X-ray Absorbing Outflows

Astro 597: High Energy Astrophysics

September 27, 2004

Brendan Miller

The Big Picture

• Outflows carry off mass, energy, and angular momentum from the accretion disk

• Feedback could regulate growth of black hole and host galaxy, even pollute IGM

• Observed as blueshifted absorption lines (mostly in UV spectra)

• Probably all AGN have associated absorption, although the details vary by class and object

Where are we in the AGN zoo?

• Radio loud quasars: UV absorption blueshifted by up to about 5000 km/s

• Radio quiet quasars: ~10% show broad absorption lines (max shift to about 0.2c)

• Seyfert 1: ~50% show narrower absorption features to about 1000 km/s

Why outflows are necessary (Arav 2003)

• Clouds require turbulence (thermal not broad enough)

• Clouds should have velocity independent absorption profiles

• Clouds don’t obviously explain “detached troughs” (shifted absorption features)

Numbers• To be self-consistent, I’ll just use results from

hydrodynamical modeling done by Proga, Stone, and Kallman (2000)

• For a central black hole accreting at a rate of , the wind launches from a radius of 1e16 cm and is accelerated by UV line radiation to a speed of 15,000 km/s at a distance of 1e17 cm. Mass loss from outflow is Column densities are a few times 1e23 and the covering factor is about 0.2.

Pretty pictures

Ionization in the outflows

• Nasty equations from Krolik reveal that ionization parameter is very large

• Need to prevent gas from becoming completely ionized (no line pressure)

• Murray et al (1995) postulate “hitchhiking gas” interior to wind to shield outflow

• Nasty equations from Murray describe properties of winds

Geometry of wind for BALQSOs

• Outflow streams off accretion disk, driven by radiative UV line pressure

• Shielding gas protects the wind from being completely ionized

Gallagher 2002

Model can explain:

• The ~10% of RQQ showing broad absorption lines is due to likelihood of viewing source through the wind

• This is supported by polarization fraction increasing in absorption troughs (photons can “detour” around outflow)

• The lack of RLQ with broad lines may be due to complete ionization of gas in the inner disk forcing outflow to launch further out

No RLQ with BAL

Non BAL quasars

BALQSOs

Velocity of the outflow

• The velocity of the outflow increases with radial distance

• Dashed line is approximate analytical function, top solid line is numerically integrated radial velocity, and bottom solid line is vertical speed

• See Murray 1995 for details

We Report, You Decide:A Fair and Balanced Look at Krolik

• UV absorption suggests column density of H atoms of about 1e20, or a density of 100-1000.

Force from a line

• The acceleration due to radiation line pressure can be evaluated by solving radiative transfer equation (looks like plane-parallel form because geometry is cylindrical, integrated over azimuthal angle)

Sobolev approximation

• Photons can interact with resonant line only at a localized region, since Doppler shifts from changing angle of outflow to ray and increasing speed with distance make the optical depth at other regions negligible

Punchline• It is then trivially left as an exercise to the

audience to show that the solution is given by

My head hurts

• Write total radiative acceleration as a “force multiplier” times the ordinary radiation acceleration and get equation of motion

Is there a point somewhere?

• Integrate equation of motion, pretending force multiplier is constant, get something pretty close to these expressions (Laor & Brandt, 2002) and discover that to get a high outflow velocity you should start closer in to disk

Absorption line profile

• Optical depth decreases as velocity increases

• Also, ionization fraction decreases

• Combined effect is that absorption is strongest at lower velocity

Murray 1995

Discussion of Gallagher paper

Soft X-ray absorption means:

ox becomes more negative (steeper power law) as absorption depresses X-rays

• The hardness ratio (H-S)/(H+S) increases as the soft X-rays are absorbed much more than the hard X-rays

Correlation with UV?• Vmin describes angle at

which you view wind relative to disk, since streams turn over

• Would expect absorption to increase (ox becomes more negative) as Vmin

decreases (line of sight closer to along disk)

On the other hand

• Low ionization quasars showing Mg II absorption have greater X-ray absorption

• This is because: (discuss)

Nice job everyone

• The low ionization-state Mg II forms further out in the disk than the C IV, leading to a smaller covering factor for the Mg II wind

• So can see C IV without Mg II but if you see Mg II, must be looking fairly close to along disk (and you’ll definitely see C IV)

• Which means X-ray absorption should also be higher (although some might occur out beyond disk)

• Example of how the presence of UV absorbers and the presence of X-ray absorbers are linked

Complications

• PG 2112+059: X-ray absorption variability not echoed in UV; need different absorbers

• APM 08279+5255: Fe absorption features at speeds of 0.2 and 0.4c suggest very close launching point (Chartas 2002)

• Is highly ionized X-ray absorption coming from shielding gas? Shielding gas is expected to fall back in…

Brandt paper: X-ray spectroscopy

• NGC 3783: Nearby Seyfert 1

• Brandt & Kaspi: 10.4 day Chandra HETGS observation

• Resolve blueshifted absorption features

Absorption in AGN

• Equivalent width of C IV increases as ox

decreases• Correlation

between presence of UV and X-ray absorption

Laor and Brandt, 2002

X-ray spectrum of NGC 3783

Multiple component outflows

• Combined line spectra show two absorption components in O VII in NGC 3783

• O VII and Ne X have different kinematic structure

• Purple lines show UV• Hard to measure

blueshift accurately

Outflow velocities for ions

Behind the scenes

• Kaastra et al (2002) required 3 distinct components, each with a different ionization phase, to fit X-ray spectrum

• Not immediately clear how the ionization components correlate to the UV velocity components

• Inclination of ~45 deg

What’s going on?• Maybe lower ionization

states are more responsive to driving radiation (more lines, lower transition energies)

• Maybe line of sight coincides with direction of low ionization outflow

• (from class: Ken suggests recombination becomes important; makes sense)

Note that these lines could very well be clumps or clouds, possibly the same material that scatters radiation into line of sight in Seyfert 2 galaxies

Lack of X-ray resolution

Conclusions• The presence of UV absorption and X-ray

absorption is clearly linked• UV and X-ray absorbers are not necessarily the

same; even in X-rays, require multiple ionization components to model absorption

• In BALQSOs, X-ray absorption associated with inner regions of disk can act to shield UV wind. Lack of correlation with UV properties, differing variability, and fast iron lines indicate that X-ray and UV absorption is probably from distinct components here as well