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

Abstract

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.