The X-Ray Reflectors in the Nucleus of the Seyfert Galaxy NGC 1068

Based on observations of the Seyfert nucleus in NGC 1068 with ASCA, RXTE, and BeppoSAX, we report the discovery of a flare (increase in flux by a factor of ~1.6) in the 6.7 keV Fe K line component between observations obtained 4 months apart, with no significant change in the other (6.21, 6.4, and 6.97 keV) Fe Kα line components. During this time, the continuum flux decreased by ~20%. The RXTE spectrum requires an Fe K absorption edge near 8.6 keV (Fe XXIII-XXV). The spectral data indicate that the 2-10 keV continuum emission is dominated (~ of the luminosity) by reflection from a previously unidentified region of warm, ionized gas located ≲0.2 pc from the AGN. The remaining ~ of the observed X-ray emission is reflected from optically thick, neutral gas. The coronal gas in the inner narrow-line region and/or the cold gas at the inner surface of the obscuring "torus" are possible cold reflectors. The inferred properties of the warm reflector are size (diameter) ≲0.2 pc, gas density n ≳ 105.5 cm-3, ionization parameter ξ ≈ 103.5 ergs cm s-1, and covering fraction 0.003(L0/1043.5 ergs s-1)-1 < (Ω/4π) < 0.024(L0/1043.5 ergs s-1)-1, where L0 is the intrinsic 2-10 keV X-ray luminosity of the AGN. We suggest that the warm reflector gas is the source of the (variable) 6.7 keV Fe line emission and the 6.97 keV Fe line emission. The 6.7 keV line flare is assumed to be due to an increase in the emissivity of the warm reflector gas from a decrease (by 20%-30%) in L0. The properties of the warm reflector are most consistent with an intrinsically X-ray-weak AGN with L0 ≈ 1043.0 ergs s-1. The optical and UV emission that scatters from the warm reflector into our line of sight is required to suffer strong extinction, which can be reconciled if the line of sight skims the outer surface of the torus. Thermal bremsstrahlung radio emission from the warm reflector may be detectable in Very Long Baseline Array radio maps of the NGC 1068 nucleus.

[1]  A. C. Brinkman,et al.  XMM-Newton Reflection Grating Spectrometer Observations of Discrete Soft X-Ray Emission Features from NGC 1068 , 2002, astro-ph/0203290.

[2]  Shai Kaspi,et al.  High-Resolution X-Ray Spectroscopy and Modeling of the Absorbing and Emitting Outflow in NGC 3783 , 2001, astro-ph/0101540.

[3]  T. Heckman,et al.  The Seyfert-Starburst Connection in X-Rays. I. The Data , 2000, astro-ph/0012035.

[4]  D. Crenshaw,et al.  Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. III. Physical Conditions in the Emission-Line Gas , 2000, astro-ph/0007018.

[5]  G. Matt,et al.  Mapping the circumnuclear matter of NGC 1068 in X-rays , 2000 .

[6]  D. Crenshaw,et al.  Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. I. The Nature of the Continuum Emission , 1999, astro-ph/9910547.

[7]  D. Crenshaw,et al.  Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. II. Physical Conditions near the NGC 1068 “Hot Spot” , 1999, astro-ph/9910548.

[8]  Makoto Kishimoto,et al.  The Location of the Nucleus of NGC 1068 and the Three-dimensional Structure of Its Nuclear Region , 1999, astro-ph/9902346.

[9]  F. Macchetto,et al.  The Ionization Structure of the Narrow-Line Region of NGC 1068 and Its Relationship to the Extended Radio Emission , 1997 .

[10]  S. Baum,et al.  A direct image of the obscuring disk surrounding an active galactic nucleus , 1997, Nature.

[11]  C. Reynolds,et al.  An X-ray spectral study of 24 type 1 active galactic nuclei , 1997 .

[12]  A. Fabian,et al.  The iron K line complex in NGC1068: implications for X-ray reflection in the nucleus , 1997, astro-ph/9703159.

[13]  R. Antonucci,et al.  VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068 , 1996, astro-ph/9609082.

