HETG
Follow-on Science Instrument
Contract NAS8-01129
Monthly Status Report No. 004
June 2002
HETG Science Theme: Active Galactic Nuclei and Jets
Prepared in accordance with DR 972MA-002
DPD #972
Prepared for
National Aeronautics and Space Administration
Marshall Space Flight Center, Alabama 35812
Center for Space
Research; Massachusetts Institute of Technology; Cambridge, MA 02139
Active Galactic Nuclei and Jets Progress
Introduction
In the center (“nucleus”) of many galaxies is a massive black hole, with a mass of one million to one billion times the mass of the Sun. The strong gravitational field of this black hole attracting surrounding gas and stars leads to complex energetic structures around the black hole: torus, disk, and jets. These hot structures can be seen in the radio, optical and x-ray bands. There are many categories of observed AGN depending on the mass of the black hole, the environment around it, and also the orientation that we view it from. The same fundamental process may be behind all of these different-appearing objects.
Introductory material on
AGN and the figures above are available starting at:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html
Summary of AGN GTO Observations
There were several types of Active Galactic Nuclei (AGN) observed with the GTO time. One source is a BL Lac type object (PKS 2155-304), several are radio loud AGN (M 87, NGC 1275, PKS 2149-306, Q0836+7104, 3C 279), many are normal Seyfert (Sy) 1 galaxies (MCG-6-30-15, NGC 4151, NGC 7469, 1H1821+643, Mrk 766, 1ES1028+511) and the remainder are low luminosity Sy 2 galaxies, whose nuclei are obscured (NGC 1068, NGC 5506, IRAS18325-5926). The scientific objectives varied with the source type.
BL Lac Objects
The
BL Lac object PKS 2155-304 was observed
in order to verify and analyze absorption features against the bright continuum
spectrum. Canizares & Kruper (1984) discovered a deep absorption feature
in the spectrum of this source in an Einstein Objective Grating Spectrometer
observation. It was interpreted as blue-shifted absorption from ionized oxygen
in a jet along the line of sight. The HETGS observations were proposed in
order to resolve this feature better to perhaps determine the velocity
structure of the jet. The actual spectra have not shown the expected
absorption though Marshall (2000, in proceedings of a symposium on
Astrophysical Phenomena Revealed by Space VLBI see figure) showed that the
spectrum of PKS 2155-304 and another BL Lac object, Mrk 421, were well fit by
power law models without significant broad absorption features. Later detailed
study revealed a narrow absorption feature that can be attributed to material
between us and the BL Lac (Fang et al. 2002, Ap. J. Letters, 572,
L127).
The spectrum of PKS 2155-304, at left, is colloquially described as “a boring power law”: the clean straight spectra are a tribute to instrument calibration but contain little high-resolution spectral information.
Radio Loud Quasars
These sources were observed for three reasons. First, two of the targets reside in the deep potentials of large galaxy clusters (M 87 in the Virgo cluster and NGC 1275 in the Perseus cluster) so there was the expectation of observing the cooling gas in emission from an extended region and in absorption against the AGN point source continuum. Four of these sources have extended radio emission and jets that were expected to be detectable. Finally, the bright sources were good "lamps" for observing absorption from intervening material in the intergalactic medium (IGM).
The observation of NGC 1275 showed the core X-ray nicely but the spectrum was disappointing, being simply a pure power law and it was not strong enough to provide good absorption lines. Marshall (2000) mentioned the HETGS results on NGC 1275 but there is no other report yet. The imaging data were superseded by direct ACIS observations, published by Fabian et al. (2000, MNRAS, 318, 65) with one member of the HETGS team (P. Ogle). The spectra of two of the radio loud AGN were published in an IGM study (PKS 2149-306 and Q0836+71) but were otherwise unremarkable.
The zeroth order data for M 87 showed significant detail in the X-ray jet (Marshall et al. ApJ, 564, L683). There was a good match between features in the radio and optical images (see Figure on next page) so that the spectral energy distributions (SEDs) over the radio—optical—X-ray range could be measured for many of the knots. These SEDs showed significant evolution along the jet in the sense that the X-ray flux drops relative to the optical flux systematically along the jet. The interpretation of this effect in terms of jet models is not yet understood.
The jet in M 87 is shown, top to bottom, in i) Radio, ii) Optical (HST), iii) X-Ray (Chandra) images.
