The transient hard X-ray tail of GX 17+2: New BeppoSAX results

Abstract

Abstract. We report on results of two BeppoSAX observations of the Z source GX 17+2. In both cases the source is in the horizontal branch of the colour-intensity diagram. The persistent continuum can be fit by two-component models consisting of a blackbody plus a Comptonization spectrum. With one of these models, two solutions for the blackbody temperature of both the observed and seed photons for Comptonization are equally accepted by the data. In the first observation, when the source is on the left part of the horizontal branch, we observe a hard tail extending up to 120 keV, while in the second observation, when the source moves towards right in the same branch, the tail is no longer detected. The hard ( > ∼ 30 keV) X-ray emission can be modeled either by a simple power-law with photon index Γ ∼ 2.8, or assuming Comptonization of ∼1 keV soft photons off a hybrid thermal plus non-thermal electron plasma. The spectral index of the non-thermal injected electrons is p ∼ 1.7. The observation of hard X-ray emission only in the left part of the horizontal branch could be indicative of the presence of a threshold in the accretion rate above which the hard tail disappears. An emission line at 6.7 keV with equivalent width ∼ 30 eV is also found in both observations. We discuss these results and their physical implications.

INTEGRALSpectral Variability Study of the Atoll 4U 1820−30: First Detection of Hard X‐Ray Emission

“…The resulting purely thermal model provides a worse fit, 2 (d:o:f :) ¼ 1:09(54), with kT e ' 24 keV, ' 1:9, and kT in ' 1:4 keV, similar to the parameters found in the atoll source 4U 1608À52 (Gierliński & Done 2002). To improve the fit of this model, in particular to account for the high-energy residuals (appearing as a high-energy tail), we have added nonthermal electrons to the thermal plasma, as is done to account for some spectra of both black hole (Gierliński et al 1999;Wardziński et al 2002) and neutron star (Farinelli et al 2005) binaries. We have obtained 2 (d:o:f :) ¼ 1:07(52) for a model with the nonthermal tail with the power-law index of p ' 1 above the electron Lorentz factor of 1.6 (and the Maxwellian distribution below it) and other parameters similar to those of the previous model.…”

“…Nonthermal tails have commonly been seen in soft states of black hole binaries, e.g., Cyg X-1 (Gierliński et al 1999) and neutron star Z sources, e.g., GX 17+2 (Di Salvo et al 2000;Farinelli et al 2005), Sco X-1 (D' Amico et al 2001), GX 349+2 (Di Salvo et al 2001), and GX 5À1 (Asai et al 1994), while they are difficult to detect in the atoll sources, e.g., 4U 0614+091 (Piraino et al 1999). On the other hand, nonthermal tails have been found in the hard state of some black hole binaries, e.g., Cyg X-1 (McConnell et al 2002) and GX 339À4 (Wardziński et al 2002).…”

A Hard X-Ray View of Scorpius X-1 with INTEGRAL : Nonthermal Emission?

“…The most probable origin of these components is therefore Comptonization in a hybrid thermal/nonthermal corona (where the Maxwellian velocity distribution of the electrons have a nonthermal high-velocity tail; e.g., Poutanen & Coppi [1998]; see Farinelli et al [2005] and D'Aí et al [2006] for a successful application of this model to the case of GX 17ϩ2 and Sco X-1, respectively). For instance, it is possible that a hard population of electrons is accelerated in internal shocks at the base of a jet.…”

The origin of the hard X-ray tail in neutron-star X-ray binaries

“…The main difficulty of the models that seek to reproduce the high-energy X-ray spectral continuum in NSXB lies in explaining the absence of a cutoff, because the spectrum resulting from Comptonization is expected to roll over at a certain energy (an effect known as Compton recoil), when the energy of the photons approaches that of the electrons. Various models have been put forward to explain the absence of a cutoff in the hard tails: bulk-motion Comptonization (Bradshaw et al 2003;Paizis et al 2006;Farinelli et al 2007Farinelli et al , 2008, Comptonization by a hybridthermal-non thermal corona (Gierliński et al 1999;Özel et al 2000;Zdziarski et al 2001;Farinelli et al 2005;Revnivtsev et al 2014), or synchrotron emission from the electrons of a jet (Markoff et al 2005). Because of the limited sensitivity of current detectors at energies above ∼100 keV, statistically significant detections are given up to ∼200 keV, typically.…”

“…The temperature of the neutron star photons is expected to be higher (kT ∼ 2−3 keV) than that of the disk soft photons (kT ∼ 1 keV), because the effective area of the neutron star surface is more compact than that of the accretion disk (Farinelli et al 2007). Above ∼20 keV, the most prominent spectral feature in the X-ray spectrum of NSXB is a power-law distribution (the hard tail), that usually extends up to 200−300 keV without evidence of a cutoff (Di Salvo et al 2000Iaria et al 2001;D'Amico et al 2001;Farinelli et al 2005;Migliari et al 2007;Ding et al 2011). …”