{'fraction_non_alphanumeric': 0.03676709832758508, 'fraction_numerical': 0.03448949046658424, 'mean_word_length': 4.243091095189355, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 2, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Suzaku broad-band spectrum of 4U 1705-44: Probing the Reflection component in the hard state

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.

which depends on the inclination and on how deep the accretion disk extends into the NS gravitational potential (e.g. Fabian et al. 1989;Matt et al. 1992).
If the origin of this line is from disk reprocessing, one would also expect the presence of other discrete features (such as emission lines and absorption edges from the most abundant elements) and a bump between 20 and 40 keV due to direct Compton scattering of the primary spectrum by the electrons in the disk. Indeed this reflection bump has been observed in the spectra of some NS LMXBs (see e.g. Barret et al. 2000;Piraino et al. 1999;Yoshida et al. 1993;Fiocchi et al. 2007;Egron et al. 2013;Miller et al. 2013), usually with reflection amplitudes (defined in terms of the solid angle Ω/2π subtended by the reflector as seen from the corona) lower than 0.3. In some cases an anti-correlation has been claimed between the photon index of the primary spectrum and the reflection amplitude of the reprocessed component (Zdziarski et al. 1999; Barret et al. 2000;Piraino et al. 1999), the same observed in Seyfert galaxies and galactic Black Hole (hereafter BH) candidates. This is probably caused by variations in the position of the inner rim of the disk.
The disk origin of the iron line in NS LMXBs is, however, debated in literature because of the brightness of these sources, which may cause photon pile-up and systematics in CCD spectra (Ng et al. 2010), making the detection of any asymmetry in the line profile somewhat controversial. However, a large number of simulations using a statistical model of photon pile-up to assess its impacts on relativistic disk line and continuum spectra suggest that severe photon pile-up acts to falsely narrow emission lines, leading to falsely large disk radii . These simulations also indicate that relativistic disk spectroscopy is generally robust against pile-up when this effect is modest. Moreover, several authors (e.g. Cackett et al. 2012;Egron et al. 2013) have shown that the CCD-based spectra from Suzaku and XMM-Newton are compatible with gas-based spectra from EXOSAT, BeppoSAX, and RXTE, demonstrating that the broad profiles seen are intrinsic to the line and not due to instrumental issues. They also report that a few BeppoSAX observations show evidence for asymmetric lines, with a relativistic diskline model providing a significantly better fit than a Gaussian line (see also Piraino et al. 1999).
Nevertheless alternatives have been proposed to explain the profiles of these features. In particular, Ng et al. (2010) propose that Compton broadening may be sufficient to explain the large width of the line. However, when self-consistently included in the fit, Compton broadening alone appears to be insufficient to explain the observed line profile (see e.g. explored the observational signatures expected from broadening in a wind. In this case the iron line width should increase with increasing the column density of the absorber (due to an increase in the number of scatterings). They show that there is no significant correlation between line width and column density, favoring an inner disk origin for the line broadening rather than scattering in a wind.
4U 1705-44 is a well-studied atoll source (see Hasinger & van der Klis 1989;Olive et al. 2003), which also shows type-I X-ray bursts. Similarly to X-ray binaries containing BHs, this source regularly shows state transitions: from a high/soft state, where the X-ray spectrum is dominated by soft spectral components with typical temperatures less than a few keV, to low/hard states where the X-ray spectrum is dominated by a hard thermal Comptonisation (e.g. Barret & Olive 2002;Piraino et al. 2007). The presence of broad discrete features in this source has been often reported in literature. A broad iron line was observed, in the soft and/or hard state, with moderate/high spectral resolution by the Chandra/HETG (Di Salvo et al. 2005), BeppoSAX (Piraino et al. 2007), Suzaku (Reis et al. 2009), and XMM/pn D'Aì et al. 2010;Egron et al. 2013). The XMM observation, taken in August 2008 during a soft state (45 ks effective exposure time), showed one of the highest signal-to-noise ratio iron line profile ever observed in a NS LMXB. The line profile is clearly broad and could be fitted equally well with a relativistic line profile, such as diskline (Fabian et al. 1989) or relline (Dauser et al. 2010), or with self-consistent reflection models, such as reflionx (Ross & Fabian 2005), refbb (Ballantyne 2004), and xillver (García & Kallman 2010). All these models gave parameters of the inner disk with unprecedented precision and all compatible with each other within the small statistical uncertainties (see Di D'Aì et al. 2010;Egron et al. 2013). The line is identified with the Kα transition of highly ionised iron, Fe XXV; the inner disk radius is R in = 14±2 R g (where R g is the Gravitational radius, GM/c 2 ), the emissivity index of the disk is −2.27±0.08 (compatible with a disk illuminated by a central source), the inclination angle with respect to the line of sight is i = 39 ± 1 degrees. This, together with the presence of other low-energy features from S XVI, Ar XVIII, Ca XIX and a smeared iron edge at 8.4 keV, which all are compatible with being smeared with the same inner disk parameters, makes 4U 1705-44 the best source for a detailed spectroscopic study, in order to address the disk origin of the observed iron line and of the whole reflection component.
