Of all known persistent stellar-mass black hole candidates, only LMC X-1 and
LMC X-3 consistently show spectra that are dominated by a soft, thermal
component. We present results from long (170ksec) Rossi X-ray Timing Explorer
(RXTE) observations of LMC X-1 and LMC X-3 made in 1996 December. The spectra
can be described by a multicolor disk blackbody plus an additional high-energy
power-law. Even though the spectra are very soft (Gamma is about 2.5), RXTE
detected a significant signal from LMC X-3 up to energies of 50keV, the hardest
energy at which the object was ever detected.
  Focusing on LMC X-3, we present results from the first year of an ongoing
monitoring campaign with RXTE which started in 1997 January. We show that the
appearance of the object changes considerably over its ~200d long cycle. This
variability can either be explained by periodic changes in the mass transfer
rate or by a precessing accretion disk analogous to Her X-1.Comment: 4 pages, 5 figures, also available at
  http://aitzu3.ait.physik.uni-tuebingen.de/publications/preprints1998.html to
  be published in "Highlights of X-Ray Astronomy, a symposium in honour of
  Joachim Truemper" (B. Aschenbach et al., eds.), MPE Repor

Begelman, M. C.

Dove, J. B.

Heindl, W. A.

Nowak, M. A.

Pottschmidt, K.

Staubert, R.

Wilms, J.

English

arXiv

Of all known persistent stellar-mass black hole candidates, only LMC X-1 and LMC X-3 consistently show spectra that are dominated by a soft, thermal component. We present results from long (170 ksec) Rossi X-ray Timing Explorer (RXTE) observations of LMC X-1 and LMC X-3 made in 1996 December. The spectra can be described by a multicolor disk blackbody plus an additional high-energy power-law. Even though the spectra are very soft (Gamma approximately 2.5), RXTE detected a significant signal from LMC X-3 up to energies of 50 keV, the hardest energy at which the object was ever detected. Focusing on LMC X-3 , we present results from the first year of an ongoing monitoring campaign with RXTE which started in 1997 January. We show that the appearance of the object changes considerably over its approximately 200 d long cycle. This variability can either be explained by periodic changes in the mass transfer rate or by a precessing accretion disk analogous to Her X-1

NASA Technical Reports Server

RXTE Observations of LMC X-1 and LMC X-3

Of all known persistent stellar-mass black hole candidates, only LMC X-1 and LMC X-3 consistently show spectra that are dominated by a soft, thermal component. We present results from long (170 ksec) Rossi X-ray Timing Explorer (RXTE) observations of LMC X-1 and LMC X-3 made in 1996 December. The spectra can be described by a multicolor disk blackbody plus an additional high-energy power-law. Even though the spectra are very soft (Gamma approximately 2.5), RXTE detected a significant signal from LMC X-3 up to energies of 50 keV, the hardest energy at which the object was ever detected. Focusing on LMC X-3, we present results from the first year of an ongoing monitoring campaign with RXTE which started in 1997 January. We show that the appearance of the object changes considerably over its approximately 200d long cycle. This variability can either be explained by periodic changes in the mass transfer rate or by a precessing accretion disk analogous to Her X-1

