Project Overview

This is the web site for the Chandra X-ray Visionary Project to observe Sgr A*, and the surrounding inner few arcminutes, with the Chandra High Energy Transmission Gratings.  We have obtained 3 Msec of observations over the course of Chanda Cycle 13, between February and October 2012.  Here we provide a brief description of the goals of this project.

  1. We will obtain the first high-resolution spectrum of Sgr A*.  There is a known Fe line in the quiescent spectrum from the inner 1.5”, from the inflowing material (Advection Dominated Accretion Flow? Or emission from a cusp of stars in the inner region?).  There are other hypothesized lines from an ADAF-type flow, e.g., Sulfur, Silicon, Magnesium, Calcium, Argon.  We will resolve the energy and width of the Fe line, detect S & Si lines if they are at the minimum levels suggested by ADAF models, and have a shot at Mg, Ca, and Ar lines. (See this HETG observation of M81* for a possible, brighter analogy.)  This will be used to test ADAF models of the accretion flow.  Alternatively, we could resolve the Fe line into a 6.4 keV fluorescent line and a 6.7 keV Fe XXV line, lending support to the stellar cusp model.
  2. Given that Sgr A* is a hard source, these observations will have about 1/2 the count rate as the current Chandra CCD observations.  About half of the counts go into the gratings arms, and half go into the 0th order image.  (Soft sources lose a bit more in the 0th order spectrum.)  Thus the combined spectrum will more than double the total, quiescent counts collected from Sgr A*, with half of those new counts being at gratings resolution.
  3. We also will more than double the counts obtained from the flare states of Sgr A*. Thanks to the presence of the gratings, far fewer of these flares will be piled up.  The very brightest flares (~factors of 100) have yet to be detected by Chandra, and are piled up beyond use without insertion of the gratings.  The flare spectra will be compared to ADAF and jet models of Sgr A*.
  4. We will obtain 3 Msec in a single year of observations, which means a greatly increased sampling density of the Sgr A* lightcurve. These lightcurves can be compared to relativistic MHD models of the Sgr A* accretion flow (e.g., such as Chris Fragile and Charles Gammie have been performing).
  5. The increased lightcurve sampling also means that there will be increased opportunities for multi-wavelength monitoring.  Multi-wavelength monitoring has been crucial in Sgr A* studies, and we plan to continue along those lines.  NuSTAR will be flying for most of the time period of these observations.
  6. At the same time as we are obtaining spectra and lightcurves of Sgr A*, we will be obtaining spectra of Sgr A East, and spectra and lightcurves (with 3.2 sec resolution) of any point sources within the inner ~1 arcminute.  For the brightest of these we might have some interesting spectra at gratings resolution.