Figure 1 shows a 5 PCU light curve for NGC 1068
Following an inquiry by Iossif Papadakis, I have investigated
NGC 1068 (RXTE obs-id 10322-01-01-***). IP observed large variability
in the light curve, inconsistent with previous observations of this
object. Figure 1 shows the light curve (5 detectors, first layer,
channels 0-27) after standard background subtraction (i.e. ftools
4.2, models pca_bkgd_faintl7_e3v19990824.mdl.gz and
pca_bkgd_faint240_e3v19990909.mdl.gz, elevation > 10, offset < 0.02,
time since SAA > 30 min, electron flag < 0.1, 5 pcu on).
Figure 1 shows a 5 PCU light curve for NGC 1068
I verified that the variability is present in each detector,
so undetected detector breakdown is ruled out.
I characterized the "excess" counts by dividing the data in
figure 1 into intervals where the rate was greater or less than
4 cts/sec. I used XSPEC to analyze 2 spectra: the "low" spectrum, using
the associated PCABACKEST derived background, and the "high" spectrum using
the "low" spectrum as a background. (Note that this assumes that the average
instrument background associated with the low and high spectra is the same, and that the
subtraction removes it completely; a proper analysis of the excess counts should
do this properly). Figure 2 shows the results of fitting a power-law + gaussian +
absorption model to the two spectra. (The response matrix was created
with pcarmf v7.01 released with FTOOLS 5.0).
Figure 2.
A snapshot of the XSPEC conditions shows:
XSPEC> sh all
15: 1: 5 16-Feb- 0
Auto-saving is done after every command.
Fit statistic in use is Chi-Squared
Minimization technique is Lev-Marq
Convergence criterion = 0.01
Querying enabled
Prefit-renorming enabled
Solar abundance table is angr
Information for file 1
belonging to plot group 1, data group 1
telescope = XTE , instrument = PCA , channel type = PHA
Current data file: data_5pcu_0_27_l1_l.pha
Background file :sky_bkgd_5pcu_0_27_l1.pha
No current correction
Response (RMF) file : ../ngc1068_5b/5pcu_l1.rsp
Auxiliary (ARF) file : none
XSPEC filter : NONE
Weighting method is standard
Noticed channels 4 to 57
File integration time 3.0864E+04
and effective area 1.000
File observed count rate 4.145 +/-4.54167E-02 cts/s
Model predicted rate : 4.040
Information for file 2
belonging to plot group 2, data group 2
telescope = XTE , instrument = PCA , channel type = PHA
Current data file: data_5pcu_0_27_l1_h.pha
Background file :data_5pcu_0_27_l1_l.pha
No current correction
Response (RMF) file : ../ngc1068_5b/5pcu_l1.rsp
Auxiliary (ARF) file : none
XSPEC filter : NONE
Weighting method is standard
Noticed channels 4 to 57
File integration time 6928.
and effective area 1.000
File observed count rate 0.7511 +/-8.15345E-02 cts/s
Model predicted rate : 0.9801
mo = ( powerlaw[1] + gaussian[2] )wabs[3]
---------------------------------------------------------------------------
---------------------------------------------------------------------------
mo = ( powerlaw[1] + gaussian[2] )wabs[3]
Model Fit Model Component Parameter Unit Value Data
par par comp group
1 1 1 powerlaw PhoIndex 1.380 +/- 0.6523E-01 1
2 2 1 powerlaw norm 8.2973E-04 +/- 0.2330E-03 1
3 3 2 gaussian LineE keV 6.566 +/- 0.1787E-01 1
4 4 2 gaussian Sigma keV 0.3575 +/- 0.3441E-01 1
5 5 2 gaussian norm 1.7249E-04 +/- 0.1125E-04 1
6 6 3 wabs nH 10^22 6.2755E-08 +/- 1.475 1
7 7 4 powerlaw PhoIndex 2.424 +/- 0.2405 2
8 8 4 powerlaw norm 1.6718E-03 +/- 0.1077E-02 2
9 3 5 gaussian LineE keV 6.566 = par 3 2
10 4 5 gaussian Sigma keV 0.3575 = par 4 2
11 9 5 gaussian norm 3.1240E-06 +/- 0.1591E-04 2
12 6 6 wabs nH 10^22 6.2755E-08 = par 6 2
---------------------------------------------------------------------------
---------------------------------------------------------------------------
Chi-Squared = 171.0383 using 108 PHA bins.
Reduced chi-squared = 1.727660 for 99 degrees of freedom
Null hypothesis probability = 9.478E-06
XSPEC> log none
The powerlaw differs by 1 in the index and the Fe line is all but absent,
strengthening the conclusion that the excess emission is unrelated to NGC 1068.
The ROSAT All Sky Survey shows 4 sources within 2 deg of NGC 1068, none
brighter than 0.1 NGC 1068 in the survey. This doesn't rule out variability in
these sources, or an unrelated transient.
We will continue to explore other explanations for the "excess" emission associated with the observation of NGC 1068. While it is unlikely to be associated with the target source, we do not currently have a good explanation. Keith Jahoda 16 February 2000