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Register a new userA Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with $emph{NuSTAR}$
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We present $emph{NuSTAR}$ observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, $F_{mathrm{Edd}}$, of 0.01; GX 17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K$_{alpha}$ line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be $R_{in}=1.03pm0.03$ ISCO (innermost stable circular orbit) assuming a dimensionless spin parameter $a_{*}=cJ/GM^{2}=0.0$, and $R_{in}=1.08pm0.06$ ISCO for $a_{*}=0.3$ (errors quoted at 1 $sigma$). This value proves to be model independent. For $a_{*}=0.3$ and $M=1.4 M_{odot}$, for example, $1.08pm0.06$ ISCO translates to a physical radius of $R=10.8pm0.6$ km, and the neutron star would have to be smaller than this radius (other outcomes are possible for allowed spin parameters and masses). For GX 17+2, $R_{in}=1.00-1.04$ ISCO for $a_{*}=0.0$ and $R_{in}=1.03-1.30$ ISCO for $a_{*}=0.3$. For $a_{*}=0.3$ and $M=1.4 M_{odot}$, $R_{in}=1.03-1.30$ ISCO translates to $R=10.3-13.0$ km. The inner accretion disk in 4U 1705-44 may be truncated just above the stellar surface, perhaps by a boundary layer or magnetosphere; reflection models give a radius of 1.46-1.64 ISCO for $a_{*}=0.0$ and 1.69-1.93 ISCO for $a_{*}=0.3$. We discuss the implications that our results may have on the equation of state of ultradense, cold matter and our understanding of the innermost accretion flow onto neutron stars with low surface magnetic fields, and systematic errors related to the reflection models and spacetime metric around less idealized neutron stars.
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Aims: A hard X-ray shortage, implying the cooling of the corona, was observed during bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart from these four sources, we investigate here an atoll sample, in which the number of bursts for each source is larger than 5, to explore the possible additional hard X-ray shortage during {it Rossi X-ray timing explorer (RXTE)} era. Methods: According to the source catalog that shows type-I bursts, we analyzed all the available pointing observations of these sources carried out by the {it RXTE} proportional counter array (PCA). We grouped and combined the bursts according to their outburst states and searched for the possible hard X-ray shortage while bursting. Results: We found that the island states of KS 1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5 and 4.7 $sigma$ and a systematic time lag of $0.9 pm 2.1$ s and $2.5 pm 2.0$ s with respect to the soft X-rays, respectively. While in their banana branches and other sources, we did not find any consistent shortage.
We analysed the X-ray spectra of six observations, simultaneously taken with XMM-Newton and Rossi X-ray Timing Explorer (RXTE), of the neutron star low-mass X-ray binary 4U 1636-53. The observations cover several states of the source, and therefore a large range of inferred mass accretion rate. These six observations show a broad emission line in the spectrum at around 6.5 keV, likely due to iron. We fitted this line with a set of phenomenological models of a relativistically broadened line, plus a model that accounts for relativistically smeared and ionised reflection from the accretion disc. The latter model includes the incident emission from both the neutron-star surface or boundary layer and the corona that is responsible for the high-energy emission in these systems. From the fits with the reflection model we found that in four out of the six observations the main contribution to the reflected spectrum comes from the neutron-star surface or boundary layer, whereas in the other two observations the main contribution to the reflected spectrum comes from the corona. We found that the relative contribution of these two components is not correlated to the state of the source. From the phenomenological models we found that the iron line profile is better described by a symmetric, albeit broad, profile. The width of the line cannot be explained only by Compton broadening, and we therefore explored the case of relativistic broadening. We further found that the direct emission from the disc, boundary layer, and corona generally evolved in a manner consistent with the standard accretion disc model, with the disc and boundary layer becoming hotter and the disc moving inwards as the source changed from the hard in to the soft state. The iron line, however, did not appear to follow the same trend.
Iron emission lines at 6.4-6.97 keV, identified with fluorescent Kalpha transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper we present a recent XMM observation of the X-ray burster 4U 1705-44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kalpha transition of highly ionised iron, Fe XXV, the inner disc radius is Rin = (14 pm 2) R_g (where R_g is the Gravitational radius, GM/c^2), the emissivity dependence from the disc radius is r^{-2.27 pm 0.08}, the inclination angle with respect to the line of sight is i = (39 pm 1) degrees. Finally, the XMM spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced.
4U 1705-44 is one of the most-studied type I X-ray burster and Atoll sources. This source represents a perfect candidate to test different models proposed to self-consistently track the physical changes occurring between different spectral states because it shows clear spectral state transitions. The broadband coverage, the sensitivity and energy resolution of the BeppoSAX satellite offers the opportunity to disentangle the components that form the total X-ray spectrum and to study their changes according to the spectral state. Using two BeppoSAX observations carried out in August and October 2000, respectively, for a total effective exposure time of about 100 ks, we study the spectral evolution of the source from a soft to hard state. Energy spectra are selected according to the source position in the color-color diagram (CCD) Results. We succeeded in modeling the spectra of the source using a physical self-consistent scenario for both the island and banana branches (the double Comptonization scenario). The components observed are the soft Comptonization and hard Comptonization, the blackbody, and a reflection component with a broad iron line. When the source moves from the banana state to the island state, the parameters of the two Comptonization components change significantly and the blackbody component becomes too weak to be detected. We interpret the soft Comptonization component as emission from the hot plasma surrounding the neutron star, hard Comptonization as emission from the disk region, and the blackbody component as emission from the inner accretion disk. The broad feature in the iron line region is compatible with reflection from the inner accretion disk.
Iron emission lines at 6.4-6.97 keV, identified with Kalpha radiative transitions, are among the strongest discrete features in the X-ray band. These are one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disk around a compact object. In this paper we present a recent Suzaku observation, 100-ks effective exposure, of the atoll source and X-ray burster 4U 1705-44, where we clearly detect signatures of a reflection component which is distorted by the high-velocity motion in the accretion disk. The reflection component consists of a broad iron line at about 6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these features are consistently fitted with a reflection model, and we find that in the hard state the smearing parameters are remarkably similar to those found in a previous XMM-Newton observation performed in the soft state. In particular, we find that the inner disk radius is Rin = 17 +/- 5 Rg (where Rg is the Gravitational radius, GM/c^2), the emissivity dependence from the disk radius is -2.5 +/- 0.5, the inclination angle with respect to the line of sight is i = 43 +/- 5 degrees, and the outer radius of the emitting region in the disk is Rout > 200 Rg. We note that the accretion disk does not appear to be truncated at large radii, although the source is in a hard state at about 3 % of the Eddington luminosity for a neutron star. We also find evidence of a broad emission line at low energies, at 3.03 +/- 0.03 keV, compatible with emission from mildly ionized Argon (Ar XVI-XVII). Argon transitions are not included in the self-consistent reflection models that we used and we therefore added an extra component to our model to fit this feature. The low energy line appears compatible with being smeared by the same inner disk parameters found for the reflection component.
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