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The XMM-Newton View of NGC 6251

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The XMM-Newton View of NGC 6251 A&A 413, 139–144 (2004) Astronomy DOI: 10.1051/0004-6361:20034053 & c ESO 2003 Astrophysics The XMM-Newton view of NGC 6251 M. Gliozzi1, R. M. Sambruna1,W.N.Brandt2, R. Mushotzky3, and M. Eracleous2 1 George Mason University, Dept. of Physics & Astronomy & School of Computational Sciences, 4400 University Drive, MS 3F3, Fairfax, VA 22030, USA 2 The Pennsylvania State University, Department of Astronomy & Astrophysics, 525 Davey Lab, University Park, PA 16802, USA 3 NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA Received 4 July 2003 / Accepted 20 September 2003 Abstract. We report on the nuclear X-ray properties of the radio galaxy NGC 6251 observed with XMM-Newton. NGC 6251 is a well-known radio galaxy with intermediate FRI/II radio properties. It is optically classified as a Seyfert 2 and hosts a supermassive black hole with mass 6 108 M . The 0.4–10 keV EPIC pn continuum is best fitted by two thermal components ∼ × (kT 0:5 and 1.4 keV, respectively), plus a power law with photon index Γ 1:9 absorbed by a column density NH ∼20 2 ∼ ∼ 5 10 cm− . We confirm the previous ASCA detection of a strong iron line. The line, resolved in the EPIC pn spectrum, is× adequately fitted with a broad (σ 0:6 keV) Gaussian at rest-frame energy 6.4 keV with EW 220 eV. We also detect, ∼ ∼ for the first time, short-term, low-amplitude variability of the nuclear flux on a timescale of a few ks. The spectral properties argue in favor of the presence of a standard accretion disk, ruling out the base of the jet as the sole origin of the X-rays. The moderate X-ray luminosity and lack of strong intrinsic absorption suggest that NGC 6251 is a “pure” type 2 AGN which lacks a broad-line region. Key words. galaxies: active – galaxies: nuclei – X-rays: galaxies 1. Introduction of a strong (EW 400 eV), narrow line at 6.7 keV (Sambruna et al. 1999; Turner∼ et al. 1997). ∼ NGC 6251 (z = 0.024) is a giant elliptical galaxy hosting a 8 Despite the intensive study of this source at all wave- supermassive black hole with mass MBH 4–8 10 M lengths, the nature of the accretion process in NGC 6251 is (Ferrarese & Ford 1999), as measured with∼ HST.Atradio× still a matter of debate. Based on the radio-to-X-ray spectral wavelengths, the source has been well studied. It shows the energy distribution, Ho (1999) suggested that an Advection- typical morphology of a Fanaroff-Riley II (FR II) with a very Dominated Accretion Flow (ADAF) is present in the nucleus narrow and long (>4 ) jet extending N–W (e.g., Saunders 0 of NGC 6251. On the other hand, Ferrarese & Ford (1999) and et al. 1981), which emits at X-ray wavelengths (Birkinshaw & Melia et al. (2002) favored a standard accretion disk. Similarly, Worrall 1993; Mack et al. 1997). However, based on its radio the origin of the X-ray emission from the nucleus is uncer- power at 178 MHz, NGC 6251 is classified as an FR I (e.g., tain: Turner et al. (1997) advocated a typical obscured Seyfert 2 Owen & Laing 1989). Thus, NGC 6251 occupies an interest- spectrum based on the ASCA data, while Hardcastle & Worrall ing niche in the study of radio galaxies as an intermediate ob- (2002) argued in favor of an origin from the base of the rel- ject (from the radio perspective) between low- and high-power ativistic jet for the soft X-rays. Further support for the jet- sources. dominated hypothesis was recently claimed by Chiaberge et al. In the optical, NGC 6251 has been classified as a Seyfert 2, (2003) based on spectral energy distribution arguments. On the based on the presence of permitted and forbidden emission other hand, Guainazzi et al. (2003) favor a scenario with two lines with FWHM 600 km s 1 (e.g., Shuder & Osterbrock − main spectral components: a blazar-like spectrum dominating 1981). Previous X-ray∼ studies of this galaxy with ROSAT the high-flux state and a Seyfert-like spectrum emerging during showed the presence of an unresolved nuclear source embed- the low-flux state. dedindiffuse thermal emission associated with the galaxy’s halo (Birkinshaw & Worrall 1993). ASCA observations showed With its larger sensitivity in the 0.3–10 keV band, the that the nuclear spectrum could be described as a moderately EPIC camera on-board XMM-Newton is an ideal instrument absorbed power law continuum and