In this paper we present a high statistics, 100-ks effective exposure, Suzaku observation of 4U 1705-44 during a hard state: these data allow us a detailed study of the reflection Suzaku broad-band spectrum of 4U 1705-44 5 features and the fit, with a self-consistent reflection model, of both the iron line profile and the associated Compton reflection bump at energies above 10 keV. In this spectrum, which includes hard-band data (up to 200 keV), the overall fractional amount of reflection is well determined by fitting the Compton bump. We can therefore test whether the observed iron line is consistent with this fractional amount of reflection. In this way we confirm independently (fitting a different spectral state and using different instruments) the inner disk parameters already obtained with XMM-Newton in the soft state.

OBSERVATIONS
Suzaku (Mitsuda et al. 2007) Koyama et al. (2007)) and the Hard X-ray Detector (HXD, 10-600 keV; Takahashi et al. (2007)) instruments were used during these observations. There are four XIS detectors, numbered as 0 to 3. XIS0, XIS2 and XIS3 all use front-illuminated CCDs and have very similar responses, while XIS1 uses a back-illuminated CCD. At the time of this observation the available CCDs were three due to the loss of the XIS2. The HXD instrument consists of two types of collimated (non-imaging) detectors, the PIN diodes (10-70 keV) and the GSO scintillators (30-600 keV).
We reprocessed the data using the aepipeline tool provided by Suzaku FTOOLS version 20 updated with the latest calibration files (2013 November). As second step, in order to obtain a more accurate estimate of the Suzaku attitude, we have calculated a new attitude using the free tool aeattcor.sl created by J. E. Davis. Then we have applied the new attitude to XIS event files using the FTOOLS xiscoord. During the observation, XIS0, XIS1, and XIS3 were operated using the 1/4 window option. The effective exposure time of each XIS CCD is 96.67 ks. In order to estimate the pile-up in the XIS spectra we have used the public available tool pile-estimate.sl created by M. A. Novak. Using a circular region with radius equal to 105 ′′ , we have found that in each XIS CCD, the pile-up fraction is ∼ 3% at most. The pile-up fraction is sufficiently small that we can neglect its effects on our spectral fitting results. In fact, we have checked that spectral results do not change significantly if we exclude a central circle in the extraction region in order to further reduce the pile-up fraction. Therefore, we have extracted the source and background spectra from a circular extraction region of radius 105 ′′ each, the background circle being centered close to the edge of the CCD, where no significant contaminating photons from the source were present. The response files of each XIS spectrum have been generated using the xisrmfgen and xisarfgen tools. Since the response of XIS0 and XIS3 are very similar, we have combined their spectra and responses using the tool addascaspec.
The PIN spectrum has been extracted using the tool hxdpinxbpi. Both the non X-ray and cosmic X-ray backgrounds are taken into account. The non X-ray background (NXB) is calculated from the background event files distributed by the HXD team. The cosmic X-ray background (CXB) is from the model by Boldt & Leiter (1987). The response files provided by the HXD team are used. We selected the HXD/PIN events in the energy range 12-30 keV and produced the HXD/PIN background-subtracted light curve using the SUZAKU tool hxd-pinxblc and adopting the background event files distributed by the HXD team. Since this light curve follows the XIS0 light curve, we conclude that no contaminating flare was present in the data. We have also extracted the GSO spectrum running the tool hxdgsoxbpi.
For the background we have used the 'tuned' non X-ray background, whereas for the response file we have used the latest version provided on 2011-06-01.
We have extracted the XIS0 light curves in the energy range 0.9 − 2.8, 2.8 − 10 and 0.9 − 10 keV, respectively (see Fig. 1). Nine type-I X-ray bursts are observed in the total 250-ks light curve. The source (persistent) count rate gradually increases by about 40% during the observation. Since no changes are observed in the hardness ratio (given by the ratio of the source counts in the 2.8 − 10 keV range to the source counts in the 0.9 − 2.8 keV, see Fig. 1) we conclude that the X-ray spectral shape of the source does not change significantly during the observation.

