In Paul, Raichur and Mukherjee (2005) we had reported a spectral mode change in the X-ray binary pulsar Cen X-3 using the then available ASM light curves of Cen X-3. Being mindful of the fact that instrument calibration or other issues like gain changes, software issues etc. might give rise to some of the artifacts, we had used the ASM light curves of three other X-ray binary pulsars namely Her X-1, SMC X-1 and Vela X-1 for comparison for the same observation times. In light of the recent finding of M\"{u}ller et al. (2011); which reports the non-detection of such features in the current ASM light curves available on the HEASARC website and with other instruments like MAXI, SWIFT and INTEGRAL, we would like to point out the inconsistency between the ASM ligthcurves available in 2005 and now. This information will be very useful for other users of ASM light curves.
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arXiv:1111.2106v1 [astro-ph.HE] 9 Nov 2011
RXTE-ASM light curves of Cen X-3
Biswajit Paul and Harsha Raichur
Raman Research Institute, Bangalore 560080, India
Abstract
In Paul, Raichur and Mukherjee (2005) we had reported a spectral
mode change in the X-ray binary pulsar Cen X-3 using the then available
ASM light curves of Cen X-3. Being mindful of the fact that instrument
calibration or other issues like gain changes, software issues etc. might give
rise to some of the artifacts, we had used the ASM light curves of three
other X-ray binary pulsars namely Her X-1, SMC X-1 and Vela X-1 for
comparison for the same observation times. In light of the recent finding
of M¨uller et al. (2011); which reports the non-detection of such features
in the current ASM light curves available on the HEASARC website and
with other instruments like MAXI, SWIFT and INTEGRAL, we would
like to point out the inconsistency between the ASM ligthcurves available
in 2005 and now. This information will be very useful for other users of
ASM light curves.
Introduction
Using multi-band light curve of the RXTE-ASM, we had reported a significant
change in the X-ray spectrum of Cen X-3 in the period December 2000 to April
2004. This was interpreted as a change in the accretion mode in this period
(Paul, Raichur & Mukherjee, 2005). Recently, M¨uller et al. (2011) reported
non detection of this feature in the RXTE-ASM light curve of the same time
span and also in some additional data from RXTE-ASM, MAXI, Integral and
Swift. We fully agree with the findings of M¨uller et al. (2011)
Here we report that the earlier finding was not due to any error in analysis
of the light curves used in Paul, Raichur & Mukherjee (2005) but due to a
discrepancy in the ASM light curve for the same period that was made available
in 2005 and what is available now.
RXTE-ASM light curves
We would like to note here that the files used by us for the report of 2005 were
all for observations from 4th or 5th January 1996 to 7th November 2004 and all
the fits files were created on 9th November 2004.
Below we show a plot of the light curves in the three energy bands that
was available in 2005 (with no further analysis done on the original lightcurves)
and the light curves now available from the NASA’s High energy Astrophysics
Science Archive Center (HEASARC). All these light curves are the dwell-by-
dwell data as used in the M¨uller et al (2011) research note. The time bin used
1
to plot the light curves is 2.08702 d. The red points are from the older light
curves and the black points are from the current light curves. Only data points
upto TJD 13600 are ploted for clarity. While small differences are seen between
the two sets for the 1.5-3 keV and 3-5 keV bands, the two light curves in the
5-12 keV band are significantly different in the second half of the plot. We have
also verified that this difference is present only for the light curves of Cen X-3
and not in a significant way for the other sources namely Vela X-1, Her X-1 and
SMC X-1 for which we have the older version of light curves to compare. The
older version of the light curves for Cen X-3, Her X-1, Vela X-1 and SMC X-1
are available with the authors and can be shared on request.
0
2
4
6
1.5−3 keV
0
2
4
6
3−5 keV
ASM Count rate in energy bands
104
1.1×104
1.2×104
1.3×104
0
10
20
5−12 keV
TIME (TJD)
Figure 1: The light curves of Cen X-3 are shown here with a binsize of 2.087
days.
Conclusion
The multiband RXTE-ASM light curves of Cen X-3 that were available at
HEASARC in 2005 are different from the light curves that are now available
at the same site. We would also like to note here that before publishing the
results in 2005, we had discussed with the RXTE-ASM team members about
any possible artifacts. The difference in the light curves was noticed in 2010 and
the same was communicated to the RXTE-ASM team. As yet, no clarification
is available. This information about possible artifacts will be useful for users of
ASM light curves.
References
[1] Paul, B., Raichur, H., Mukherjee, U., 2005, A&A, 442, 15
[2] M¨uller, D., Klochkov, D., Santangelo, A., Mihara, T., & Sugizaki, M., 2011,
A&A, in press [arXiv:1111.0438v1]
2
