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Register a new userA BeppoSAX LECS observation of the super-soft source CAL83
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Authors
A. N. Parmar
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We report on a BeppoSAX Low-Energy Concentrator Spectrometer (LECS) observation of the super-soft source (SSS) CAL83. The X-ray emission in SSS is believed to arise from nuclear burning of accreted material on the surface of a white dwarf (WD). The LECS spectrum of CAL83 can be well fit by both absorbed blackbody and WD atmosphere models. If the absorption is constrained to be equal to the value derived from Hubble Space Telescope measurements, then the best-fit blackbody temperature is 46.4 +/- 1.4 eV while a Non Local Thermal Equilibrium (NLTE) WD atmosphere model gives a lower temperature of 32.6 +/- 0.7 eV. In contrast to CAL87, there are no strong absorption edges visible in the X-ray spectrum with a 68% confidence upper limit of 2.3 to the optical depth of a Cvi edge at 0.49 keV predicted by WD atmosphere models. The luminosity and radius derived from the NLTE fit are consistent with the values predicted for stable nuclear burning on the surface of a ~0.9-1.0 solar mass WD.
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We report on a BeppoSAX Concentrator Spectrometer observation of the super-soft source (SSS) CAL87. The X-ray emission in SSS is believed to arise from nuclear burning of accreted material on the surface of a white dwarf (WD). An absorbed blackbody spectral model gives a chi^2_v of 1.18 and a temperature of 42 +/- ^13 _11 eV. However, the derived luminosity and radius are greater than the Eddington limit and radius of a WD. Including an O viii edge at 0.871 keV gives a significantly better fit (at > 95% confidence) and results in more realistic values of the source luminosity and radius. We also fit WD atmosphere models to the CAL87 spectrum. These also give reasonable bolometric luminosities and radii in the ranges 2.7-4.8 10^{36} erg/s and 8-20 10^7 cm, respectively. These results support the view that the X-ray emission from CAL87 results from nuclear burning in the atmosphere of a WD.
In this paper I will briefly review what are, in my view, the main contributions of BeppoSAX to the understanding of the class of sources known as Soft Gamma Repeaters. These enigmatic sources were firmly identified as steady pulsars just during the operating lifetime of BeppoSAX. All the instruments onboard BeppoSAX have at some level contributed in this field with specific observations, always allowing high quality - sometimes unprecedented - studies of the quiescent counterparts or the bursting behavior of these sources. I will try to stress the results that were uniquely achieved by BeppoSAX and identify their impact on the knowledge of the physics at work in these sources.
We present a study of the source positioning accuracy of the LECS and MECS instruments on-board BeppoSAX. From the analysis of a sample of archival images we find that a systematic error, which depends on the spacecraft roll angle and has an amplitude of ~17 for the LECS and ~27 for the MECS, affects the sky coordinates derived from both instruments. The error is due to a residual misalignment of the two instruments with respect to the spacecraft Z axis arisen from the presence of attitude inaccuracies in the observations used to calibrate the pointing direction of LECS and MECS optical axes. Analytical formulae to correct LECS and MECS sky coordinates are derived. After the coordinate correction the 90% confidence level error radii are 16 and 17 for LECS and MECS respectively, improving by a factor of ~2 the source location accuracy of the two instruments. The positioning accuracy improvement presented here can significantly enhance the follow-up studies at other wavelengths of the X-ray sources observed with LECS and MECS instruments.
The Super Soft Source RX J0925.7--4758 was observed by BeppoSAX LECS and MECS on January 25--26 1997. The source was clearly detected by the LECS but only marginally detected by the MECS. We apply detailed Non-Local Thermodynamic Equilibrium (Non-LTE) models including metal line opacities to the observed LECS spectrum. We test whether the X-ray spectrum of RX J0925 is consistent with that of a white dwarf and put constraints upon the effective temperature and surface gravity by considering the presence or absence of spectral features such as absorption edges and line blends in the models and the observed spectrum. We find that models with effective temperatures above ~1e6 K or below ~7.5e5 K can be excluded. If we assume a single model component for RX J0925 we observe a significant discrepancy between the model and the data above the NeIX edge energy at 1.19 keV. This is consistent with earlier observations with ROSAT and ASCA. The only way to account for the emission above ~1.2 keV is by introducing a second spectral (plasma) component. This plasma component may be explained by a shocked wind originating from the compact object or from the irradiated companion star. If we assume log g = 9 then the derived luminosity is consistent with that of a nuclear burning white dwarf at a distance of ~4 kpc.
We have used the BeppoSAX satellite to study the broad band (0.5-100 keV) X-ray spectrum of the Seyfert 1.5 galaxy MKN6. The source is characterized by a power law of Gamma=1.7 [+0.08, -0.07] and there is no strong evidence for either a reflection bump or a high energy cut-off. We have detected a narrow iron line at 6.4 keV (rest frame) with an equivalent width of 98 [+33, -35] eV. MKN6 also exhibits strong and complex absorption. At least two components (NH_1 =1.34 [+0.4,-0.4] x 10^(22) cm^(-2) and NH_2 = 4.18 [+2.2, -1.3] x 10^(22) cm^(-2)) are present and they both partially cover the source with covering fractions of ~90% and ~50% respectively. Comparison with a previous ASCA observation indicates that in both absorbing columns the NH is variable over a 2 year timescale, while the covering fractions are constant over the same amount of time. The state of each absorber is cold or mildly photoionized. The Broad Line Region (BLR) is suggested as the possible location for this complex absorption.
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