GRO J1655-40: from ASCA and XMM-Newton Observations
**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
GRO J1655-40: from ASCA and XMM-Newton
Observations
Xiao-Ling Zhang1, Shuang Nan Zhang2,3,4, Gloria Sala1,
Jochen Greiner1, Yuxin Feng3,4, Yangsen Yao5
Abstract. We have analysed four ASCA observations (1994–1995, 1996–
1997) and three XMM-Newton observations (2005) of this source, in all of which
the source is in high/soft state. We modeled the continuum spectra with rel-
ativistic disk model kerrbb, estimated the spin of the central black hole, and
constrained the spectral hardening factor fcol and the distance. If kerrbb model
applies, for normally used value of fcol (1.7), the distance cannot be very small,
and fcol changes with observations.
1. Background
GRO J1655-40, the second microquasar (after GRS 1915-105), had X-ray out-
bursts in 1994-1995, 1996-1997, 2005. Its geometric parameters are considered
best determined: mass MBH = 7.0± 0.2M⊙, inclination angle θ = 69.50
± 0.08
(Orosz & Bailyn 1997), distance D = 3.2 ± 0.2 kpc (Hjellming & Rupen 1995),
which makes it a very good laboratory of studying black holes and environments.
The spin of the central black hole has been estimated by various authors
with various methods (see, e.g., Zhang et al. 1997; Abramowicz & Kluźniak 2001;
Aschenbach 2004; Shafee et al. 2006), and the reported value range from 0.2
(Abramowicz & Kluźniak 2001) to 0.996 (Aschenbach 2004).
In estimating black hole spin from continuum spectral modeling, the color
correction factor fcol = Tcol/Teff , is one of the key factors. The normally used
value of fcol is 1.7, following Shimura & Takahara (1995), while many authors
believe it should not be constant (see, e.g., Merloni et al. 2000). The distance
is also very important. The widely accepted value 3.2± 0.2 kpc was challenged
by Foellmi et al. (2006), who gave an upper limit of 1.7 kpc.
2. Observations, data reduction and model fitting
We analysed three ASCA observations during the 1994–1995 and the 1996–1997
outbursts, and three XMM-Newton observations during the 2005 outburst, in all
of which the source was in high/soft state. For ASCA, only GIS2 data were used,
after gain correction and deadtime correction. For XMM-Newton, only Epic-pn
MPE, Postfach 1312, 85741 Garching, Germany, zhangx@mpe.mpg.de
Tsinghua Univ, 100084, Beijing, China
U. of Alabama, Huntsville, AL 35899, USA
NSSTC, Sparkman Dr. 320, Huntsville AL 35805, USA
MIT Kavli Inst. for Astro. and Space Research, 70 Vassar Street, Cambridge, MA 02139
http://arxiv.org/abs/0704.0734v1
2 X.-L. Zhang et al
data were used, after correction for rate-dependent Charge-Transfer-Efficiency
(Sala et al. 2006).
The classical way of estimating black hole spin from the continuum spectral
fitting is to fit the spectra with disk models, and obtain the spin directly or
indirectly. All models take the source distance as parameter, and most models
treat the disk as multi-temperature black-body rings and the derived spin value
depends on the apparent/effective temperature ratio.
The relativistic disk model kerrbb in XSPEC was used in the fitting. We
let fcol vary from 1.0 to 3.0, and D vary from 1.0 kpc and 3.2 kpc. For each
combination of fcol and D, we fitted the data sets and obtained a spin value, if
the fit was acceptable (χ2/dof < 2). The contour of the derived spin a over D
and fcol are shown in the Fig. 1.
3. Conclusion
From Fig. 1 we can see, 1. for the normally used fcol value 1.7, kerrbbmodel does
not favor small distance; 2. because the black hole spin and the source distance
should be constant, fcol changes dramatically between these observations.
Figure 1. Contour of black hole specific angular momentum a versus dis-
tance D and fcol. The two dotted lines indicate D=1.7kpc, and fcol = 1.7.
References
Abramowicz, M. A., & Kluźniak, W. 2001, A&A, 374, L19
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Foellmi, C., Depagne, E., Dall, T. H., & Mirabel, I. F. 2006, A&A, 457, 249
Hjellming, R.M. & Rupen, M.P. 1995, Nat, 375, 464
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Orosz, J. A., & Bailyn, C. D. 1997, ApJ, 477, 876
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http://arxiv.org/abs/astro-ph/0606272
