Volume 530, June 2011
|Number of page(s)||39|
|Published online||05 May 2011|
During the spectral fit, those sources which are not well fitted using a simple power law model can be usually well fitted by using different additional components. Here, we describe how we decided between the different models that are an acceptable fit for each source:
Leaky model: all sources for which a leaky model was selected asour best-fit model share a common spectral shape:a power law shape at high energiesthat drops around ~ 2−3 keV and an additional soft component. These sources are four type 2 AGN (XBSJ021822.2–050615, XBSJ033845.7–322253, XBSJ040758.9–712833 and XBSJ112026.7+431520) and two type 1 AGN (XBSJ091828.4+513931 and XBSJ095218.9–013643). XBSJ095218.9–013643 is a NLSy1 (narrow line Seyfert 1) whose intriguing X-ray spectral shape (a very steep photon index and large amount of absorption that partially covers the central source) and variability (variability of a factor of 4 in the soft X-rays) have been already studied in detail and presented in Grupe et al. (2004). In Fig. A.1 is shown an example of a leaky model fit. We find that leaving the soft photon index free to vary for all these sources does not significantly improve the fit. However, in all cases, this soft photon index steepens if it is left free to vary, which suggests the contribution of an additional soft component, most likely a thermal component given the low luminosity observed for this sources. In no case adding a thermal component to the leaky model significantly improves the fit, and by fitting a simple absorbed power law plus a thermal component always gives worse residuals at low energies than the leaky model.
Warm absorption: we find that an additional ionized absorber gives a best fit in five cases (XBSJ030641.0–283559, XBSJ052543.6–334856, XBSJ140100.0–110942, XBSJ140127.7+025605, XBSJ223547.9–255836, all type 1 AGN), although the ionized absorber parameters, mainly the ionization state of the absorber, are not well constrained in all cases. We selected this model as our best-fit model when the power-law residuals at low energies showed some evidence of an structured shape resembling absorption lines or edges. In two cases, the source also displays a soft-excess (XBSJ030641.0–283559 and XBSJ223547.9–255836). One example of this model is again shown in Fig. A.1. The ionized absorber was added to the neutral one because of the way the spectral fit is carried out, i.e., our base-line model is a simple power law including neutral intrinsic absorption. It is worth noting, however, that none of the sources for which the best-fit model includes warm absorption need significant additional cold absorption, as can be seen in Table 7.
Absorption edges: in three cases (XBSJ100926.5+533426, XBSJ102412.3+042023 and XBSJ204159.2–321439), an absorption edge has to be added to the simple power law model to obtain an acceptable fit. It is not clear whether these edges are real or an instrumental effect given the energies at which they are found, but they could be caused by a warm absorber that our simple Xspec absori model is not able to fit properly. See again Fig. A.1.
Reflection component: we find that a simple power law plus a neutral reflection component is a good fit in four cases (XBSJ031311.7–765428, XBSJ043448.3–775329, XBSJ052108.5–251913 and XBSJ101922.6+412049; all type 1 AGN). We use neutral reflection in all cases (pexrav model in Xspec) since our intention is not to determine where this reflection component originates in. Given the data quality, we can only estimate the amount of reflection by the reflection fraction R in the pexrav model. Nevertheless, we find that in all cases but one (XBSJ031311.7–765428) the reflection component is most likely coming from Compton-thick material far away from the central source, the putative torus in unified models, given the spectral shape, a rather flat continuum at high energies, and the characteristics of a possible Fe Kα line. Note also that all but XBSJ043448.3-775329 show a soft-excess.
Black body: a phenomenological black body component is needed to obtain an acceptable fit in 13 cases (XBSJ000532.7+200716, XBSJ005031.1–520012, XBSJ015957.5+003309, XBSJ021808.3 − 045845, XBSJ023530.2–523045, XBSJ023713.5–522734, XBSJ 031851.9–441815, XBSJ065839.5–560813, XBSJ074312.1+ 742937, XBSJ141736.3+523028, XBSJ 153456.1+013033, XBSJ225118.0–175951 and XBSJ231601.7–424038; all type 1 AGN), all showing soft-excess. The physical origin for this soft component is not clear although a host galaxy thermal contribution is ruled out given its high luminosity in all cases but XBSJ000532.7+200716. In that case the low luminosity found for the black body component (~4 × 1042 erg s-1 in the 0.5 − 2.0 keV energy range) could be caused by thermal emission, but adding an Xspec mekal component does not improve the simple power law fit. More complex models recently proposed in the literature (Crummy et al. 2006; Middleton et al. 2007) cannot be used in our case given the data quality, and in any case, they are indistinguishable in the EPIC-covered energy range. In some cases, the need for a black body component instead of a more physically motivated model, could be just due to the data quality. For example, in seven cases, XBSJ005031.1–520012, XBSJ015957.5+003309, XBSJ021808.3–045845, XBSJ065839.5–560813, XBSJ074312.1+742937, XBSJ153456.1+013033 and XBSJ225118.0–175951 (see Table 7), an ionized absorber is also a good fit, but gives worse residuals that the black body model. This could be because of to both the data quality and the need of a better representation of the ionized absorber. And for XBSJ021808.3-045845, XBSJ023713.5–522734, XBSJ074312.1+742937, XBSJ141736.3+523028 and XBSJ231601.7–424038 (see Table 8, note that XBSJ021828.3–045845 and XBSJ074312.1+742937 can be also fitted by using ionized absorption), the addition of a reflection component instead of a black body also significantly improves the fit. For the first two cases, this reflection component could derive from ionized material.
Sources for which no best fit is found: we are unable to find an acceptable fit in 11 cases. We do not find that these sources share any common characteristic, and that an acceptable fit is not found could be simply due to our selection criteria based on the resulting null hypothesis probability. The simple power law fit for these sources is shown in Fig. A.2. In the case of XBSJ021822.2–050615 and XBSJ153456.1+013033, the fit corresponds to a leaky model and a power law plus a black body, which significantly improve the fit, but not enough to obtain a probability > 10%.
Unfolded spectra corresponding to the different additional components used during the spectral fit. From top to bottom and left to right: power law plus thermal component, power law plus black body, leaky model, power law and absorption edge, power law plus reflection component and power law and ionized absorber.
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Data and residuals corresponding to the eleven sources for which no acceptable fit is found.
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Summary of XMM-Newton observations used.
Power law fit results.
© ESO, 2011
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