Abstract
BeppoSAX observed the Seyfert 1.9 galaxy NGC 4258 in 1998 December, when its 2-10 keV luminosity was about 1041 ergs s-1. Large amplitude (100%) variability is observed in the 3-10 keV band on timescales of a few tens of thousands of seconds, while variability of ~20% is observed on timescales as short as 1 hr. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5 ± 1.2) × 1022 cm-2; this component is detected at up to 70 keV with a signal-to-noise ratio of 3 and with a steep power-law energy spectral index of αE = 1.11 ± 0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in advection-dominated accretion flow models with strong winds, is ruled out by the data. The ratio between the nuclear radio (22 GHz) luminosity and the X-ray (5 keV) luminosity is consistent with that of radio-quiet quasars and Seyfert galaxies. X-ray variability, spectral shape, and radio/X-ray and near-IR/X-ray luminosity ratios suggest that the nucleus of NGC 4258 could be a scaled down version of a Seyfert nucleus and that the X-ray nuclear luminosity can be explained in terms of Comptonization in a hot corona. The soft (E 2 keV) X-ray emission is complex. There are at least two thermal-like components with temperatures of 0.6 ± 0.1 keV and 1.3 keV. The cooler (L0.1-2.4 keV~ 1040 ergs s-1) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI (Cecil et al. 1994). The luminosity of the second component, which can be modeled equally well by an unobscured power-law model with αE = 0.2, is L0.1-2.4 keV~ 7 × 1039 ergs s-1, consistent with that expected from discrete X-ray sources (binaries and supernova remnants) in the host galaxy. Observations of NGC 4258 and other maser active galactic nuclei (AGNs) show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of water maser emission from X-ray illumination. So a large column density absorbing gas may be a necessary property of masing AGNs.
Highlights
The nearbyrobsrcigohptic(Bal0Tly\cla8s.5s)iÐSeAdBabs ca galaxy SeyfertNGC 4258 (M106) is 1.9 galaxy (Ho, Filippenko, & Sargent 1997 ; they Ðnd [O III]/Hb \ 10.3, and [S II]/Ha \ 0.94). Wilkes et al (1995) Ðnd relatively broad (1000 km s~1) emission lines that are strongly polarized (5%È10%), supporting the existence of an obscured active nucleus in this galaxy
The observations were performed with the BeppoSAX Narrow Field Instruments LECS (0.1È10 keV ; Parmar et al 1997), MECS (1.3È10 keV ; Boella et al 1997), HPGSPC (4È60 keV ; Manzo et al 1997), and PDS (13È200 keV ; Frontera et al 1997)
We can include in L(22 GHz) the emission slightly o†set from the dynamical center found by Herrnstein et al (1998 ; 3 and 0.5 mJy at 0.4 and 1 mas from the dynamical center, respectively). Even including this radio emission, the NGC 4258 radio/X-ray ratios are in the region occupied by radioquiet AGNs
Summary
The nearby (distance 7.2 ^ 0.3 Mpc ; Herrnstein et al.1sp9e9c9t)robsrcigohptic(Bal0Tly\cla8s.5s)iÐSeAdBabs ca galaxy Seyfert. Reynolds et al (2000) report the detection of a narrow 6.4 keV iron Ka line of equivalent Because width of the somf 1a0ll7l~`u43m27ineVos. ity in terms of the Eddington luminosity, the nuclear emission of NGC 4258 can be explained either by a standard accretion disk with a very small accretion rate or by radiatively inefficient accretion models, such as the advection-dominated accretion Ñow model (ADAF ; e.g., Narayan & Yi 1995). It drastically reduces the synchrotron emission in the radio band and the Compton emission in the optical-UV and X-ray bands (Di Matteo et al 2000) In both modelings, the near-IR (NIR) luminosity is generated by a standard geometrically thin disk, external to the ADAF transition radius. The paper is organized as follows : ° 2 presents the data and gives information on their reduction, ° 3 presents a variability analysis, ° 4 presents the spectral analyses and the broadband radio/X-ray spectral indices, and ° 5 discusses the main results on the hard nuclear component and low-energy components
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