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 αΕ = 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 (L0.1-2.4keV ∼ 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 αΕ = 0.2+0.8-0.2, is L0.1-2.4keV ∼ 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.
The BeppoSAX view of the X-ray active nucleus of NGC 4258 / Fiore F.; Pellegrini S.; Matt G.; Antonelli L.A.; Comastri A.; Della Ceca R.; Giallongo E.; Mathur S.; Molendi S.; Siemiginowska A.; Trinchieri G.; Wilkes B.. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 0004-637X. - STAMPA. - 556:1(2001), pp. 150-157. [10.1086/321530]
The BeppoSAX view of the X-ray active nucleus of NGC 4258
Pellegrini S.;Comastri A.;
2001
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 αΕ = 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 (L0.1-2.4keV ∼ 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 αΕ = 0.2+0.8-0.2, is L0.1-2.4keV ∼ 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
