information on the temporal evolution of the photospheric parameters. Our spectral analysis with line-blanketed LTE models ˚ , of the suggests that the short-wavelength part, k < 40 A spectrum may be strongly modified by the white dwarf’s radiation-pressure driven wind, and by radiation from the accretion-disc corona. This, in turn, suggests that per˚ ) of the spechaps only the long-wavelength part (k > 40 A trum should be used to obtain reliable values of the effective temperature and radius of RX J0513. On the whole, our model fits are still far from perfect and do not include the possible contribution of emission lines to the spectrum. The observed features could be understood as a superposition of X-ray emissions from a hot, high-gravity stellar atmosphere and an optically thin, corona-like plasma enshrouding the system. References Cowley, A., Schmidtke, P.C., Hutchings, J.B., et al. Detection, identification, and observed properties of large magellanic cloud supersoft X-ray sources. ApJ 418, L63–L66, 1993.
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