HETG

Follow-on Science Instrument

Contract NAS8-01129

Monthly Status Report Numbers 019

September 2003

Science Theme: Active Galactic Nuclei and Jets, Part II

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, Part II (July’02 – September’03)

 

Summary of AGN Observations and Activities

 

Chandra guaranteed time observations of AGN with the HETG continue at a high pace, and over the past year or so the AGN group has been busy analyzing new data sets, and performing more sophisticated in depth analysis and modeling of previous observations. New observations have been made of bright quasars (QSO’s) to look for intervening absorption line systems, Seyfert 1 galaxies to study the accretion flow and the warm gas obscuring the nucleus, Seyfert 2 galaxies to study the photo- and collisionally-ionized gas in the vicinity of the obscured nucleus, and blazers and radio galaxies to study their jets. These are providing a much more quantitative understanding of the AGN environment than has been possible in the past.

 

Figure 1: AGN cartoon from Urry & Padovani (1995) illustrating the “unified model” of Seyfert type 1 and 2 AGN showing the obscuring “torus” which has been cut away to show the accretion disk and active nucleus, the bipolar jet (which extends out to vastly larger scales), and the broad and narrow emission line clouds. HETG observations are improving our quantitative understanding of such systems.

 

The Table below summarizes the HETG GTO observations and publications arising from them. Almost all objects either have a draft paper in preparation or their results have already been published.

 


 

Status of GTO AGN Targets

AO

Obsid(s)

Object

Brief description

Z

Publications, [cda] = obsid-to-ADS

5

1

---

433

MCG –6-30-15

Sy 1, warm absorber, iron line.

Cycle 5 for 540 ks!

 

Lee et al. 2003 (in prep.); Lee et al. 2002c agnspec; Lee et al. 2002 ApJ 570 L47 [cda]; Lee et al. 2001 ApJ 554 L13 [cda]

4

3568

H1426+428

QSO, IGM

0.13

(T. Fang ?)

4

3567,4399,4441-2

Mrk 290

Sy 1

 

---

3

2977

MR 2251-178

QSO / Sy 1

.064

Gibson, et al. (in prep.)

3

3147

NGC 7469

Sy 1.2

 

Kriss et al. (in prep.)

3

2969, 4284

1H 0414+009

QSO, IGM

0.28

(T. Fang ?)

3

2970, 3472

1ES 1028+511

QSO, IGM

0.36

(T. Fang ?)

3

2971

3C 279

RL QSO, IGM, jet

0.64

Marshall et al. (in prep.); (T. Fang ?)

3

3148, 3452

IRAS 18325-5926

Sy 2, iron line

 

Lee et al. (in prep.)

2

1597

Mrk 766

Sy 1, warm absorber

 

Lee et al. (in prep.)

2

1599

H 1821+643

QSO / Sy 1

0.30

Crawford & Fabian 2003 MNRAS 339 1163 [cda]; Fang et al. 2002 ApJ 565 86 [cda]

2

1598

NGC 5506

Sy 1.9, Compton thin

 

Bianchi et al. 2003 A&A 402 141 [cda]

Gallagher et al. (in prep.)

1

332

NGC 1068

Sy 2, plasma diag.s

 

Ogle et al. 2003 A&A 402 849 [cda]

Brinkman et al. 2003 A&A 396 761[cda]

1

241

M87, NGC 4486

BLRG, Jet; Virgo

 

Marshall et al. 2002 ApJ 564 L683[cda]

1

335

NGC 4151

Sy 1.5

 

Ogle et al. 2000 ApJ 545 L81 [cda]

1

1481, 336

PKS 2149-306

QSO

2.34

Fang et al. 2001 ApJ 555 356 [cda]

1

337, 1703, 1705

PKS 2155-304

BL Lac

0.12

Fang et al. 2002 ApJ 572 L127 [cda]

Marshall et al. 2002 ApJ 564 941 [cda]; (*)

1

1450, 334, 1802

S5 0836+710

QSO

2.17

Davis 2001 ApJ 562 575 [cda]

Fang et al. 2001 ApJ 555 356 [cda]; (*)

1

333, 428

NGC 1275

Sy2, Perseus cluster

 

Marshall 2000 (*)

(*) Marshal 2000 in “Astrophysical Phenomena Revealed by Space VLBI” eds. Hirabayashi, Edwards and Murphy (Sagamihara:ISAS), p. 207

 


The Jet of 3C 279

 

The Chandra X-ray observatory got off to a good start with the unexpected discovery of an X-ray jet in its first exposure! It has since become clear that one-sided X-ray jets are common features of extragalactic radio sources, and the growing number of Chandra observations of jets will soon allow questions about this entire class of objects to be addressed. Recent GTO jet targets include 3C 273 and 3C 279, and we shall briefly discuss the latter here. 3C 279 (at z=0.64) was observed in Cycle 3 for approximately 100 ksec with the Chandra HETG, and the 0th order image Figure 2 (at right) immediately shows a powerful jet to the southwest of the nucleus; an overlay of radio emission contours is shown in green. The radio image was obtained with the VLA at 15 GHz. The knots D, E, F, and G (using the notation from de Pater & Perley 1983) are well resolved.