tqf c-5--73q
":'i "
RXTE Observations of LMC X-1 and LMC X-3
J. Wilms 1, M.A. Nowak 2, J.B. Dove 3, K. Pottschmidt 1, W.A. Heindl 4, M.C. Begelman _, and
R. Staubert 1
1 Institut f/Jr Astronomie und Astrophysik - Astronomie, Waldhiiuser Str. 64, D-72076 Tiibingen, Germany
JILA, University of Colorado, Boulder, CO 80309-440, U.S.A.
3 CASA, University of Colorado, Boulder, CO 80309-389, U.S.A.
CASS, University of California, San Diego, La Jolla, CA 92093, U.S.A.
_, Abstract'. Of all known persistent stellar-rnas_ black he:'e
candidates, only LMC X-1 and LMC X-3 consistently
show spectra that are dominated by a soft, thermal com-
_._ponent. We present results from long (170ksec) Rossi
t,9 X-ray Timing Explorer (RXTE) observations of LMC X-1
{::)and LMC X-3 made in 1996 December. The spectra can
k_be described by a multicolor disk blackbody plus an ad-
ditional high-energy power-law. Even though the spectra
are very soft (F --- 2.5), RXTE detected a significant sig-
[-_nal from LMC X-3 up to energies of 50 keV, the hardest
energy at which the object was ever detected.
Focusing on LMC X-3 , we present results from the
first year of an ongoing monitoring campaign with RXTE
(_,_which started in 1997 January. We show that the appear-
ance of the object changes considerably over its _,200d
¢_ long cycle. This variability can either be explained by pe-
i*_iodic changes in the mass tra_,sfer rate or by a precessing
O accretion disk analogous to Her X-1.
1. Introduction
Since the discovery of Cygnus X-1 in 1964 (Bowyer et al.,
1965), the s_udy of galactic black hales (BHs) has shown
that these objects reveal a large variety of states, which are
characterized by their distinct spectral shapes and tempo-
ral behaviors. The most important states which have been
identified are the so called "hard state", which is charac-
terized by a hard X-ray spectrum with a photon index
F = 1.7 and large variability (rms = 30%, see contribu-
tions by Pottschmidt et al. and Belloni in this volume),
and the "soft state", which is spectrMly softer (F -_ 2.5)
and characterized by less variability. The overall luminos-
ity of sources in the soft state appears to be higher than
that of sources in the hard state (Nowak, 1995).
Despite the fact that the soft state is very common in
galactic BHs, most observational attention has been con-
centrated on the hard state, since most of the brighter
galactic BHs are found in this state and only show oc-
casional state switches to the soft state. Only two of the
persistent galactic BHs, LMC X-1 and LMC X-3, are al-
ways found in the soft state. These objects were discov-
_red by Uhuru duAng scans of the Large Magellanic Cloud
(Leong et al., 1971). Although their intrinsic luminosity is
quite high (a few 103serg/sec), the large distance of the
LMC prevented the detailed study of these objects for
a long time. Such a study became feasible with the ad-
vent of detectors with large effective areas. Ginga results
on LMC X-1 and LMC X-3 revealed that both sources
exhibit very interesting physical behavior, such as a pos-
sible low-frequency QPO and long term spectral variabil-
ity (Ebisawa et al., 1991, 1993). No systematic monitor-
ing was done by Ginga, however, and the absence of an
instrument sensitive above ,-.20 keV prohibited gathering
information about the high energy spectrum.
To enable a systematic study of the soft state we have
monitored LMC X-1 and LMC X-3 with the Rossi X-ray
Timing Explorer (RXTE) since the end of 1996 in three
to four weekly intervals, which has provided an unique
opportunity to study their long term behavior. To facil-
itate the understanding of the spectrum, the campaign
started with 170ksec long observations of both sources
In this contribution we present first results from the spec-
tral analysis of the long observations. Note that the results
presented here are preliminary since the campaign will be
continued throughout all of 1998. We start, in Sect. 2, with
a description of our data analysis methodology. We then
_resent the outcome 6f the analysis of the long observa-
tions of LMC X-1 (Sect. 3) and LMC X-3 (Sect. 4.1). The
spectral variability of LMC X-3 during the first year of
the campaign is described in Sect. 4.2.
2. Data Analysis
The data presented here were obtained with the Rossi
X-ray. Timing Explorer (RXTE). Onboard RXTE are
two pointed instruments, the Proportional Counter Array
(PCA) and the High Energy X-ray Timing Experiment
(HEXTE), as well as the All Sky Monitor (ASM). We used
the standard RXTE data analysis software, ftools 3.5.
Spectral modeling was done using XSPEC, version 10.00s
(Arnaud, 1996). In the meantime a revised release of the
data analysis software and of the response matrices has
become available. The results of an analysis using these
https://ntrs.nasa.gov/search.jsp?R=19990070940 2020-06-15T21:50:48+00:00Z
J.Wilmsetal.:RXTEObservationsofLMCX-1andLMCX-3
improvedtoolsarethe subjectof a forthcomingpaper
