Issue |
A&A
Volume 439, Number 2, August IV 2005
|
|
---|---|---|
Page(s) | 575 - 583 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20042231 | |
Published online | 29 July 2005 |
The BeppoSAX 0.1–18 keV spectrum of the bright atoll source GX 9+1: an indication of the source distance
1
Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy e-mail: iaria@fisica.unipa.it
2
Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
3
Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, NL 1098 SJ Amsterdam, The Netherlands
Received:
22
October
2004
Accepted:
5
March
2005
We report the results of a long, 350 ks, BeppoSAX observation of the
bright atoll source GX 9+1 in the 0.12–18 keV energy range. During
this observation GX 9+1 showed a large count rate variability in its
lightcurve. From its color–color diagram we selected six zones and
extracted the source energy spectrum from each zone. We find that
the model, composed of a blackbody plus a Comptonized component
absorbed by an equivalent hydrogen column of ~1.4 1022 cm-2, fits the spectra in the energy range 1–18 keV well; however, below 1 keV a soft excess is present. We find that the spectrum of GX 9+1, in the 0.12–18 keV energy range, is well
fitted by the model above, if we use an equivalent hydrogen column of
~0.8
1022 cm-2, together with several
absorption edges from ionized matter (
,
,
,
, and
) and an absorption
line from
. From the study of these features we deduce
that the electron density of the plasma and the equivalent hydrogen
column density associated with the ionized matter is ~1012 cm-3 and ~1023 cm-2, respectively, at a distance
from the central object of
cm, while at larger
distances the equivalent hydrogen column density associated to the
ionized matter decreases down to ~1022 cm-2.
The value of the equivalent hydrogen column of ~0.8
1022 cm-2 associated to the interstellar matter implies
that GX 9+1 is at a distance of 5 kpc, and not 8.5–10 kpc as
usually assumed. This result implies that the atoll source GX 9+1
does not emit at the Eddington limit, as supposed until now, but
has a luminosity of 6
1037 erg/s in the 0.1–18 keV
energy range, typical of LMXBs belonging to the bright atoll class.
Key words: accretion, accretion discs / stars: individual: GX 9+1 / stars: neutron / X-ray: stars / X-ray: binaries / X-ray: general
© ESO, 2005
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