SPECTRAL ANALYSIS AND RESULTS
In order to extract spectra for the persistent emission, we have excluded the type-I bursts that occurred during the observation. In particular we have excluded approximately 100 s of data starting from the onset of each burst. We adopt 0.7 − 11 keV energy range for the XIS0+XIS3 (hereafter XIS03) and XIS1 spectra, 15−50 keV energy range for the HXD/PIN spectrum and 50 − 200 keV energy range for the HXD/GSO spectrum. We excluded the energy interval between 1.7 and 2.0 keV from the XIS03 and XIS1 spectra because of the  presence of systematic features associated with neutral silicon and neutral gold which give a mismatch between the two spectra. The XIS spectra were grouped by a factor 4 in order not to oversample too much the instrumental energy resolution. The HXD/PIN and HXD/GSO spectra were grouped in order to have at least 25 photons per energy channel. We fitted the spectra using XSPEC version 12.7.0.
We started to fit the continuum in the 0.7-200 keV energy range with the typical model used for NS LMXBs of the atoll class, which revealed to be the best fit continuum for this source too (see e.g. Di Piraino et al. 2007;Barret & Olive 2002). This model consists of a soft blackbody and a thermal Comptonised component, in this case modelled by nthComp (Życki et al. 1999), modified at low energy by photoelectric absorption caused by neutral matter and modeled by phabs in XSPEC. This continuum model gave, however, an unacceptable fit, corresponding to a χ 2 /dof = 2425.13/1511, because of the presence of evident localised residuals at 2.5-3.5 keV, 6-9 keV and 15-30 keV. The most prominent is a clear iron line profile at energies from 5 to 7 keV and an absorption feature at 7-8 keV (see Fig. 2).
In order to fit these residuals, we first added to our continuum model the pexriv component (Magdziarz & Zdziarski 1995) which takes into account the iron edge and Compton bump present in the residuals. Note that pexriv does not self-consistently include any emis- this component gave a significant improvement of the fit reducing the χ 2 /dof to the value 2009.67/1509 (∆χ 2 = 415 for the addition of two parameters). Therefore, the presence of the Compton hump and the iron edge are detected with high statistical significance (an F-test would give a probability of chance improvement of the fit negligibly small, ∼ 2.7 × 10 −62 ).
We then added a Gaussian line at 6.4 keV obtaining again an improvement of the fit. As a first step we decided to fix the energy of the line at 6.4 keV because, otherwise, it tends to get broad (Gaussian sigma about 1 keV) and its energy tends to decrease to 6.2 keV.
This fit gives a χ 2 /dof = 1956.50/1507 (∆χ 2 = 53 for the addition of two parameters). This line can be identified with fluorescence from neutral iron. The addition of another Gaussian at about 3 keV again improves the fit, giving χ 2 /dof = 1829.89/1504 (∆χ 2 = 127 for the addition of three parameters). This line can be identified with emission from mildly ionized Argon, Ar XVI-XVII. In order to check whether the iron line energy was stable or not, we let the iron line energy free to vary obtaining χ 2 /dof = 1818/1503. We also tried to substitute the Gaussian line at ∼ 6 keV with a diskline. In this case we had to fix all the smearing parameters but the inner radius of the disk. This did result in a slight improvement Table 1. The best fit parameters of the spectral fitting of the Suzaku spectrum of 4U 1705-44 in the 0.7 − 200 keV energy range with phenomenological models. The blackbody luminosity is given in units of L 35 /D 2 10 , where L 35 is the bolometric luminosity in units of 10 35 ergs/s and D 10 the distance to the source in units of 10 kpc. The blackbody radius is calculated in the hypothesis of spherical emission and for a distance of 7.4 kpc. Fluxes in the nthComp and pexriv components are calculated in the 1 − 16 keV range, while total flux is calculated in the 1 − 10 keV band. Uncertainties are given at 90% confidence level.
total Flux (10 −10 ergs cm −2 s −1 ) 3.3 ± 0.7 3.34 ± 0.02 3.34 ± 0.03 3.34 ± 0.03 total χ 2 (dof) 2425 (1511) 1990 (1509) 1818 (1503) 1783 (1501) of the fit, since we get χ 2 /dof = 1803/1503 fixing the emissivity index at −2.4, the outer radius at 400 R g , the inclination angle at 40 deg. Also in this case the centroid energy of the line remained at 6.1 − 6.3 keV, but we get an upper limit to the inner radius of the disk of 25 R g . On the other hand, we get a more significant improvement of the fit if we add a (mild) relativistic smearing to the whole reflection component (i.e. the emission lines at ∼ 3 and ∼ 6 keV and the pexriv component) convolving all these three components with rdblur, the kernel of diskline. In this case we get χ 2 /dof = 1783/1501 (∆χ 2 = 35 for the addition of three parameters, corresponding to an F-test probability of chance improvement of ∼ 4 × 10 −7 ). The results of these phenomenological models are reported in Table 1.
