Instruments: HETG

HETG Calibration

The HETG spectrometer continues to provide high-quality dispersed spectra with stable characteristics. In the past year, progress was made on the effective area calibration of the HETGS by including an update to the HEG and MEG efficiencies and a correction for the HRMA's Ir M-edge response. These updates were released on 15 December 2005 in the CALDB version 3.2.1. The efficiency changes bring the HEG and MEG spectra into agreement: the error between them was apportioned to the HEG below and the MEG above a cross-over point of 1 keV. Using these updated calibration products, fractional residuals in spectral fits should now be less than 5% from 0.5 to 8 keV - provided that one is fitting the "real" source model! The major remaining degree of calibration freedom, the absolute effective area, is being studied through on-going cross-calibration, in particular with the XMM-Newton instruments and the RXTE PCA.
With these calibration improvements, the six years of HETGS data form a unique and coherent spectral resource. We're also entering a new era of HETGS calibration focussing more on calibration's effects on science conclusions, e.g., along the lines begun in the special session on "Incorporating Calibration Uncertainties into Data Analysis" at the Chandra 2005 Calibration Workshop [0]. As always, up-to-date HETG calibration status and details are available on the web [1].

We note for HETG fan[atic]s that the infamous "HETG paper" has been published: Canizares et al. 2005 [2]; it includes many gory details of the design, fabrication, ground and flight calibration of the HETG. Although not required reading for the general observer it is a useful reference and may give future instrument builders a hint of what they're getting into.

HETG Science: The AGN NGC 3783

The object NGC 3783, an active galactic nucleus (AGN), is thought to harbor a 30 million solar-mass black hole which is surrounded by an accretion disk, out flowing ionized material, and an obscuring torus; Figure 12 illustrates the geometry. If viewed at the orientation of this illustration, the central source would be obscured by the torus and this would be classified as a Seyfert 2. NGC 3783, in contrast, is a Seyfert 1 where our viewing angle is above/below the torus and lets us see directly to the bright, central source. In terms of its physical size, the inner diameter of the torus is of the order of parsecs, roughly the same size as the Cas A supernova remanant in our Galaxy. Located about ten thousand times further away from us than Cas A, NGC 3783 appears as an unresolved point source to the Chandra telescope and so its spectrum is the main source of detailed information.

FIGURE 12: An illustration of a generic Seyfert AGN. At the distance of NGC 3783 this image would subtend one-tenth of an arc second on a side. Interestingly, the AGN's physical scale is of the same order as that of Cas A, as shown.

Back in the years 2000 and 2001, HETGS observations totaling 900 ks were dedicated to NGC 3783, a cycle 1 GTO (PI: G. Garmire) and a cycle 2 GO (PI: I. George.) The observations were carried out and the spectra were analyzed and published in a timely fashion [3,4]. The bright continuum spectrum emitted by the central source was seen to contain many absorbtion lines, generally blue-shifted from the galaxy's rest frame at z=0.00976. These are interpreted as being due to outflowing gas along the line of sight. Much of the absorbtion is not from cold, neutral atoms but from ionized atoms, i.e., a "warm absorber".

FIGURE 13: HETGS spectrum of NGC 3783 in the Fe-M unresolved transition array (UTA) region fitted with a detailed ion-by-ion model, red line. (From Holczer et al., Fig.2)

Even with the extensive analysis given, this data set is not depleted: during the past year there have been at least 5 further papers explicitly making use of these HETGS data to carry out new analyses [5,6,7,8,9]. Example spectra and highlights from these papers are shown in Figures 13 through 17. Clearly, these are spectra that tell a story - and it's still in translation.

FIGURE 14: Best-fit model (solid lines) to MEG and HEG first-order and MEG third-order absorption-line profiles. The asymmetric model line shape before instrumental convolution is shown as well (dashed.) (From Chelouche et al., Fig.11)

Of course, NGC 3783 is not the only Seyfert AGN to be observed with the HETGS; it is just one of many "good reads" available in the Chandra archive. To make these data more available to the general astronomy community, a Chandra archival grant was awarded to make a database of HETGS AGN products that are in "ready to go" format, e.g., rest-frame fluxed spectra. Although in development now, keep your eye on the "Hot Gas" web page [10] and/or send a request to the e-mail address on that web page to be informed when it is online. And... enjoy the stories!
Dan Dewey, for the HETG team.

FIGURE 15: (a) HETGS spectra of NGC 3783 in the UTA region for spectrally-identified "low" and "high" states. (b) Plotted here are spectra from a photoionization model for a nominal central source flux and for a flux change by a factor of 2. The model change is in agreement with the low-high state difference seen in (a). (From Krongold et al., Fig.1(a)(b))

FIGURE 16: Ratios of the HEG spectrum to a simple power-law continuum model. The Fe-K emission line is shown for data separated into two states and hints at a slightly broader core in one set ("+" symbols) than in the other ("*" symbols.) (From Yaqoob et al., Fig.5)

FIGURE 17: Histogram data of Ne X and neighboring absorption lines with model and continuum overplotted, shown in both wavelength (left) and velocity (right) space. (From Ramirez et al., Fig.3)

References

[0] Chandra2005 Cal Workshop: http://cxc.harvard.edu/ccw/proceedings/05_proc/index.html
[1] HETG calibration: http://space.mit.edu/CXC/calib/
[2] Canizares et al, 2005, PASP 117, 1144.
[3] Kaspi et al. 2001, ApJ, 554, 216.
[4] Kaspi et al. 2002, ApJ, 574, 643.
[5] Holczer, T., Behar, E., and Kaspi, S. 2005, ApJ, 632, 788.
[6] Chelouche, D. and Netzer, H. 2005, ApJ, 625, 95.
[7] Ramirez, J.M., Bautista, M. and Kallman, T. 2005, ApJ, 627, 166.
[8] Krongold, Y., Nicastro, F., Brickhouse, N.S., Elvis, M., and Mathur, S. 2005, ApJ, 622, 842.
[9] Yaqoob, T., Reeves, J.N., Markowitz, A., Serlemitos, P.J., and Padmanabhan, U. 2005, 627, 156.
[10] Hot Gas web page: http://hotgas.pha.jhu.edu