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Release of new XRT responses,
and comparison of ascaarf v2.53 and v2.62


1. Introduction and Summary

ascaarf v2.62, as well as the XRT response files, xrt_ea_v2_0.fits and xrt_psf_v2_0.fits, was released on July 1996 through ASCA GOF Previous versions of ascaarf (v2.53 and earlier) combined with older XRT responses (xrt_ea_v1_1.fits/xrt_psf_v1_1.fits and earlier), has been known to have the following problems:

These problems were corrected in ascaarf v2.62 and xrt_ea_v2_0.fits/xrt_psf_v2_0.fits. Essentially, two changes have been made:

Please note that ascaarf v2.62 has to be used with xrt_ea_v2_0.fits and xrt_psf_v2_0.fits to make the latest and most reliable ARFs. The correction filters are included in the ascaarf v.2.62 source code, and can be turned off by specifying the hidden parameter "arffil = no". By turning off the ARF fillter and combining with older XRT calibration files (xrt_ea_v1_1.fits and xrt_psf_v1_1.fits), ascaarf v2.62 is able to make the same old ARFs as were made with ascaarf v2.53 and ealier.

2. Direct Comparison of ARFs

Figure 1 shows ratios of ARFs made by ascaarf v2.62 (with xrt_ea_v2_0.fits and xrt_psf_v2_0.fits) and v2.53 (with xrt_ea_v1_1.fits and xrt_psf_v1_1.fits). The observation was 3C273 (June 9, 1993), and the point source method was used with the tex2html_wrap_inline347 4 and tex2html_wrap_inline347 6 arcmin integration radii for SIS and GIS respectively.

New GIS ARFs give 15 - 20 % smaller effective area values than old ones, hence GIS normalizations will become higher using the new ARFs. New SIS ARFs are also smaller than old ones by about 5 %. Overall, the ratio decreases from 2 keV toward 8 keV, resulting in slightly flatter spectral indices with the new ARFs. There are alo local structures with amplitudes of a few percents in the ratios.

  
figure1

Figure 1: Ratios of the ARFs made with ascaarf v2.62 (with xrt_ea_v2_0.fits and xrt_psf_v2_0.fits) and v2.53 (with xrt_ea_v1_1.fits and xrt_psf_v1_1.fits) for the same observation (3C273; June 9, 1993). Upper-left:SIS0, upper-right:SIS1, lower-left:GIS2 and lower-right:GIS3.

3. Effect of the filters

As explained in section 1, ARF correction filters are introduced in the ascaarf v2.62 to absorb the normalization differences and local spectral structures the revised XRT response could not account for. GIS2 and 3 filters are made independently so that each Crab spectrum can be fitted with a power-law. Average of the two filters is used for both SIS, besides the normalization adjustment.

ascaarf v2.62 has an option not to apply this ARF filter to see its effect. In figure 2, we show ratios of the v2.62 ARFs (for 3C273 in June 1993) with the ARF filter to those without the filter.

  
figure2

Figure 2: Ratios of the v2.62 ARFs with the ARF filters to those without the filters. Upper-left:SIS0, upper-right:SIS1, lower-left:GIS2 and lower-right:GIS3.

4. Consistency among the Four Detectors

3C273 (observed on June 9, 1993) and EXO2030+375 (observed on July 1, 1993) were used to check consistency of spectral parameters of the four sensors. The former source has a small low energy absorption, while the latter suffers from a significant absorption. In Table 1 - 4, we show results of model fitting for individual sensors (Table 1 and 3) and for four sensors simultaneously (Table 2 and 4), using ARFs with v2.53 and v2.62. The model for 3C273 is an absorbed power-law, and that for EXO2030 is an absorbed 'highecut'. Results of the model fitting for for 3C273 were shown in figure 3. In figure 5, we show results of simultaneous fitting for four sensors using ARFs with v2.53 and v2.62. GIS deadtime was corrected for EXO2030, which affects the normalization by tex2html_wrap_inline347 2 %.