[14]  K. Nandra,et al.  ASCA Observations of Seyfert 1 Galaxies. II. Relativistic Iron Kα Emission , 1996, astro-ph/9606169.

[15]  Giorgio Matt,et al.  The iron Kα line complex in Compton-thick Seyfert 2 galaxies , 1996 .

[16]  D. A. Verner,et al.  Atomic data for astrophysics. II. New analytic fits for photoionization cross sections of atoms and ions , 1996 .

[17]  W. Sparks,et al.  Hubble Space Telescope Imaging Polarimetry of the Inner Nuclear Region of NGC 1068 , 1995 .

[18]  A. Zdziarski,et al.  Angle-dependent Compton reflection of X-rays and gamma-rays , 1995 .

[19]  C. Reynolds,et al.  Warm absorbers in active galactic nuclei , 1995, astro-ph/9502006.

[20]  J. Krolik,et al.  Observable Properties of X-Ray--heated Winds in Active Galactic Nuclei: Warm Reflectors and Warm Absorbers: Erratum , 1995, astro-ph/9501089.

[21]  A. Kinney,et al.  Multiwavelength tests of the dusty torus model for Seyfert galaxies , 1994 .

[22]  M. S. Oey,et al.  Atlas of quasar energy distributions , 1994 .

[23]  E. Pier,et al.  The intrinsic nuclear spectrum of NGC 1068 , 1994 .

[24]  K. Nandra,et al.  GINGA observations of the X-ray spectra of Seyfert galaxies. , 1994 .

[25]  G. Matt,et al.  The Contribution of the obscuring torus to the x-ray spectrum of Seyfert galaxies: A Test for the unification model , 1994, astro-ph/9401044.

[26]  Julian H. Krolik,et al.  X-ray bumps, iron K-alpha lines, and X-ray suppression by obscuring tori in Seyfert galaxies , 1994 .

[27]  D. Leahy,et al.  Monte Carlo simulations of X-ray spectra for internally illuminated spherical matter distributions , 1993 .

[28]  C. Done,et al.  Unified theories of active galactic nuclei: the hard X-ray spectrum of NGC 1068 , 1993 .

[29]  T. Phillips,et al.  High-resolution measurements, line identification, and spectral modeling of K-alpha transitions in Fe XVIII-Fe XXV , 1993 .

[30]  R. Petre,et al.  A new X-ray spectral observation of NGC 1068 , 1993 .

[31]  Robert Antonucci,et al.  Unified models for active galactic nuclei and quasars , 1993 .

[32]  J. Bland-Hawthorn,et al.  An X-ray image of the Seyfert galaxy NGC 1068 , 1992 .

[33]  K. Weaver,et al.  Hard X-Ray Tests of the Unified Model for an Ultraviolet-detected Sample of Seyfert 2 Galaxies , 1992 .

[34]  M. Whittle.  Virial and Jet-induced Velocities in Seyfert Galaxies. I. A Compilation of Narrow Line Region and Host Galaxy Properties , 1992 .

[35]  R. Goodrich,et al.  Multidirectional views of the active nucleus of NGC 1068 , 1991 .

[36]  Andrew C. Fabian,et al.  X-ray reflection from cold matter in Active Galactic Nuclei and X-ray binaries , 1991 .

[37]  J. Dickey,et al.  H I in the Galaxy , 1990 .

[38]  T. J. Turner,et al.  The EXOSAT spectral survey of AGN , 1989 .

[39]  J. Krolik,et al.  Fe K Features as Probes of the Nuclear Reflection Region in Seyfert Galaxies , 1987 .

[40]  J. Miller,et al.  Spectropolarimetry and the nature of NGC 1068 , 1985 .

[41]  R. Mushotzky.  The X-ray spectrum and time variability of narrow emission line galaxies , 1982 .

[42]  U. Feldman,et al.  High-resolution X-ray spectra of the 1979 March 25 solar flare , 1980 .

[43]  C. K. Seyfert.  Nuclear Emission in Spiral Nebulae. , 1943 .