XMM–Newton observations of seven soft X-ray excess quasi-stellar objects

K. L. Page 1 N. Schartel 0 M. J. L. Turner 1 P. T. O'Brien 1 0 XMM-Newton Science Operations Centre, European Space Agency , Villafranca del Castillo, Apartado 50727, E-28080 Madrid, Spain 1 X-Ray and Observational Astronomy Group, Department of Physics & Astronomy, University of Leicester , Leicester LE1 7RH A B S T R A C T XMM-Newton observations of seven quasi-stellar objects (QSOs) are presented and the EPIC spectra analysed. Five of the active galactic nuclei (AGN) show evidence for Fe K emission, with three being slightly better fitted by lines of finite width; at the 99 per cent level they are consistent with being intrinsically narrow, though. The broad-band spectra can be well modelled by a combination of different temperature blackbodies (BBs) with a power law, with temperatures between kT 100 and 300 eV. On the whole, these temperatures are too high to be direct thermal emission from the accretion disc, so a Comptonization model was used as a more physical parametrization. The Comptonizing electron population forms the soft excess emission, with an electron temperature of 120-680 eV. Power-law, thermal plasma and disc BB models were also fitted to the soft X-ray excess. Of the sample, four of the active galactic nuclei are radio quiet and three radio loud. The radio-quiet QSOs may have slightly stronger soft excesses, although the electron temperatures cover the same range for both groups. 1 I N T R O D U C T I O N At energies below 2 keV, the spectra of most active galactic nuclei (AGN) show an upturn, away from the extrapolation of the highenergy (210 keV) power law. This so-called soft excess emission is thought to be common in both Seyfert galaxies and QSOs (quasi-stellar objects). The first such soft excess was identified in Mrk 841 by Arnaud et al. (1985); Turner & Pounds (1989) then found that 50 per cent of their EXOSAT sample showed steeper spectral slopes at low energies. Likewise, Walter & Fink (1993) and Schartel et al. (1996) found that the ROSAT Position Sensitive Proportional Counter spectral index tends to be significantly steeper than that measured above 2.4 keV (typically 1.9; e.g. Nandra & Pounds 1994). More recently, Pounds & Reeves (2002) have discussed the frequent presence of soft excesses in XMMNewton data. Many papers have been published about the soft excess, covering both observational results with ROSAT (e.g. Fiore et al. 1994; Piro, Matt & Ricci 1997), Ginga, EXOSAT (e.g. Saxton et al. 1993) and Einstein (e.g. Masnou et al. 1992; Zhou & Yu 1992) and theoretical work (e.g. Czerny & Elvis 1987; Czerny & Zycki 1994; Xia & Zhang 2001). It is generally thought that the soft excess may be linked to the hot tail-end of the Big Blue Bump (BBB), or is an extension of the ultraviolet (UV) band. The BBB/UV excess is likely to be owing to thermal emission from the accretion disc surrounding the black hole (e.g. Shields 1978; Malkan & Sargent 1982). However, this thermal emission is not hot enough to also account for the soft X-ray flux; hence, Comptonization is often invoked to explain the resultant emission. In this scenario, the direct thermal emission from the accretion disc is observed as the optical and UV spectrum. Some of the disc photons, however, undergo inverse Compton scattering with a population of hot electrons, thus gaining energy and producing a broader spectrum, which appears similar to a power law over a limited energy range (assuming unsaturated Comptonization). All seven objects in this paper, listed in Table 1, have been previously observed by ROSAT (Schartel et al. 1996) and were each noted to have steep photon indices over the 0.12.4z,keV ROSAT band. The Galactic absorption in the direction of each low-redshift QSO is small and it has been previously found that there is no significant evidence for additional, intrinsic absorption in any of the objects. These QSOs, therefore, represent a useful sample to investigate the soft excess in both radio-quiet and radio-loud AGN. 2 X M M N E W T O N O B S E RVAT I O N S The QSOs in this paper were observed by XMMNewton between revolutions 105 and 315 (Table 2). SAS (Science Analysis Software) v. 5.4 was used to produce the event lists for the MOS and PN EPIC (European Photon Imaging Camera) instruments, which were then filtered using XMMSELECT. For most of the objects, events covering patterns 012 (single, double, triple and quadruple events) were selected for the MOS exposures, while 04 (singles and doubles) were used for the PN, after ensuring that there were no problems with pile up in either instrument. PG 0804+761 did appear to show pile-up effects (using the SAS task EPATPLOT), though, so pattern zero spectra (i.e. single events only) were used for this object. Spectra were extracted within a circular region of 40 arcsec centred on V band 1.4-GHz flux (mJy)a Q 0056363 PG 0804+761 Mrk 1383 Mrk 876 B2 1028+31 B2 1128+31 B2 1721+34 aRadio fluxes from the NVSS. Q 0056363 PG 0804+761 Mrk 1383 Mrk 876 B2 1028+31 B2 1128+31 B2 1721+34 Observation date (rev.) 2000-07-05 (105) 2000-11-04 (166) 2000-07-28 (116) 2001-04-13 (246) 2001-08-29 (315) 2000-12-06 (182) 2000-11-22 (175) 2001-02-13 (216) 2001-02-26 (223) Exposure time (ks) MOS1 MOS2 aLW, large window; SW, small window; FF, full frame. each object; background spectra were obtained from an adjacent area of blank sky. Finally, the spectra were binned in such a way that each individual bin contained a sufficient number of counts to apply the 2 statistic (the minimum used was 25) and the energy range of each bin is larger than one third of the FWHM to avoid oversampling effects (i.e. to ensure that the bins were uncorrelated). Version 11.1.0 of XSPEC was then used to analyse the data. Table 1 lists whether the QSOs are radio loud or quiet, their redshift and Galactic absorption. The 1.4-GHz radio fluxes were taken from the National Radio Astronomy Observatories Very Large Array Sky Survey (NVSS; Condon et al. 1998) catalogue browser. The completeness limit of the survey is 2.5 mJy so, as no object was catalogued at the coordinates of Q 0056363, 2.5 mJy is taken as the upper limit to the radio flux. Table 2 gives information about the set-up of XMMNewton for each observation, while Table 3 then gives the right ascension and declination of the objects, together with source count rates and net counts. Although the objects will be referred to as QSOs throughout this paper, from their definition in the VeronVeron catalogue, their magnitudes are on the border between bright Seyfert galaxies and QSOs (M B 23) and they are thus sometimes referred to as type 1 Seyfert galaxies in the literature. H0 has been taken as 70 km s1 Mpc1, and q 0 = 0.5; errors are given at the 1 level (e.g. 2 = 2.3 for two interesting parameters). 3 S P E C T R A L A N A LY S I S 3.1 212 keV As is conventional, each background-subtracted spectrum was first simply fitted with a single power law over the entire bandpass of Gal. abs. (1020 cm2) (...truncated)