Vadim Burwitz (chair, ROSAT, Chandra, XMM,eROSITA)
Jelle Kaastra (Chandra, Modeling)
Herman Marshall (Chandra)
Norbert Schulz (Chandra)
Jeremy Drake (Chandra)
Steve Sembay (XMM EPIC-MOS)500ks OBS_IDs: 00113 03380 03381 03382 03399
120ks OBS_IDs: 14418 15293
Chandra LETG + ACIS-S
RX J1856.5-3754 based on the CHANDRA LETGS + HRCS Data
delchisqr = 1 ( = 1 sigma for 1 parameter)
tbabs*bbodyrad
chiqs = 692.6phabs*bbodyrad
chiqs = 696.0
The Beuermann et al 2006 paper uses the TMAP white dwarf atmosphere model of Werner et al for fitting. For the purposes of calibration efforts, C. Markwardt developed a fitting function that can be used in place of the full atmosphere model. The fit was done in the 0.1 - 1 keV range. Note that Beuermann et al INCLUDE interstellar absorption in their model, so it is not necessary to provide an additional absorption component.
The model can be used in XSPEC using "mdefine" statements. The model is a modified blackbody spectrum, whose "energy" is a polynomial of several terms. The kT value is the white dwarf temperature in keV, and the norm is the expected norm. (Note that a 2021 version of XSPEC or later is required)
mdefine poly_hz43 e*(0.955968 + e*(-0.699684 + e*(0.555432 + e*(-0.292088 + e*0.069582))))+dummy
mdefine hz43 8.0525*poly_hz43(0.0)**2/kT**4/(exp(poly_hz43(0.0)/kT)-1.0)
and then defined as
model hz43
0.0130643 -0.0001(0.000130643) 0.01 0.01 0.02 0.02
0.0395467 0.001(0.000395467) 0 0 1e+20 1e+24
Here the temperature of ~0.013 keV is the best-fit temperature using this fitting function, and does not represent the true white dwarf temperature. The norm of ~0.039 is the best-fit norm, and again is physically meaningless other than as a fitting coefficient. Using this model results in the following curve in the photon spectrum. Solid is the Beuermann et al curve and dashed is the model. The residual panel below demonstrates that the worst-case error in this fitting function approximation is 1%.
