The HET Echo Mapping Project

The Latest HEMP News:
The current status of HEMP.

ABSTRACT:
Many QSOs are highly variable sources. Some of these show a pronounced time delay between variations seen in their optical continuum and in their emission lines. ``Echo mapping'' (also known as "reverberation mapping") is a technique that uses these time delays to measure the geometry and flow of the gas inside the QSO, near the supermassive black hole. The technique is immensely powerful, but the results so far have been ambiguous due to insufficent quality data.

We are starting a long-term project to echo map QSOs using the brand new Hobby-Eberly Telescope (HET) . The HET is a 9-m telescope located at McDonald Observatory in West Texas, and is operated by a consortium of universities. The HET will allow us to map nearby (but intrinsically faint) QSOs with far greater detail than before. We can also map QSOs at high redshifts; these QSOs are very much more luminous than their nearby "local" counterparts. Echo mapping of these high-luminosity, high redshift QSOs has not been attempted before.

The HET Echo Mapping Project Team is:
Bill Welsh, "Rob" Robinson, Gary Hill, Greg Shields & Bev Wills (UT);
Niel Brandt & Mike Eracleous (Penn State); Wolfram Kollatschny (Goettingen);
and Keith Horne (St. Andrews, Scotland)

ECHO MAPPING - A Brief Introduction:
The idea of using echoes of light to learn about the geometry and structure of gas around the illuminating source goes back at least to the early 1970s. In the context of AGN, the seminal paper was written by Blandford and McKee in 1982. But it wasn't until 1990 that echo mapping really took off. This was due, in no small part, to the availability of high quality data: good signal-to-noise ratio spectrophotometry with a sampling rate high enough to resolve the fast variability. Many astronomers at many different sites were involved in these campaigns to monitor AGN variability, e.g. the International AGN Watch. In addition to the ground-based optical observations, ultraviolet data was obtained with the IUE and HST satellites.
So, just how does echo mapping work? Well, it is similar to radar, sonar or `echo location' used by bats: a radio or sound signal is sent out from a source, and the returning echoes are measured. The echoes contain amplitude and timing information, which can be used to infer the shape and size of the object that is doing the reflecting. However, unlike in these analogies, in AGN echo mapping it is the AGN itself that is the source of the signal.
An AGN consists of several components, the most important for echo mapping being (i) the "central engine" and (ii) the surrounding gas clouds (the so-called "broad line region" or BLR). The central engine is believed to contain an accretion disk spiraling into a supermassive black hole. The accretion disk produces the tremendous luminosity of a QSO. The BLR gas clouds are further away from the black hole, surrounding the accretion disk. The gas in the BLR is illuminated and photoionized by the high energy photons from the accretion disk.
The accretion disk is unstable, and as a consequence, wild fluctuations in luminosity occur (the light curve is something like a random walk). When the disk emission varies, so does the amount of photoionization of the surrounding BLR gas - the BLR emission is slaved to the disk emission. When the BLR gas "cools" (i.e. recombines), it emits light at very specific wavelengths (in spectral emission lines). So the BLR emission can be distinguished from the disk continuum emission. By measuring the time delay between the continuum fluctuations and the emission line response, we get a measure of the light-travel time from the black hole to the BLR. (The BLR gas effectively recombines instantly, so it is only the light travel time that causes the delay). Typical time delays range from 1 day to several hundred days for low luminosity QSOs.
This simplified description is perhaps both too simple and too complex, depending upon your background. For more information, see a more detailed simplified description or check out a slightly more technical summary of echo mapping and the HEMP. (Also, see the bottom of this page for some figures.)

QSO Monitoring Project
While the variability properties of nearby QSOs are fairly well known, this is not true of the high-z QSOs. This is a problem for the HEMP - we don't know which QSOs are variable sources, so we don't know which ones to observe. Since time on the HET is extremely precious, we can't possibly survey dozens of QSOs. We must know ahead of time which targets are feasible for echo mapping, and limit ourselves to those few. Thus we have started a campaign to monitor many high-redshift QSOs for the purposes of determining their variability characteristics. This multi-site, multi-university collaborative effort is known as the HEMP QSO Monitoring Project.

Check out some overhead transparencies I made for an "astrophysics lunch talk":
Intro viewgraph: AGN schematic & NGC 5548 light curves (p1)
Echo mapping basic equations (page 2)
1-d tranfer functions and HEMP strengths (page 3)
2-d transfer function (page 4).

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This page was last updated on 2000 Jan 20