THE OPTX PROJECT. III. X-RAY VERSUS OPTICAL SPECTRAL TYPE FOR ACTIVE GALACTIC NUCLEI

Journal Article · Thu Oct 01 04:00:00 EDT 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/703/2/2160· OSTI ID:21371814

Trouille, L; Barger, A J ^[1]; Cowie, L L ^[2]; Yang, Y ^[3]; Mushotzky, R F ^[4]

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1. Department of Astronomy, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706 (United States)
2. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)
3. Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801 (United States)
4. NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771 (United States)

We compare the optical spectral types with the X-ray spectral properties for a uniformly selected (sources with fluxes greater than the 3sigma level and above a flux limit of f {sub 2-8keV} > 3.5 x 10{sup -15} erg cm{sup -2} s{sup -1}), highly spectroscopically complete (>80% for f {sub 2-8keV} > 10{sup -14} erg cm{sup -2} s{sup -1} and >60% below) 2-8 keV X-ray sample observed in three Chandra fields (CLANS, CLASXS, and the CDF-N) that cover approx1.2 deg{sup 2}. For our sample of 645 spectroscopically observed sources, we confirm that there is significant overlap of the X-ray spectral properties, as determined by the effective photon indices, GAMMA{sub eff}, obtained from the ratios of the 0.5-2 keV to 2-8 keV counts, for the different optical spectral types. For example, broad-line active galactic nuclei (AGNs) are expected to be unobscured and hence X-ray soft (GAMMA{sub eff} >= 1.2), yet we find 20%+- 3% have GAMMA{sub eff} <1.2. Non-broad-line AGNs are expected to be obscured and hence X-ray hard (GAMMA{sub eff} < 1.2), yet we find 33% +- 4% have GAMMA{sub eff} >= 1.2. Thus, one cannot use the X-ray spectral classifications and the optical spectral classifications equivalently. Since it is not understood how X-ray and optical classifications relate to the obscuration of the central engine, we strongly advise against a mixed classification scheme, as it can only complicate the interpretation of X-ray AGN samples. We confirm the dependence of optical spectral type on X-ray luminosity, and for z < 1, we find a similar luminosity dependence of GAMMA{sub eff}. However, this dependence breaks down at higher redshifts due to the highly redshift-dependent nature of GAMMA{sub eff}. We therefore also caution that any classification scheme which depends on GAMMA{sub eff} is likely to suffer from serious redshift bias.

• OSTI ID: 21371814
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 703; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BOSONS
CLASSIFICATION
COSMOLOGY
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
GALAXIES
GALAXY NUCLEI
IONIZING RADIATIONS
LUMINOSITY
MASSLESS PARTICLES
OPTICAL PROPERTIES
PHOTONS
PHYSICAL PROPERTIES
RADIATIONS
RED SHIFT
X RADIATION