A Quick Description of Echo Mapping
ECHO MAPPING:
The cores of many galaxies harbor an exceedingly luminous, highly variable, compact, non--stellar energy source. These enigmatic ``active galactic nuclei'' (AGN) are important because they require exotic physics --- accretion onto supermassive black holes. AGN and the broad emission line--producing region (BLR) around them are too distant to resolve with any current or near--future telescope. However, we can deduce the structure of the BLR though the ``reverberation'' or ``echo'' mapping technique (Blandford & McKee 1982; Gondhalekar, Horne & Peterson 1994). Conceptually, echo mapping is simple: the accretion process produces a flash of emission near the black hole. As the light propagates outward, it photoionizes the BLR and, when the gas recombines, an emission--line ``light echo'' is produced. The time delay between the continuum flash and emission line response gives information on the size and geometry of the BLR. It is analogous to sonar, radar, and echo--location used by bats. In addition, the response of different parts of the line profile provides a measure of the velocity flow of the echoing gas. In short, echo mapping is an indirect imaging technique that uses information contained in the time delays across the emission line profiles. Like direct imaging which gives two dimensions on the sky (x,y) out of the 6-d position-velocity phase space, echo mapping also gives two dimensions, (r,V_z), from which the geometry and kinematics of the flow of gas inside an AGN can be deduced. Structure at microarcsecond scales can be measured. For example, for the Seyfert 1 galaxy NGC 5548, an observed time delay of 20 days for the H_beta line (Peterson et al. 1999) corresponds to an angular resolution of 50 microarcsec. For more distant QSOs, the effective angular resolution can be even smaller.

The HET Echo Mapping Project:
To date, only small (1-3m class) telescopes have been used for echo mapping, so all previous attempts were limited to very nearby and low luminosity AGN. Usually only the 1-d mapping case (integrated line flux) was possible --- velocity information in the line profile was not usable due to low S/N. Furthermore, it is known that the limiting factor in the echo mapping technique is the low S/N of the data (Vio et al. 1994). To make progress, a long-term program of spectroscopic measurements with much higher S/N is needed. Fortunately, the queue-scheduled HET+LRS is perfectly suited to meet these demands. Thus the ``HET Echo Mapping Project'' (HEMP) was created. The goal of HEMP is to fully exploit the potential of the echo mapping technique to determine the geometry and kinematics of the BLR and hence determine the flow of gas in the environment around supermassive black holes in QSOs. From the gas flows we can determine properties of the black hole itself, e.g., its mass. While HEMP will produce vastly superior echo maps, its real contribution lies in achieving qualitatively difference science: (1) the full 2-d velocity-resolved problem can be solved; and (2) we will echo map much more intrinsically luminous AGN at high redshifts --- this is a completely unexplored realm. Thus HEMP will focus on three classes of targets: (i) the nearby, very bright and highly variable Seyfert-type AGN; (ii) the low-redshift QSOs; and (iii) the high-redshift QSOs. The HEMP project is one clear example of how the HET can outperform Keck or any other big telescope that is not queue scheduled.

The HEMP QSO Monitoring Project:
HEMP QSOs must be bright ( <20th mag), radio-quiet, as point-like as possible, and variable. For Seyferts and low redshift QSOs, candidate HEMP targets were readily found, as there is a wealth of data on variability in the literature. For the high-z QSOs, we specifically choose QSOs with z ~2.4-2.8 so that Ly_alpha through CIV appear in the optical and that the H_beta and H_alpha lines fall in the infrared H and K bands. Given the above constraints, we have a list of ~40 candidate high-z QSOs for HEMP. Unfortunately, very little is known about the variability properties of QSOs at these redshifts. Thus we are embarking on a long-term photometric study of the candidate HEMP QSOs. Of the 100 or so high and low-z QSOs we will monitor, the most variable ones will be selected to be observed with the HET. The observations are being made at the 0.8m at McDonald and with collaborators at the Univ. of Wyoming, Alfred University (NY), and the University of Central Arkansas.

References:
Blandford, R.D. \& McKee, C.F. 1982 ApJ 255, 419
Gondhalekar, P.M., Horne, K. \& Peterson, B.M. (eds.) 1994 ``Reverberation Mapping of the Broad-Line Region in Active Galactic Nuclei'', ASP Conf. Ser. 69, (San Francisco: ASP)
Peterson, B.M., et al. 1999, ApJ, 510, 659
Vio, R., Horne, K. \& Wamsteker, W. 1994 PASP 106, 1091

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