(Wilmset al., 1998b).
ThePCAconsistsof fiveco-alignedXenonpropor-
tionalcounterunits (PCUs)with a total effectivearea
of about6500cm2.Theinstrumentissensitivein theen-
ergyrangefrom2keVto ,-, 60keV (Jahoda et al., 1997),
although response matrix uncertainties currently limit the
usable energy range to 2.5-30keV. We used a pre-release
version of the PCA response matrices, version 2.2.1, for
the spectral analysis (Jahoda, 1997, priv. comm.). The
spectral calibration of the instrument appears to be un-
derstood on the 2% level for this version of the matrix.
Even for count-rates as small as those of LMC X-1 and
LMC X-3, the systematic uncertainty affects the data.
Therefore, we added a 2% systemat;c error to ':he data. See
Dove et al. (1998) and Wilms et al. (1998a) for an in-depth
discussion of the PCA calibration issues. Background sub-
traction of the PCA data was performed analogously to
our previous study of Cyg X-1 (Dove et al., 1998). Since
the major uncertainty of the PCA background model is in
the description of the radioactivity induced by spallation
of the detector material during passages of the satellite
through the South Atlantic Anomaly (SAA), we ignored
data measured in the 30 minutes after SAA passages.
HEXTE consists of two clusters of four NaI/CsI-
phoswich scintillation counters that are sensitive from 15
to 250 keV. A full description of the instrument is given by
Rothschild et al. (1998). Background subtraction is done
by source-background switching. We used the standard
HEXTE response matrices of 1997 March 20. Data mea-
sured above 20 keV were used. In our spectral fits we took
care of the intercalibration of the instruments by intro-
ducing a multiplicative constant. We consistently found
the HEXTE fluxes to be 75% of the PCA fluxes. Most
probably, this offset between fluxes is due to a slight mis-
alignment of the HEXTE honeycomb collimators (Heindl,
1998, priv. comm.).
3. LMC X-l: The Long Observation
LMC X-1 is a good candidate for a black hole. Using a
large number of ROSAT HRI observations, Cowley et al.
(1995) were able to identify the counterpart with "star
number 32" of Cowley et al. (1978). This object has a
mass function of only f = 0.144 Mo, but including other
evidence the mass of the compact object appears to be
M > 4 M O (Hutchings et al., 1987). The luminosity of the
object is about 2 x 103Serg/s (Long et al., 1981) and was
not found to be variable (Syunyaev et al., 1990). We could
verify the latter statement in our monitoring campaign.
We therefore concentrate here on the results from the long
RXTE observation from 1996 December 6 to 8, which is a
typical example for the spectrum of LMC X-1. The object
was circumpolar during the three days of the observation.
We only use data from time intervals when all five PCA-
PCUs were turned on. Taking also our 30minutes SAA
s
8
10
1.05
_,oo
o.95
1.00
0.95
' ....... '+''' a)
- ------" " +1 i
.:. c):
4 6 8 10 12 14 16
Channel Energy [keV]
Fig. 1. a PCA spectrum of 80 ksec of on-source data on
LMC X-1. b Ratio between the data and the best fit spec-
tral model without and e with a smeared edge feature.
exclusion time interval into account, a total of 80 ksec of
data were obtained.
In Fig. 1 we show the total PCA spectrum measured
du:ing tha_ tiine. The spectrum can be well described by
either a pure black-body or a muiticolor disk blackbody
(MCD) with kT = "-0.04++°°2keVto which a high energy
power-law with a photon index F -- 3 65 +°'°s is added. In
" -0.07
both cases, :_2/dof = 50.5/30. The multicolor disk black
body is an approximation to the spectrum of an accretion
disk with T(r) o( r -s/4, i.e., a simple a-disk. See Mitsuda
et al. (1984, eq. 4) for a detailed description of this model.
The parameters are consistent with those found in pre-
vious investigations (Schlegel et al., 1994; Ebisawa et al.,
1991, and references therein). Introduction of a smeared
edge in the region around 7.5keV, as was required in the
Ginga and BBXRT analysis, slightly increases the qual-
ity of the fit (Fig. lb and c), although the uncertainty of
the PCA response matrix prohibits any statement whether
the improvement is real. Note, however, that in this en-
ergy region the soft disk radiation and the power-law have
the same strength, so that the edge might be just a feature
associated with this region of overlap.
J.Wilmsetal.:RXTEObservationsofLMCX-1andLMCX-3 3
 Mo×3 , ,°'I
1 2 31996 Dece4mber 5
'_' 10" + -i
_- 10-2
_.2o ,_ 10_3
10 .4 .... , I -
" i I ........
0.95_ ill I
...... _.o5 I
• ,B ,,, .... ,2 1._ .... _-_'-_÷_+_m_tt4t_ i .........
0.95 /0,0F
Fig. 2. Background-subtracted light-curve of LMC X-3 for 3 10 20 50
the long observation in 1996 December. The short gaps are Energy[keV]
due to SAA passages and Earth occultations. The "spikes"
are due to the PCA background model which is insufficient
directly after the SAA passage (these data were excluded