With v2.53 ARFs, it is clear that GIS gives tex2html_wrap_inline347 20 % smaller normalizations than those with SIS. With v2.62 ARFs, GIS normalization increases by 20-30 %, and so does SIS normalization by tex2html_wrap_inline347 5 %. SIS and GIS normalizations are consistent within tex2html_wrap_inline347 5 % with the v2.62 ARFs.

5. Test with the Crab Nebula

The Crab Nebula was observed on September 28, 1994 for tex2html_wrap_inline347 35 ksec. It is known that GIS gain tends to become higher than real for bright sources such as Crab. Hence we adjusted the gain using the instrumental features at 2.2 keV and 4.8 keV as fiducial marks. Thus GIS2 and GIS3 gains were respectively made 99.4 % and 98.6 % of the original gain values determined in the REV1 processing. Deadtime correction is carried out (using the ftool 'deadtime').

In figure 6, results of the fitting with a power-law function are shown. It is seen that the residuals are less than tex2html_wrap_inline347 2.5 % with the v2.62 ARFs, although they are as much as tex2html_wrap_inline347 5 % with the v2.53 ARFs.

In Table 5, best-fit parameters are shown. Using the v2.62 ARFs, the 2 - 10 keV energy fluxes are 2.06 tex2html_wrap_inline377 ergs s tex2html_wrap_inline379 cm tex2html_wrap_inline381 and 2.01 tex2html_wrap_inline377 ergs s tex2html_wrap_inline379 cm tex2html_wrap_inline381 for GIS2 and GIS3 respectively. These are close enough to the standard value by Toors and Seward (Ast.J.79,995), 2.16 tex2html_wrap_inline377 ergs s tex2html_wrap_inline379 cm tex2html_wrap_inline381 .

 
ARF version Sensor tex2html_wrap_inline395 a N b tex2html_wrap_inline399 Flux c tex2html_wrap_inline401 /dof (dof)
v2.53 SIS0 2.9 tex2html_wrap_inline403 1.5 2.72 tex2html_wrap_inline403 0.07 1.65 tex2html_wrap_inline403 0.02 3.46 1.66 (179)
SIS1 <0.5 2.60 tex2html_wrap_inline411 1.65 tex2html_wrap_inline413 3.40 1.25 (179)
GIS2 <0.7 2.25 tex2html_wrap_inline417 1.61 tex2html_wrap_inline403 0.01 3.03 1.10 (94)
GIS3 <1.2 2.37 tex2html_wrap_inline423 1.63 tex2html_wrap_inline425 3.14 1.34 (94)
v2.62 SIS0 <1.6 2.65 tex2html_wrap_inline429 1.61 tex2html_wrap_inline431 3.58 1.52 (179)
SIS1 <0.2 2.72 tex2html_wrap_inline403 0.03 1.63 tex2html_wrap_inline437 3.59 1.26 (179)
GIS2 <0.5 2.69 tex2html_wrap_inline441 1.58 tex2html_wrap_inline437 3.71 1.11 (94)
GIS3 <0.4 2.84 tex2html_wrap_inline403 0.03 1.62 tex2html_wrap_inline413 3.79 1.44 (94)
Table1: 3C273 fit with ascaarf v2.53 and v2.62 ; for individual sensor

a:10 tex2html_wrap_inline451 cm tex2html_wrap_inline381 .
b:10 tex2html_wrap_inline381 photons s tex2html_wrap_inline379 keV tex2html_wrap_inline379 cm tex2html_wrap_inline381 at 1 keV.
c:10 tex2html_wrap_inline381 photons s tex2html_wrap_inline379 cm tex2html_wrap_inline381 for 0.9 - 10 keV. Energy range is 0.9 - 10 keV.