also suggested the presence for investigating the origin of the X-rays and the nature of the accretion process in NGC 6251 through timing and spec- Send offprint requests to: M. Gliozzi, tral analyses. Motivated by the above considerations, we ob- e-mail: [email protected] served NGC 6251 with XMM-Newton in AO1 for 50 ks. Here Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034053 140 M. Gliozzi et al.: The XMM-Newton view of NGC 6251 we report the results from the analysis of the nuclear light curve and spectrum. X-ray emission from the extended jet and halo will be discussed elsewhere (Sambruna et al. 2003). Throughout the paper we use a Friedman cosmology with H0 = 1 1 75 km s− Mpc− and q0 = 0:5. 2. Observations and data reduction We observed NGC 6251 with XMM-Newton on 2002 March 26 for a duration of 41 ks with the EPIC pn, and for 49 ks with EPIC MOS1 and∼ MOS2. All of the EPIC cameras∼ were oper- ated in full-frame mode with a medium filter for the MOS cam- eras and a thin filter for the pn. The recorded events were screened to remove known hot pixels and other data flagged as bad; only data with FLAG=0 were used. The data were processed using the latest CCD gain values, and only events corresponding to pattern 0–12 (singles, doubles, triples, and quadruples) in the MOS cameras and 0–4 (singles and dou- bles only, since the pn pixels are larger) in the pn camera were accepted. Arf and rmf files were created with the lat- est available release of the XMM-Newton Science Analysis Software (SAS 5.4). Investigation of the full–field light curves revealed the presence of a period of background flaring at the end of the observation. These events were excluded, reducing Fig. 1. EPIC pn+MOS1+MOS2 light curves of the background- the effective total exposure time to 36 ks for the EPIC pn subtracted count rate in the 0.8–10 keV band (top panel) and of the and 43 ks for the MOS cameras. The∼ RGS data of NGC 6251 X-ray color 1.8–10 keV/0.8–1.5 keV (bottom panel). The extraction have∼ a signal-to-noise ratio (S=N) that is too low for a mean- radius is 3500; time bins are 3000 s. The dashed lines indicate the av- ingful analysis. Background spectra and light curves were ex- erage values. tracted from source-free regions on the same chip as the source. There are no signs of pile-up in the pn or MOS cameras ac- XMM-Newton, using an extraction radius of 3500 and a time cording to the SAS task epatplot. With an extraction radius bin of 3000 s (changing the extraction radius or the time bin of 3500 the detected count rates in the energy range 0.4–10 keV has a negligible impact on our results). At energies below 1 1 are (0:519 0:003) s− for the MOS1, (0:533 0:004) s− for 0.8 keV no statistically significant variability is detected ac- ± 1 ± 2 the MOS2, and (1:810 0:007) s− for the pn. For compari- cording to a χ test. This might be partly due to a combina- son with previous broad-band± X-ray satellites the count rates tion of dilution from the thermal emission and intrinsic absorp- measured by the SIS0 on-board ASCA and the MECS on- tion. However, the small intrinsic absorption and the relatively 1 1 board BeppoSAX were 0:07 s− and 0:06 s− , respectively small contribution of the thermal component in the low energy (Sambruna et al. 1999; Guainazzi∼ et al.∼ 2003). range (see Sect. 4) suggest that the very-soft (0.4–0.7 keV) Inspection of an archival Chandra observation of flux might be intrinsically constant or, at most, variable at a NGC 6251 indicates that no serendipitous sources are present very low level. We therefore focus on the 0.8–10 keV en- within the EPIC extraction radius and that the contribution ergy band. The combination of pn, MOS1, and MOS2 light from the resolved kpc jet is negligible. However, no entirely re- curves increases the statistical significance of the variabil- liable information on the nuclear properties of NGC 6251 can ity. Figure 1 (top panel) shows the 0.8–10 keV background- be drawn from Chandra data, due to the unfortunate location subtracted EPIC pn+MOS1+MOS2 light curve of NGC 6251. of the nucleus on the CCDs’ gaps and to the pile-up. The bottom panel shows a plot of the hardness ratio, defined as The EPIC spectra were rebinned such that each spectral the ratio of the 1.8–10 keV count rate to the 0.8–1.5 keV count bin contains at least 100 counts for the pn and 40 counts for rate, versus time.
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