A comparison between the X-ray image and a 15 GHz VLA map shows a good spatial correlation between the X-ray and radio jets, with X-ray emission associated with knots D, E, F and G at nuclear distances of approximately 0.6”, 1.5”, 2.0” and 4.5”, respectively. The radial profile of the jet is shown in Figure 3 (at left) in which the X-ray excess at the location of knots E, F and G is clearly seen. The radio jet is relatively brighter than the X-ray jet at smaller radii. Profiles were derived from the X-ray data in a 1.4” wide region. The jet region (position angle 202°) contains a clear detection of knot G at 4.5” from the core as well as an excess about 2” from the. The profile of the radio emission is overlaid for comparison. Knots E and F are somewhat confused in the Chandra X-ray image but appear to contribute equally.

 




 

Combining the radio, optical and X-ray flux densities allows the emission mechanism of the jet to be constrained. The X-ray emission of the knots cannot be produced a simple extension of the radio synchrotron emission because this would pass above the optical upper limits (see Fig. 4 below). Instead, the X-ray emission may be produced inverse-Compton (i-C) scattering of microwave background photons by relativistic electrons - a common explanation for the X-ray emission of jets and hot spots in radio galaxies. A combination of a few factors helps make the i-C X-ray emission so strong: (i) the energy density of the microwave background scales as (1+z)^4, so for high redshift galaxies there is "more" background, (ii) in the frame of the relativistic jet the energy density of the background is further enhanced, and (iii) the i-C emission from the jet is beamed towards us. A further advantage of this particular model is that it allows the magnetic field to be large enough to be in "equipartition" (i.e. equal energy in the magnetic field and the particles), which (while not really justified) is a state many people like their jets and hot spots to be in!

 

 

 

Figure 4: Spectral energy distribution for knots E, F, and G, assuming that E and F contribute equally to the X-ray emission 2” from the core. A simple synchrotron emission model is fitted through each set of radio fluxes but must cut off strongly before the optical data points. The strong X-ray detections indicate that these knots are likely to result from inverse-Compton scattering of cosmic microwave background photons in a relativistic jet with G~10-15.



 

The Archetypical Seyfert 2 Galaxy: NGC 1068

NGC 1068 is the archetypical Seyfert 2 galaxy in which polarized, broad optical emission lines were first observed, indicating the presence of a “buried” active nucleus seen only in scattered (and hence polarized) light. This is great supporting evidence for the “unification model” which hypothesizes that Seyfert 1 and 2 galaxies are the same physical objects (i.e. AGN surrounded by an optically thick “torus”; see Fig. 1) and that their observed differences are a consequence of their orientation to our line-of-sight.

 

 

 

NGC 1068 was observed in Cycle 1 for almost 50 ksec with the HETG, with the paper describing its high resolution spectrum appearing in press this year (Ogle et al. 2003). The 0th order image of the nucleus of NGC 1068 is shown at right in Figure 5. This image shows highly ionized emission associated with the nucleus (blue/white) and less highly ionized gas extending towards the northeast (colored red. The red, green and blue color bands correspond to energy ranges of 0.4-0.6 keV, 0.6-0.8 keV and 0.8-1.3 keV, respectively.

 


The high-resolution (wonderful) Chandra HEG and MEG spectra of the nucleus are shown in Figs. 6 and 7 below, respectively. The wealth of emission lines provides powerful diagnostics of the physical conditions in the X-ray emitting plasma. For example, the resonant, intercombination and forbidden line triplets of He-like ions are indicative of the ionization mechanism (in this case photoionization), and the width of the resolved radiative recombination continuum emission line gives the electron temperature (in this case ~105 K). Using a grid of XSTAR photoionization models the column densities and temperatures of the different ions can be computed. Furthermore, it is found that the spectrum is well described by a photoionization model with roughly equal mass in all ionization states between log x = 1-3. The plasma is photoionized by the hidden nucleus as expected in the unification scheme.

 

Figure 6: Chandra HEG spectrum of the nucleus of the Seyfert 2 galaxy NGC 1068, the x-axis given in wavelength (Å) and energy (keV) units. The equivalent width of the neutral iron line, Fe I K, is significantly larger than that of a Seyfert 1 galaxy because the primary continuum is absorbed by the “torus” surrounding the nucleus of NGC 1068.

 


 

Figure 7: MEG spectrum of the nucleus of NGC 1068 (black line) with a photoionized plasma model overlayed (red line). Individual line identifications are shown. The relative strengths of the resonant, intercombination and forbidden lines (r, i, f, respectively) of He-like ions can be determined, and the strong forbidden lines indicate that the plasma is predominantly photoionized rather than collisionally ionized. The width of the radiative recombination continuum features (RRC) are a measure of the electron temperature.