 
ARF version Sensor tex2html_wrap_inline395 a N b tex2html_wrap_inline399 Flux c tex2html_wrap_inline401 /dof (dof)
v2.53 < 0.07 - 1.625 tex2html_wrap_inline479 - 1.41 (552)
SIS0 - 2.61 tex2html_wrap_inline481 - 3.47 -
SIS1 - 2.57 tex2html_wrap_inline481 - 3.41 -
GIS2 - 2.28 tex2html_wrap_inline481 - 3.03 -
GIS3 - 2.36 tex2html_wrap_inline487 - 3.14 -
v2.62 < 0.01 - 1.610 tex2html_wrap_inline491 - 1.40 (552)
SIS0 - 2.65 tex2html_wrap_inline493 - 3.57 -
SIS1 - 2.68 tex2html_wrap_inline493 - 3.60 -
GIS2 - 2.76 tex2html_wrap_inline493 - 3.71 -
GIS3 - 2.81 tex2html_wrap_inline493 - 3.79 -
Table2: 3C273 fit with ascaarf v2.53 and v2.62 ; fitting simultaneously for all the sensors

a: 10 tex2html_wrap_inline451 cm tex2html_wrap_inline381 .
b: 10 tex2html_wrap_inline381 photons s tex2html_wrap_inline379 keV tex2html_wrap_inline379 cm tex2html_wrap_inline381 at 1 keV.
c: 10 tex2html_wrap_inline381 photons s tex2html_wrap_inline379 cm tex2html_wrap_inline381 for 0.9 - 10 keV. Energy range is 0.9 - 10 keV. Only normalizations are made free for individual sensors.

 
ARF version Sensor tex2html_wrap_inline395 a N b tex2html_wrap_inline399 tex2html_wrap_inline525 tex2html_wrap_inline527 Flux c tex2html_wrap_inline401 /dof (dof)
v2.53 SIS0 1.91 tex2html_wrap_inline431 tex2html_wrap_inline533 0.80 tex2html_wrap_inline425 6.7 9.9 1.24 1.51 (179)
SIS1 1.91 tex2html_wrap_inline417 tex2html_wrap_inline539 0.83 tex2html_wrap_inline493 6.6 18.5 1.24 1.66 (179)
GIS2 1.81 tex2html_wrap_inline543 tex2html_wrap_inline545 0.82 tex2html_wrap_inline493 6.4 11.0 1.05 1.17 (113)
GIS3 1.91 tex2html_wrap_inline417 tex2html_wrap_inline551 0.91 tex2html_wrap_inline437 6.5 9.6 1.05 1.24 (113)
v2.62 SIS0 1.91 tex2html_wrap_inline417 tex2html_wrap_inline557 0.77 tex2html_wrap_inline425 6.8 9.0 1.30 1.38 (179)
SIS1 1.90 tex2html_wrap_inline417 tex2html_wrap_inline563 0.79 tex2html_wrap_inline493 6.8 16.7 1.33 1.50 (179)
GIS2 1.85 tex2html_wrap_inline567 tex2html_wrap_inline569 0.81 tex2html_wrap_inline493 6.5 11.6 1.31 1.11 (113)
GIS3 1.91 tex2html_wrap_inline417 tex2html_wrap_inline575 0.88 tex2html_wrap_inline425 6.9 7.5 1.28 1.56 (113)
Table 3: EXO2030 fit with ascaarf v2.53 and v2.62 ; for individual sensor

a: 10 tex2html_wrap_inline579 cm tex2html_wrap_inline381 .
b: 10 tex2html_wrap_inline381 photons s tex2html_wrap_inline379 keV tex2html_wrap_inline379 cm tex2html_wrap_inline381 at 1 keV.
c: 10 tex2html_wrap_inline379 photons s tex2html_wrap_inline379 cm tex2html_wrap_inline381 for 0.9 - 10 keV. Energy range is 0.9 - 10 keV.