In order to fit the high-quality Suzaku spectrum with more consistent physical models, we substitute the pexriv + Fe line components with the self-consistent reflection model reflionx (Ross & Fabian 2005), modified by a relativistic blurring component (again modeled with rdblur) to consider the relativistic and/or Doppler effects produced by the motion in the inner disk close to the compact object. In this model, emission lines from the most abundant elements or ions are also self-consistently calculated. In the reflionx model the Table 2. The best fit parameters of the spectral fitting of the Suzaku spectrum of 4U 1705-44 in the 0.7 − 200 keV energy range with the self-consistent reflection model reflionx. The blackbody luminosity is given in units of L 35 /D 2 10 , where L 35 is the bolometric luminosity in units of 10 35 ergs/s and D 10 the distance to the source in units of 10 kpc. The blackbody radius is calculated in the hypothesis of spherical emission and for a distance of 7.4 kpc. The disk blackbody normalization is given by (R in (km)/D 10 ) 2 cos i, where i is the inclination angle of the binary system. The disk blackbody inner radius R in (km) is calculated for an inclination angle of 40 • . Flux is calculated in the 1 − 10 keV band. Uncertainties are given at 90% confidence level. 18.2 ± 3.6 17.0 ± 3.4 18.6 ± 3.5 17.5 ± 3.7 Flux (10 −10 ergs cm −2 s −1 ) 3.34 ± 0.04 3.34 ± 0.15 3.34 ± 0.04 3.34 ± 0.14 total χ 2 (dof) 1831 (1503) 1841 (1503) 1846 (1503) 1852 (1503) emergent (reflected) spectrum is calculated for an optically-thick atmosphere (such as the surface of an accretion disk) of constant density illuminated by radiation with a power-law spectrum, whose photon index is fixed to that of the nthComp component, and a high-energy exponential cutoff with e-folding energy fixed at 300 keV. In order to take into account the high-energy cutoff in the illuminating spectrum, we have multiplied the reflionx spectrum  Table 2. We also checked the possibility of iron overabundance with respect to cosmic abundances (as claimed by Egron et al. 2013, for the soft state), fixing the iron abundance alternatively to the cosmic value and twice the cosmic value. We find a slightly better fit when we fix the iron abundance to the cosmic value and we use a blackbody to fit the soft thermal component (see Table 2). The total 0.5 − 200 keV luminosity of the source during the Suzaku observation was 6.15 × 10 36 ergs/s assuming a distance to the source of 7.4 kpc.
Finally, we fitted the reflection component in the Suzaku spectrum of 4U 1705-44 with the convolution model rfxconv (Kolehmainen et al. 2011), which has the advantage to take into account the exact shape of the illuminating continuum. For sake of completeness we also tried the self-consistent reflection model relxill by García et al. (2014), whose novelty is that for each point on the disk the proper reflection spectrum is chosen for each relativistically calculated emission angle. In the relxill model we fixed the photon index and e-folding energy of the reflected spectrum to those of the nthComp component. Both these models include emission lines from the most abundant elements or ions, as in the case of reflionx. Again we obtain a good fit of the 4U 1705-44 spectrum, with best-fit values of the reflection and smearing parameters well in agreement with those obtained with other reflection models (see Tab. 3, cf. with Tab. 1 and 2).

DISCUSSION
Similar to BH X-ray Binaries, NS LMXBs show clear differences in the spectral parameters during hard and soft states. Studying these differences is important in order to address different geometry or physical properties of the inner central emitting region in these two spectral states and in order to understand what causes the spectral transition. In fact, there is no general consensus on the hot corona-accretion disk geometry in these two spectral states. Theoretically the hard Comptonized component may come from either a patchy corona, possibly powered by magnetic flares (e.g. Beloborodov 1999) or the base of a centrally located jet (e.g. Markoff et al. 2005). In both the cases, the thin accretion disk may extend Table 3. The best fit parameters of the spectral fitting of the Suzaku spectrum of 4U 1705-44 in the 0.7 − 200 keV energy range with the self-consistent reflection models rfxconv and relxill. The blackbody luminosity is given in units of L 35 /D 2 10 , where L 35 is the bolometric luminosity in units of 10 35 ergs/s and D 10 the distance to the source in units of 10 kpc. The blackbody radius is calculated in the hypothesis of spherical emission and for a distance of 7.4 kpc. Flux is calculated in the 0.7 − 200 keV band. Uncertainties are given at 90% confidence level. Flux (10 −10 ergs cm −2 s −1 ) 9.0 ± 0.3 9.33 +0.17 −0.36 total χ 2 (dof) 1832 (1503) 1818 (1504) close to the last marginally stable orbit or the NS surface. Alternatively the thin disk may be truncated at large distances from the compact object, with the central region replaced by an advection-dominated accretion flow (ADAF) where, at high accretion rates, material may condense to form an inner optically thick disk (see e.g. Esin et al. 1997). This hot, inner flow can also act as the launching site of the jet. In this respect, much information may come from the study of the so-called reflection component in different spectral states of a source (see Done et al. 2007, as a review).
