Pulling it all together - the flux

Next up is to normalize the surface brightness by the source flux. This has already been done for the 3C273 observation I'm using for the PSF, but remains to be done for the  GX5-1 observation.  The tricky bit is that the 'source flux' depends both on the time the observation was done (as discussed earlier) and on how it's measured.  Chandra can measure the flux from a point source quite easily and accurately, but RXTE is a non-imaging detector and so a flux measured with RXTE will include both the point source and a significant amount of the 'scattered' flux.  Bringing these two measurements into alignment requires a determination of how much flux is scattered - which is, in fact, what our ultimate goal is.

Comparing previous results, Smith et al. (2006) found Fx(1-10 keV) = (5.2±0.1)x10^-8 erg/cm^2/s for their Chandra/ACIS observation of GX5-1 (ObsID 109), while Ueda et al (2005) found Fx(1-10 keV)=4.3x10^-8 erg/cm^2/s from their HETG data (ObsID 716).   From the fit I did of the new HETG spectrum I got Fx(1-10 keV)=3.7x10^-8 erg/cm^2/s, about 40% less than the ACIS observations and 16% less than the HETG flux.  This variation can be compared to the RXTE all-sky-monitor count rates:

ObsID   Description    1.3-3 keV     5-12.1 keV
                                     cts/s              cts/s
109      ACIS-S            13.4±1.0      35.1±1.3
716      1st HETG        14.2±1.1      32.3±1.3
5888    2nd HETG       12.3±1.3      40.8±2.2
7029    HRC-I              11.2±3.4      31.5±6.2

Of course, this emission includes the direct and scattered flux, so it's not possible to convert these into fluxes directly (using, say, WebPIMMS) and get a sensible comparison.  But the range of count rates here is 25% or less, which for an X-ray binary is practically constant.  It also suggests that while the flux from the 2nd HETG observation might be slighter larger than during the HRC-I observation, the difference is less than 25%.

In fact, plugging the total ASM count rate for the HRC-I observation (63.38 cts/s) into WebPIMMS, along with kT=10.5 keV, NH=4e22 cm^-2 (from Smith et al. 2006), gives an HRC-I count rate (including all scattered photons) of 131 cts/s; adding 55 cts/s for the background gives 186 cts/s, just the telemetry limit.  This is reasonably close to the estimate I got from the HETG fit, although keep in mind that was just the source itself and this is the source + scattered flux so I would have expected it to be larger.  Plus the deadtime measurement from the data itself of 33% suggests this is an underestimate since a true count rate of 186 cts/s should have a lower deadtime than 33%.  Hmm.  Have to think more about this.