 

 



 

NGC 5506: A “Compton thin” Seyfert 2 Galaxy

 

An HETGS and a brief ACIS-I observation of NGC 5506 were taken on New Year's Eve, 2000. One RXTE observation was taken simultaneously, with further RXTE observations being obtained over the next year. In the HETGS spectrum, an unresolved Fe K-alpha line is present, Figure 8 below left. XMM data has shown additional helium- and hydrogen-like Fe lines, but these were not detected in the HETGS data. The presence of a OVII forbidden and RRC lines indicates the importance of photo-ionization processes. Imaging spectroscopy with the Chandra data, e.g., Figure 9 at right, demonstrates the presence of ionized plasma on both large and small scales surrounding the nucleus, which produces the soft excess seen in earlier CCD X-ray spectra.

All RXTE observations were combined into a single spectrum. The RXTE data shows a 2-7 keV flux a factor of two higher than in the HETGS observation. Extending to higher energies, the RXTE data show a strong reflection continuum and a broad ( ~13500 km/sec) Fe K-alpha line. As the RXTE spectrum is a combination of observations taken over one year, the higher flux and broad iron line are not inconsistent with the HETGS spectrum, but the difference is intriguing. The RXTE data also show an apparent edge at 9.2 keV, which may be attributed to Fe XXVI. The narrow Fe K-alpha line has been generally ascribed to distant material (e.g., the molecular torus), but it is debated whether the broad line is due to reflection from a disk or to a blend of neutral and highly-ionized iron lines.

Observed-frame HEG spectrum of NGC~5506 illustrating the Fe K emission-line region. The spectrum has been fit with an absorbed power law model (red curve.) Note the systematically negative residuals at wavelengths less than 1.75 Å (7.1 keV) indicating that the strength of the neutral Fe edge is being under-predicted by the model.

 



 

MRC 2251-178: A Variable Warm Absorber

 

MRC 2251 was the original “warm absorber" source (Halpern, 1984), based on variability in the soft X-ray spectrum. It has recently been shown to have intrinsic variable ionized absorption in ultraviolet CIV lines (Ganguly et al., 2001) from an absorber within 4 kpc of the central source. Yet we have caught it in a state with little or no ionized absorption evident, though we do see forbidden lines from helium-like Ne, Figure 10 below, and O. The forbidden lines (and corresponding lack of strong intercombination lines) indicate warm photoionized gas is present. The lack of absorption coupled with information from models of photoionized plasma may allow us to constrain the covering fractin and electron density, assuming a homogeneous plasma is responsible for both the forbidden lines and (lack of) absorption.

 



 

MCG -6-30-15: The Saga Continues

 

What AGN report would be complete without the latest status of MCG -6-30-15? With a long observation planned for the near future, we're all anxious to see this galaxy yield up its secrets.

Recent work on MCG -6 has built on the fact that a single-zone ionization model (produced, e.g., by CLOUDY) is insufficient to describe the HETG spectrum. Examining the ranges of the photo-ionization parameter x = L/nR^2 where ionization levels of various elements are significant, we can immediately see that a single value of x is insufficient to support significant quantities of the ions observed. In fact, four different values of x are required to produce the range of ions seen in the HETG spectrum.

Assigning a value of x to each line present (based on where the corresponding ion would be abundant) shows an increasing tendency of log(x) with equivalent hydrogen column density (derived from curve-of-growth considerations). Furthermore, log(x) increases with observed blue shift of the line, Figure 11 above, suggesting the presence of an out ow along our line of sight. These considerations indicate that the absorber we see in MCG-6-30-15 is not a homogeneous entity, but is possibly constructed of individual clouds with separate velocities and column densities.

 



AGN Plans, Part II

 

In general, analyses and papers are in preparation for a number of the HETG GTO AGN and jet targets. In addition some specific plans for the next year or so of our AGN work are given here:

 

MCG –6-30-15

·      540 ksec of the HETG GTO allocation in Cycle 5 will be used to observe MCG –6-30-15 to further study the warm absorber and iron line.

·      Arrange coordinated observations for Cycle 5 observation: JCL submitted a Magellan proposal; Jerry Kriss and JCL will be working on an HST proposal as well.

 

 

Markarian 290

·      Understand and model the Fe XVII emission lines seen in this source.

 

 

Photo-ionized plasma modeling

·      Apply existing models (equilibrium and time-varying) to observations; characterize models analytically.

·      Use improved reaction rates (M.F. Gu results?)

·      Generalize to non-equilibrium and time-varying continuum cases. (Hypothesis is that very non-equilibrium abundances can occur from some very simple continuum variations.)

·       

 

Longer Range modeling:

·      Add more processes to models (charge transfer, Auger processes, multi-zone models (simple radiation transfer), etc.

·      Consider non-Maxwellian electron distributions.

 

 

"Get your model runnin’… And whatever comes our way…" (Steppenwolf, 1968)