Science in China Series G: Physics, Mechanics & Astronomy
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The flux modulation of INTEGRAL
Chen YuPeng†, Zhang Shu, Wang JianMin, Li TiPei & Qu JinLu
Laboratory for Particle Astrophysics, Institute of High Energy Physics, Beijing 100049, China
We have analyzed International Gamma-Ray Astrophysics Laboratory (INTEGRAL) data corresponding
to observations of the high mass X-ray binary (HMXB) V0332+53 during its huge outburst occurred in
2004－2005, and found that there is a quasi periodic signal with ultra low frequency of roughly 4－
5  10-5 Hz existing in the observational. This feature is revealed through a series of consistent analysis
of imaging and fitting the energy spectrum for each science window (SCW), and shows up in the corresponding light curves. The analysis of the power spectrum also provides a hint of a power peak located around 4  10-5 Hz, after subtracting off the linear trend of the light curve, showing consistency
with the previous analyses. Similar feature shows up in our analyzing the light curve of Crab in hexagonal dithering mode. Therefore, an instrumental effect on the flux modulation might exist in
INTEGRAL data product of the different levels: imaging, spectral fitting and light curve analysis, for at
least the observational mode of hexagonal dithering. Such an effect is, to our knowledge, not yet
available previously in the current literature
X-ray source, individual, V0332+53
Outbursts of HMXB V0332+53 were observed four
times in the hard X-rays since its discovery. With these
detections, V0332+53 was identified as a high mass
X-ray binary system, consisting of an early-type star BQ
Cam[1] and a neutron star. The distance to the source was
estimated to be 2.2－5.8 kpc[2]. A cyclotron absorption
component at 28.5 keV suggested that the magnetic field
could be as high as 2.5  1012 G on the surface of the
neutron star[3].
The most recent outburst was detected by the
all-sky-monitor (ASM) of the Rossi X-ray Timing Explorer (RXTE) in November 2004 and Target of Opportunity (ToO) observations were carried out by
INTEGRAL at hard X-rays as well. Apart from the feature of 0.05 Hz quasi-periodic oscillation (QPO) reported in Takeshima et al. (1994) [4], an additional QPO feature was discovered from the Proportional Counter Array (PCA) data of RXTE to ride on the spin frequency[5].
The orbital parameters were updated and refined by
Zhang et al. (2005) [6]. Multiple cyclotron lines of~ 25
keV, 50 keV and 72 keV were found in RXTE[7] and
INTEGRAL[8], respectively, by using the observational
data when the source was around its flux maximum
during the outburst. INTEGRAL has carried out a series
of long time durations in hexagonal dithering mode,
during which V0332+53 was as bright as Crab. These
data are therefore the good sample for investigating both
the source and the possible instrumental effect if any.
We report in this paper the discovery of a quasi periodic
signal induced by the observational mode, which is not
yet clarified in the current literature.
1 Observations and Data Analysis
INTEGRAL is a 15 keV－10 MeV gamma-ray mission
supplemented by the Joint European X-ray Monitor
(JEM-X, 3-35 keV) and the Optical Monitor Camera
Received; accepted
doi:
†
Corresponding author (email: chenyp@mail.ihep.ac.cn)
This work was subsidized by the National Natural Science Foundation of China, the
CAS key Project KJCX2-YW-T03, and 973 program 2009CB824800. Wang JianMin
thanks the Natural Science Foundation of China for support via NSFC-10325313,
10521001 and 10733010. DFT acknowledges support by Spanish MEC grant
AYA2006-00530 and CSIC-PIE 200750I029.
CHEN YuPeng Sci China Ser G-Phys Mech Astron | Jan. 2009 | vol. 52 | no. 1 | 1-?
(OMC, V, 500-600 nm). At hard X-rays to gamma-rays,
INTEGRAL payload consists of two main instruments:
the INTEGRAL on-board imager (IBIS, 15 keV－10
MeV) and spectrometer (SPI, 20 keV－8 MeV). The
CdTe array (ISGRI) of IBIS has a continuum sensitivity
in hard X-rays (15 keV－200 keV) much better than SPI.
The satellite was launched in 2002 October and has an
orbital period of 3 days. Due to the coded-mask design
of the detectors, the main detectors work normally in
dithering mode in order to suppress systematic effects on
spatial and temporal background.
The INTEGRAL ToO observations on V0332+53 are
available for 9 revolutions during January－February
2005. The data are collected with the total exposure of
396 ks and 132 SCWs. The majority of the observations
are
carried
out
within
rev.
284
(MJD=53410.053-53410.865), which have total exposure of 149 ks and 43 SCWs, carried out in hexagonal
dithering mode. Since the source emission is weak at
high energies, and the SPI has lower sensitivity than
ISGRI/IBIS at 20-200 keV, the analysis that follows focus on the observations from ISGRI/IBIS and JEMX.
Data reduction was performed by using the standard
Online Science Analysis (OSA) software of version 5.1.
The scientific products of image, spectrum and light
curve are obtained, as shown above, by running the
pipeline in the flowchart up to the corresponding analysis level. The spectra are fitted by using FTOOL software package-version 5.3.1.
