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Strongly absorbed quiescent X-ray emission from the X-ray transient XTE J0421+56

We have observed the soft X-ray transient XTE J0421+56 in quiescence with XMM-Newton. The observed spectrum is highly unusual being dominated by a broad feature at 6.5 keV and can be modeled by a strongly absorbed continuum. The spectra of X-ray transients observed so far are normally modeled using Advection Dominated Accretion Flow models, black-bodies, power-laws, or by the thermal emission from a neutron star surface. The strongly absorbed X-ray emission of XTE J0421+56 could result from the compact object being embedded within the dense circumstellar wind emitted from the supergiant B[e] companion star.


Introduction
XTE J0421+56 was discovered by the All-Sky Monitor onboard RXTE as a soft X-ray transient during its 1998 April outburst (Smith et al. 1998). This outburst, one of the shortest ever detected from a transient (∼15 days) was observed by CGRO (Paciesas & Fishman 1998), RXTE (Belloni et al. 1999), ASCA (Ueda et al. 1998) and Bep-poSAX (Frontera et al. 1998;Orr et al. 1998). The outburst spectra are complex and can not be fit by any of the models usually applied to the soft X-ray transients.
The optical counterpart of XTE J0421+56 is CI Cam, also known as MWC 84. This is a supergiant B[e] star (Clark et al. 1999(Clark et al. , 2000Robinson et al. 2001). This makes XTE J0421+56 the first high mass X-ray binary (HMXB) with such a companion. Recently, Robinson et al. (2001) estimated that the distance to the source was greater than previously thought, making XTE J0421+56 among the most luminous transients. The 2-25 keV luminosity at the peak of the outburst is 3.0 ×10 38 erg s −1 , assuming the revised distance of 5 kpc.
Furthermore, Robinson et al. (2001) suggest that the compact object is traveling in a wide orbit through the dense spherical circumstellar wind emitted from the supergiant B[e] star. Within this picture, the outburst would be caused by the same disk instability mechanism responsible for the outbursts in X-ray novae. It would thus differ from the outbursts observed in Be HMXB that recur at multiples of the orbital period, probably when the compact object crosses the equatorial wind from the Be star. Robinson et al. (2001) also favor a black hole as the compact object, as infered from the absence of neutron star signatures (X-ray bursts, periodic X-ray pulsations) and from the large ratio of peak to quiescent luminosity.

XMM-Newton observation and data analysis
XTE J0421+56 was observed in quiescence by XMM-Newton on 2001 August 19. We present the spectral analysis of the EPIC PN data in the 0.2-12 keV en- ergy band where the source is detected. The analysis was carried out using the XMM-Newton Science Analysis Software. The exposure is ∼ 7 h after filtering out high background level periods and selecting single pixel events.
Source counts were extracted from a circular region of 30 ′′ radius centered on XTE J0421+56 (Figure 1). Background counts were obtained from a circular region of 90 ′′ radius offset from the source position.
The background count rate represents ∼37% of the source count rate (in the 0.2-12 keV band). The net (source minus background) count rate has a mean value of 0.0071±0.0007 counts s −1 .
In order to ensure applicability of the χ 2 statistic with so few counts, we rebinned the extracted spectrum such that at least 20 net counts per bin were present. The obtained spectrum has 9 significant bins (Figures 2 and 3). All spectral uncertainties are given at 90% confidence.

The X-ray spectrum
The spectrum of XTE J0421+56 (Figures 2 and 3) is clearly dominated by a broad feature peaking at 6.5 keV.
Considering that the feature around 6.5 keV could be due to an emission from Fe, we first tried a model consisting of a power-law to account for the low energy component and a Gaussian to account for the 6.5 keV feature. This model does not provide an acceptable fit to the data: the reduced χ 2 (χ 2 ν ) is 3.8 for 4 degrees of freedom (dof). A similar result is obtained when including photo-electric absorption. A model with a power-law and two Gaussians does not provide a good fit either.
We then adopted a different fitting approach, considering the 6.5 keV feature not as an emission line, but as a highly absorbed continuum component. This approach is consistent with the one adopted to fit the BeppoSAX spectrum of XTE J0421+56 in quiescence during the 1999 September observation . A model consisting of a power-law (photon index, α, of 1.4±0.4) plus an absorbed power-law (N H of (72±37) × 10 22 atom cm −2 , α of 2±1) was tried. This gives a χ 2 ν of 1.35 for 4 dof. An F-test indicates that adding a Gaussian at 6.5 keV does not improve the fit: the probability P rej of rejecting the hypothesis that the fit is better including the additional Gaussian is 98%.
A better fit (χ 2 ν of 1.07 for 5 dof) is obtained using a partialy absorbed power-law model, with an N H of (61 +27 −20 ) × 10 22 atom cm −2 , α of 1.5±0.5 and a partial covering fraction of 0.99 +0.007 −0.010 . This model is shown in Figures 2 and 3. Adding a Gaussian at 6.5 keV does not significantly improve the fit (P rej of 69%).
In Figure 3, the contributions of the absorbed and covered component (dashed line) and of the unabsorbed and uncovered component (doted line) to the total model are shown separetely. The absorbed and covered component clearly dominates the emission above ∼5 keV whereas the unabsorbed and uncovered component dominates the emission below ∼5 keV.
The 1-10 keV absorbed and unabsorbed (derived by setting N H to 0) fluxes are 1.3 and 9.3 × 10 −13 erg cm −2 s −1 respectively, using the partialy absorbed power-law model. This corresponds to a luminosity of 2.8 × 10 33 erg s −1 , assuming a distance of 5 kpc.

Discussion
We have shown that the quiescent X-ray spectrum of XTE J0421+56 during the XMM-Newton observation was best modeled by a strongly absorbed power-law (N H of (61 +27 −20 ) × 10 22 atom cm −2 ). Such a spectrum is unique among soft X-ray transients, and adds to the growing list of very peculiar properties observed from XTE J0421+56.
Although we note that only simple models could be tested because of the small number of bins in the spectrum, this model is very attractive because it indicates the presence of large amounts of absorbing material within the system itself. Thus, this confirms the recently drawn picture of the compact object being embedded within a very dense wind emitted from the B[e] companion star (Robinson et al. 2001).
Furthermore, it suggests that the accretion onto the compact object is still occuring at a substantial rate in order to explain the observed quiescent luminosity. This is also consistent with a compact object continuously travelling through an extended wind and not only crossing an equatorial wind during short intervals as in Be systems.