in the analysis). Note the "flare" in the first two data
blocks.
Fig. 3. Combined PCA/HEXTE spectrum of the long ob-
servation of LMC X-3 (including the "flare" - we did not
find any variation in spectral shape over the whole obser-
vation).
4. LMC X-3
,_.1. The Long Observation
LMC X-3 is the most luminous BH in the LMC. The ob-
ject has a peak X-ray luminosity of about 4 x 303Serg/s
and is variable by a factor of about four on time scales
of 100d or 200d (Cowley et al., 1991, see also Sect. 4.2}.
The optical counterpart is a well established B3 V star in a
1.7 d orbit. The mass function of the system is f = 2.3 M O
(Cowley et al., 1983). Using the absence of X-ray eclipses
to determine an upper limit for the inclination the mass
of the compact object is found to be above 9 M O (Cow-
ley et al., 1983) and therefore a very safe candidate for a
Black Hole.
In Fig. 2 we show the lightcurve of our LMC X-3 ob-
servation. The larger countrate of the object compared
to LMC X-1 allows the inclusion of the high energy
HEXTE data. Fig. 3 shows that the object is detected
out to 50 keV, the highest energy at which LMC X-3
has ever been observed. The joint PCA/HEXTE data can
be well described by a multicolor disk black-body with
k_n = 1.25 4- 0.01 keV plus a power-law with a photon-
index of F = 2.5 4- 0.2 (x2/dof = 114/117). These values
are in agreement with previous observations by EXOSAT
and Ginga (Treves et al., 1988, 1990). Adding a smeared
edge at 7.5keV to the data improves the fit (Figs. 3b
and c), but as with LMC X-1 the feature might be caused
by the transition between the disk black-body and the
power-law, and may not be a true spectral feature in its
own right.
4.2. Spectral variability
Since the MCD plus power-law model was shown to give a
good description of the long observation we also used this
model to describe the data from our short (10 ksec) moni-
toring observations. In Fig. 4 we present the results of this
modeling for the first year of our campaign. Note that all
caveats associated with our use of the older background
model also apply to these data (Dove et al., 1998, cf. ). In
our fits we find that for the first half of the observations,
until about 1997 May, lower MCD temperatures are cor-
related with softer high energy power-laws (Fig. 4). Such
a tendency appears not to be present in the second half
of the observations. This could indicate that the soft and
hard spectral components, which we associate with the ac-
cretion disk and a Comptonizing corona as most plausible
places of origin, are produced in geometrically separate
regions of the system. We will be able to test this claim
with the data from the second year of observations which
are currently being measured.
In Fig. 5 we place the above results in the bigger pic-
ture resulting from the RXTE ASM data. The ASM data
for 1996 clearly show the pure _ 200d periodicity found
by Cowley et al. (1991). After that year, however, the sinu-
soidal variation is replaced by a much more complicated
variability pattern. Note that at the same time the ap-
4 J. Wilms et al.: RXTE Observations of LMC X-1 and LMC X-3
2;
0
1.2
1.0
0.8
35
25
5
4
L. 3
2
0.3
,_0.2
0.1
0.0
JD-2450000
4O0 5OO 60O 70O
.,,r. .... 1 ..... i .... i ....
_o o o o q o o° o °o
" 0o° • o • o ° a o * ° o °
. vo ooo _ ° • o°°o
=e,_ , , ° _ o ° o °o
. ®_ +$°.°
4p _ °• • 0 0 0 0 -
÷
• ¢...
)= : i+'
¢ ** I +
t, I i_
t " l,l
Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov
1996 1997
Fig. 4. Top: RXTE/ASM soft X-ray flux, illustrating the
long term variability. Following Plots: Temporal vari-
ability of the best-fit parameters for the first 20 mon-
itoring observations, Tin: inner disk temperature in the
multicolor disk model (keV), r _ sin 0: normalization of the
MCD model, F: photon index of the high energy power-
law, ApL: Normalization of the power-law (photon flux at
I keV).
LMC X-3: ASM ld averages
JD-;_4*_(X)O0
200 300 400 500 600 700 800
f ... -"i
= _ _'3_-.'-" .. --._ . :
5.'== .: ]
F ° ._" :.o "-;,- • 7. .,
,',..:..,.,.,1
FMAMJ J ASONDJ FMAMJ J ASOND J FMAM
1996 1997 1998
Fig. 5. RXTE ASM lightcurve since the launch of RXTE.
parent correlation between the spectral parameters shown
in Fig. 4 vanishes. Since the variability is either inter-
preted by a radiatively warped accretion disk (Maloney
et al., 1996) or by a disk warped by an accretion disk
wind (analogous to Her X-l; Schandl, 1996), the change
in the variability pattern could indicate that the accre-
tion disk of LMC X-3 is unstable. Further data, as well
as the analysis of the data from our current monitoring
campaign, might clarify this phenomenon.
Acknowledgements. The research presented in this paper has
been financed by NASA grants NAG5-3225, NAG5-4621,
NAG5-4737, and a travel grant to J.W. and K.P. from the
DAAD.
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RXTE Observations of LMC X-1 and LMC X-3

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