2 Results
To investigate the time variability, the source intensity at
20-40 keV is retrieved from the produced images for
each SCW. The intensity has a series of additional small
bursts separated by roughly 6－7 SCWs, with a time
duration of 20－25 ks. The intensity of these additional
outbursts varies by 5－10 percent and their shapes are
not self-similar. Due perhaps to the short time duration
of some revolutions and the poor statistics at low intensity level, such a series of the outburst are not present in
the remaining revolutions. The time variability of the
source intensity is investigated as well using JEMX data.
No trend of a quasi periodic burst is visible in the source
intensity on time scale around 104 seconds. As an example, Figure 1 shows the light curves for rev. 284 in two
energy bands: 5－10 keV and 10-20 keV. The large error
2
bars in the source intensity may be responsible for
missing the features in the intensity evolution, which
appear more clearly in ISGRI/IBIS data.
Figure 1 The light curves of revolution 284 obtained
from imaging analysis (panel (a) in 20－40 keV, panel (c)
in 10－20 keV and panel (d) in 5－10 keV) and spectral
fitting (panel (b)) in the energy band of 20－40 keV. Each
bin represents one SCW.
In the imaging analysis, a likely QPO signal with a
period around 20－25 ks is present only in ISGRI data
of the energy band 20－40 keV, but not at the neighboring ones. To investigate further the possible periodic
signal via spectral analysis, data of both JEMX and
ISGRI/IBIS are combined for each SCW and a spectral
fitting is then performed. Also similarly to Kreykenbohm et al. (2005)[8], we try to fit the data with model cutoffpl, available in xspec, plus two cyclotron absorption lines of the form,
CY (E) = exp (-  1 )exp (-  2 ),
where  is
 =  c exp(-0.5 (
E  Ec
c
)2 ) ,
CHEN YuPeng et al. Sci China Ser G-Phys Mech Astron | Jan. 2009 | vol. 52 | no. 1 | ?-?
with  c being the optical depth, Ec being the line
energy and  c being the line width. A constant is added
into the model to account for the different normalization
of JEMX and ISGRI/IBIS.
The resulting light curve is then plotted in Figure 1, in
comparison to that obtained with imaging analysis. The
light curves obtained from different methods are consistent with each other.
We have also run the standard IBIS pipeline with the
analysis levels up to the level LCR, where the light
curves are produced. The time is binned in slots of 3500
seconds. The produced light curve is shown in Figure 2
for revolution 284 in the energy band 20－40 keV. The
linear fit of this results in a reduced  ~16. An additional sine function can improve the fit by deducing the re2
duced  down to ~ 8. The best-fit frequency is of
3.7±0.04×10-5 Hz.
2
Figure 2 The light curves of revolution 284 generated
by the pipeline up to the level of the light curve production and the linear fit are shown in the upper panel, the fit
by a sine function on the residual in the middle panel,
and the final remainders in the lower panel, in the energy
band of 20－40 keV. The time duration of each bin is
3500.
3 Summary
We argue that this analysis has uncovered a likely periodic signal during the decay of the outburst of HMXB
V0332+53, having an ultra low frequency of 4－5 
10-5 Hz. The signal is present in the light curves produced at different level of the pipeline-imaging, spectral
fitting and light curve analysis. To investigating further
the reality of this signal, we carried out the analysis of
Crab observations in hexagonal dithering mode. The
longest duration of Crab observation published to date in
hexagonal dithering mode, consists of ~ 28 SCWs in
rev.43, corresponding roughly to the exposure of 6 ks.
The light curve as obtained for IBIS/ISGRI in 20－40
keV shows the feature of a flux modulation consistent
with the one found in V0332+53, suggesting that an instrumental effect should be at work for the obtained flux
modulation during the hexagonal dithering mode, where
the flux is modulated by roughly 8 percent. Such an effect is not yet clearly present in the observational data
from the other detectors of INTEGRAL, probably due to
the relatively poor statistics and correspondingly the
larger error bars. The hexagonal dithering observations
are carried out with 6 pointings around the source and
one centered on it. For V0332+53, by taking the typical
pointing duration of ~3500 seconds for each SCW, one
complete hexagonal cycle (7 pointings) of INTEGRAL
is approximately 25000 seconds long, which corresponds to a "frequency" of ~4×10-5 as shown above.
That the flux is modulated with the observational mode
is, to our knowledge, not yet clarified previously in the
current literature for INTEGRAL, but is very important
to those who are interesting in considering the
long-duration variability of XRB. Take V0332+53 as an
example, a QPO assumption of such a flux modulation
would either lead to a rather far accretion disk moving
around the neutron star, or have to introduce an additional object moving around the neutron star, given the
report from Zhang et al. (2005)[6]. Whatever, the herein
reported flux modulation will prevent from analyzing
delicately the INTEGRAL data for the long-duration
time evolution of XRB, at least at the variability level of
less than ~ 8 percent in the flux.
CHEN YuPeng et al. Sci China Ser G-Phys Mech Astron | Jan. 2009 | vol. 52 | no. 1 | ?-?
3
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CHEN YuPeng et al. Sci China Ser G-Phys Mech Astron | Jan. 2009 | vol. 52 | no. 1 | ?-?