 
ARF version Sensor tex2html_wrap_inline395 a N b tex2html_wrap_inline399 tex2html_wrap_inline525 tex2html_wrap_inline527 Flux c tex2html_wrap_inline401 /dof (dof)
v2.53 - 1.92 tex2html_wrap_inline493 - 0.86 tex2html_wrap_inline437 6.5 10.1 - 2.10 (592)
SIS0 - tex2html_wrap_inline613 - - - 1.22 -
SIS1 - tex2html_wrap_inline615 - - - 1.21 -
GIS2 - tex2html_wrap_inline617 - - - 1.04 -
GIS3 - tex2html_wrap_inline617 - - - 1.06 -
v2.62 - 1.92 tex2html_wrap_inline493 - 0.83 tex2html_wrap_inline437 6.8 8.9 - 2.09 (592)
SIS0 - tex2html_wrap_inline625 - - - 1.28 -
SIS1 - tex2html_wrap_inline627 - - - 1.30 -
GIS2 - tex2html_wrap_inline627 - - - 1.30 -
GIS3 - tex2html_wrap_inline627 - - - 1.29 -
Table 4: EXO2030 fit with ascaarf v2.53 and v2.62 ; fitting simultaneously for four sensors

a: 10 tex2html_wrap_inline579 cm tex2html_wrap_inline381 .
b: 10 tex2html_wrap_inline381 photons s tex2html_wrap_inline379 keV tex2html_wrap_inline379 cm tex2html_wrap_inline381 at 1 keV.
c: 10 tex2html_wrap_inline379 photons s tex2html_wrap_inline379 cm tex2html_wrap_inline381 for 0.9 - 10 keV. Energy range is 0.9 - 10 keV. Only normalizations are made free for individual sensors.

 
tex2html_wrap_inline395 a Nb tex2html_wrap_inline399 Flux c tex2html_wrap_inline401 /dof (dof)
v2.53
GIS2 3.1 8.00 2.13 1.65 4.41 (72)
GIS3 3.2 8.08 2.14 1.63 5.69 (72)
v2.62
GIS2 3.00 tex2html_wrap_inline403 0.03 9.44 tex2html_wrap_inline661 2.092 tex2html_wrap_inline403 0.003 2.06 1.83 (72)
GIS3 2.70 tex2html_wrap_inline403 0.04 9.07 tex2html_wrap_inline403 0.04 2.083 tex2html_wrap_inline403 0.003 2.01 1.64 (72)
Table 5: GIS Crab Spectral Fit

a: 10 tex2html_wrap_inline671 cm tex2html_wrap_inline381 .
b: photons s tex2html_wrap_inline379 keV tex2html_wrap_inline379 cm tex2html_wrap_inline381 at 1 keV.
c: 10 tex2html_wrap_inline681 ergs s tex2html_wrap_inline379 cm tex2html_wrap_inline381 for 2 - 10 keV. Energy range is 0.9 - 10 keV.

  

figure3

Figure 3: Model fit for 3C273 with the v2.53 ARF (left) and v2.62 ARF (right) for SIS0 (top) and SIS1 (bottom). SIS fit was carried out for 0.9-10 keV, but the spectra were shown down to 0.5 keV for references. 3C273 may have a soft-excess component over the power-law below tex2html_wrap_inline347 0.9 keV, and XRT/SIS calibration is less reliable in this energy band.

 

figure4

Figure 4: Model fit for 3C273 with the v2.53 ARF (left) and v2.62 ARF (right) for GIS2 (top) and GIS3 (bottom).

  

figure5

Figure 5: Unfolded spectra of four sensors simultaneously fitted. Normalizations are allowed to be free individually. Top two panels are for 3C273 and the bottom two panels are for EXO2030, and v2.53 ARFs left and v2.62 ARFs right. Two GIS spectra gave systematically lower normalizations than SIS with v2.53 ARFs, but GIS and SIS normalizations are consistent with v2.62 ARFs.

  

figure6

Figure 6: Spectral fitting for GIS Crab spectra (top GIS2, bottom GIS3) using the v2.53 ARF (left) and v2.62 ARF (right). Deadtime correction is carried out.


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Next: About this document

Ken Ebisawa
Fri Jul 19 19:14:08 EDT 1996


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