In the final panel iv) the X-ray image is overlaid
with contours from the optical smoothed to Chandra resolution.
Normal Seyfert 1 Galaxies and Quasars
Some of the more luminous AGN seem to have rather featureless spectra even if not radio loud, such as 1H1821+643 and 1ES1028+511. Both targets were observed in order to find absorption features in the intervening IGM (more on this in a future report). Fang et al. (2002, ApJ, 565, 86) showed that the spectrum had a strong neutral Fe K line and a weak Fe XXVI line which are thought to arise in the atmosphere of an accretion disk. The spectrum of 1ES1028+511 has not yet been analyzed.
MCG--6-30-15 has provided a bounty of spectral features in its HETGS spectrum that can be attributed to warm gas along the line of sight within the host galaxy. Lee et al. (2001, ApJ, 554, L13) presented the 0.5-1.0 keV portion of the spectrum to show that it is rich with absorption lines from a moderately ionized plasma (see figure next page). An absorption edge is observed near 0.7 keV (in the rest frame) while the O VII edge was expected near 0.74 keV. The shift is thought to result from significant absorption by neutral Fe. In addition to the absorption lines, MCG-6-30-15 has a strong Fe K line that is broad and a weaker slightly broadened component. Lee et al. (2002, ApJ, 570, L47) showed that the broad line was similar to that found in previous ASCA observations but that the lack of a strong narrow line indicated that there was little reflection off of a molecular torus, often invoked to explain the differences between Sy 1 and Sy 2 galaxies.
The 0.5-1.0 keV portion of the spectrum of MCG—6-30-15 shows spectral features that can be attributed to warm gas (moderately ionized plasma) along the line of sight within the host galaxy (Lee et al. 2001, ApJ, 554, L13.) An absorption edge is observed near 0.7 keV (in the rest frame) while the O VII edge was expected near 0.74 keV. The shift is thought to result from significant absorption by neutral Fe.
NGC 4151 shows extended emission in
both the X-ray and optical images, shown at left. This extended gas follows the structure of the
narrow line region (NLR), indicating that the X-ray and NLR gas are co-spatial.
When observed with the HETGS, it was thought that the spectrum would be
representative of normal Sy 1 but the continuum was very weak, so the soft
portion of the spectrum was dominated by emission lines,
figure below. Obscuration of the hard power law component by
neutral material with a very large optical depth is more typical of Sy 2
galaxies (next section). The unusual weakness of the hard X-ray continuum
allowed Ogle et al. (2000, ApJ, 545, L51) to measure the emission lines from
extended gas that is photo-ionized by the hard, but obscured, continuum.
Comparing the O VIII line (seen below at 19A) and Fe K line (not shown)
indicates that the width of the Fe K line is probably dominated by the spatial
size of the galaxy, instead of by Doppler motions.
Seyfert 2 Galaxies
Several early observations of Sy 2 galaxies showed
significant emission lines in the soft X-ray portion of the spectrum. The
results from NGC 1068 (Ogle et al.,
2002, ApJ, submitted) were no exception, being rich with lines from a variety
of ions as well as showing recombination features indicative of
photo-excitation (see figure above).
The line-emitting gas for ‘1068 is spatially extended as in the case of
NGC 4151 discussed previously.
AGN and Jets Plans and Further Work
* We have spectra in hand for two more Sy 2 galaxies that can be analyzed in a manner similar to that of NGC 1068: NGC 5506 and IRAS18325-5926. A preliminary look shows that the line spectra of these two are not quite as rich for reasons that are not yet understood.
* A more full analysis of the absorption line spectrum of MCG-6-30-15 is nearing completion and a similar analysis is nearly complete for Mrk 766, which has a spectrum that is comparable to MCG-6-30-15 in terms of absorption features.
* There have been and will be several more observations of continuum sources that will be analyzed primarily for absorption features due to the IGM (to be described in a subsequent report) In addition these spectra will be analyzed for other AGN features.
* The quasar MRC 2251-178 will be observed this Fall and should also show absorption lines due to a warm absorber.
* The Sy 1 galaxy NGC 7469 will be observed in October 2002 jointly with HST so an analysis of those data will require examining the detailed absorption spectra in the optical band for matches with the X-ray absorption features.
* In addition to the spectra, the images of these AGN can show X-ray emission coincident with radio jets. For example, the image of 3C 279 obtained in March shows extended emission aligned with a radio emitting jet, so a separate paper will be written about this X-ray jet.