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Session 41 - RXTE Diagnostics of Active Galactic Nuclei.
Display session, Tuesday, June 09
Atlas Ballroom,

[41.03] The X-Ray Properties of NGC4258

J. K. Cannizzo (NASA/GSFC/USRA), L. J. Greenhill (CfA), J. R. Herrnstein (VLA), J. M. Moran (CfA), R. F. Mushotzky (NASA/GSFC)

We present X-ray observations taken with the Rossi X-Ray Timing Explorer of the Seyfert galaxy NGC4258 spanning 175 days from 1996 December to 1997 May. The fit to the integrated spectrum using an absorbed power law model gives an absorbing column \sim6\times10^22 cm^-2, a photon index \gamma\sim1.7, and a 2-10 keV flux \sim10^-11 erg cm^-2 s^-1. Short term variations of \sim10-30 % in these parameters were evident in the daily observations. We cannot exclude systematic effects as being the cause of the variations. No monotonic secular changes were seen in any of the parameters, except for an increase in the mean flux by a factor \sim2 over the last \sim20 days of the campaign.

Our cumulative spectrum contains about 7\times 10^5 counts. In the post-fit residuals to an absorbed power law, we note an excess between about 6 and 7 keV, and a deficit at lower and higher energies adjacent to this band. If this is verified to be a real physical effect, it would be consistent with a broad Fe feature, and would argue against the ADAF model of NGC4258 by Lasota et al in which there is no cool gas at small radii in the accretion disk close to the central black hole.

JKC was supported through the long-term scientist program under the Universities Space Research Association (USRA contract NAS5-32484) in the Laboratory for High Energy Astrophysics at Goddard Space Flight Center.

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Program listing for Tuesday