Context. GX 9+9 (4U 1728-16) is a low mass X-ray binary source harboring a neutron star. Although it belongs to the subclass of the bright Atoll sources together with GX 9+1, GX 3+1, and GX 13+1, its broadband spectrum is poorly studied and apparently does not show reflection features in the spectrum. Aims. To constrain the continuum well and verify whether a relativistic smeared reflection component is present, we analyze the broadband spectrum of GX 9+9 using BeppoSAX and XMM-Newton spectra covering the 0.3-40 keV energy band. Methods. We fit the spectrum adopting a model composed of a disk-blackbody plus a Comptonized component whose seed photons have a blackbody spectrum (Eastern Model). A statistically equivalent model is composed of a Comptonized component whose seed photons have a disk-blackbody distribution plus a blackbody that mimics a saturated Comptonization likely associated with a boundary layer (Western model). Other trials did not return a good fit. Results. The spectrum of GX 9+9 was observed in a soft state and its luminosity is 2.3 × 1037 erg s-1 assuming a distance to the source of 5 kpc. In the Eastern Model scenario, we find the seed-photon temperature and electron temperature of the Comptonized component to be 1.14-0.07+0.10 keV and 2.80-0.04+0.09 keV, respectively, while the optical depth of the Comptonizing corona is 8.9 ± 0.4. The color temperature of the inner accretion disk is 0.86-0.02+0.08 keV and 0.82 ± 0.02 keV for the BeppoSAX and XMM-Newton spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 ± 0.07 keV and 1.01 ± 0.08 keV for the BeppoSAX and XMM-Newton spectrum, respectively. The electron temperature of the Comptonized component is 2.9 ± 0.2 keV, while the optical depth is 9.4-1.1+1.5. The blackbody temperature is 1.79-0.18+0.09 keV and 1.85-0.15+0.07 keV for the BeppoSAX and XMM-Newton spectrum, respectively. The addition of a relativistic smeared reflection component improved the fit in both the scenarios, giving compatible values of the parameters, even though a significant broad emission line in the Fe-K region is not observed. Conclusions. From the reflection component we estimated an inclination angle of about 43-4+6 deg and 51-2+9 deg for the Eastern and Western Model, respectively. The value of the reflection fraction ω/2π is 0.18 ± 0.04 and 0.21 ± 0.03 for the Eastern and Western Model, respectively, suggesting that the Comptonized corona should be compact and close to the innermost region of the system.

Reflection component in the Bright Atoll Source GX 9+9

Iaria R.;Di Salvo T.;Gambino A. F.;Sanna A.;Riggio A.;Burderi L.
2020-01-01

Abstract

Context. GX 9+9 (4U 1728-16) is a low mass X-ray binary source harboring a neutron star. Although it belongs to the subclass of the bright Atoll sources together with GX 9+1, GX 3+1, and GX 13+1, its broadband spectrum is poorly studied and apparently does not show reflection features in the spectrum. Aims. To constrain the continuum well and verify whether a relativistic smeared reflection component is present, we analyze the broadband spectrum of GX 9+9 using BeppoSAX and XMM-Newton spectra covering the 0.3-40 keV energy band. Methods. We fit the spectrum adopting a model composed of a disk-blackbody plus a Comptonized component whose seed photons have a blackbody spectrum (Eastern Model). A statistically equivalent model is composed of a Comptonized component whose seed photons have a disk-blackbody distribution plus a blackbody that mimics a saturated Comptonization likely associated with a boundary layer (Western model). Other trials did not return a good fit. Results. The spectrum of GX 9+9 was observed in a soft state and its luminosity is 2.3 × 1037 erg s-1 assuming a distance to the source of 5 kpc. In the Eastern Model scenario, we find the seed-photon temperature and electron temperature of the Comptonized component to be 1.14-0.07+0.10 keV and 2.80-0.04+0.09 keV, respectively, while the optical depth of the Comptonizing corona is 8.9 ± 0.4. The color temperature of the inner accretion disk is 0.86-0.02+0.08 keV and 0.82 ± 0.02 keV for the BeppoSAX and XMM-Newton spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 ± 0.07 keV and 1.01 ± 0.08 keV for the BeppoSAX and XMM-Newton spectrum, respectively. The electron temperature of the Comptonized component is 2.9 ± 0.2 keV, while the optical depth is 9.4-1.1+1.5. The blackbody temperature is 1.79-0.18+0.09 keV and 1.85-0.15+0.07 keV for the BeppoSAX and XMM-Newton spectrum, respectively. The addition of a relativistic smeared reflection component improved the fit in both the scenarios, giving compatible values of the parameters, even though a significant broad emission line in the Fe-K region is not observed. Conclusions. From the reflection component we estimated an inclination angle of about 43-4+6 deg and 51-2+9 deg for the Eastern and Western Model, respectively. The value of the reflection fraction ω/2π is 0.18 ± 0.04 and 0.21 ± 0.03 for the Eastern and Western Model, respectively, suggesting that the Comptonized corona should be compact and close to the innermost region of the system.
2020
Accretion, accretion disks
Stars: individual: GX 9+9
Stars: neutron
X-rays: binaries
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11584/329195
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