In this paper we have reported the results of the spectral analysis of a long Suzaku an Fe line, we find small differences in the line intensities (a factor 2 at most) and higher differences in the line equivalent width (up to a factor 8). However, when the spectrum is fitted to a self-consistent reflection model, a suitably modified version of the xillver model by García & Kallman (2010) which includes Ar and Ca transitions, we find that the Ar line is well fitted by the reflection model with a slight overabundance by a factor 1.8 with respect to Solar abundances (see Egron et al. 2013). We conclude therefore that the simple calculation above is merely an order of magnitude estimation and that the consistency of the Ar line with other reflection features should be checked using self-consistent reflection models including Ar and Ca transitions.
We also tried to fit all the reflection features (the Compton hump, the iron edge and the iron line) with a self-consistent reflection model such as reflionx, which we have modified with a high energy cutoff at the electron temperature of the Comptonizing corona to take into account the curvature of the Comptonization spectrum with respect to a simple power- Table 4. Comparison of the best-fit continuum and reflection parameters obtained for the soft state (SS) as observed in the 60-ks XMM-Newton observation and for the hard state (HS) observed by Suzaku (this paper). Continuum parameters for the SS observed by XMM-Newton are taken by Egron et al. (2013), who use a similar model for the continuum, while smearing parameters of the reflection component are taken from Di  where these parameters are obtained with smaller uncertainties. L X is the X-ray luminosity extrapolated in the 0.1 − 150 keV range for the SS, as reported by Egron et al. (2013), and in the 0.5 − 200 keV range for the HS (this work). L Edd is the Eddington luminosity for a 1.4 M ⊙ NS, L Edd = 2.5 × 10 38 ergs s −1 (van Paradijs & McClintock 1994).
In this case, we used a bbody or a diskbb to fit the soft thermal component and tried to vary the iron abundance fixing this value to 1 or 2 times the cosmic abundance. The best fit corresponds to a bbody component for the soft thermal emission and to an iron abundance of 1 (see Table 2). We also tried to fit the reflection component with the selfconsistent convolution model rfxconv, which consistently takes into account the curvature of the illuminating spectrum caused by the high-energy rollover at the electron temperature in the corona producing the primary Comptonized spectrum, or with the relxill model, in which the cutoff energy in the reflected spectrum is fixed at the electron temperature of the primary Comptonized component, fitted with nthComp. Note that both rfxconv and relxill, as well as reflionx, all include Compton broadening effects caused by Compton scattering in the surface layers of the accretion disc. The results of these fits are reported in Table 3 and are perfectly consistent with those obtained with all the other reflection models that we tried.
In order to check the stability of the best fit model with respect to the smearing parameters of the reflection component, we have let all these parameters free to vary. The most uncertain of these parameters, as expected, is the outer radius of the disk, for which we find only loose constraints. Interestingly, all of the smearing parameters of the best fit model of this observation are in good agreement with the smearing parameters already obtained with other instruments, e.g. XMM-Newton and BeppoSAX Egron et al. 2013), during a soft state. In order to facilitate the comparison, we report in Table 4  We note that there is no clear indication of a receding inner accretion disk radius in the hard state, corresponding to a luminosity of ∼ 3% of the Eddington luminosity, with respect to the soft state, which was observed at about 70% of L Edd . On the contrary, the inner disk radius as inferred from the reflection component is consistent to be the same in the two spectral states, at about 34 km from the NS center. A similar indication comes from the inner radius of the disk as inferred from the blackbody component, that we interpret as the direct emission from the accretion disk. Both in the soft and in the hard state the blackbody radius is a few tens of km, in agreement with the estimate we get from the reflection component. We caution the reader, however, that neither the color factor or the geometry of the system has been taken into account in this calculation. What is reported is just a zero-order estimation of the radius of the region (assumed to be spherical) emitting the blackbody component. In  Egron et al. (2013). These authors discussed the possibility of a truncated disk in the hard state based on the best fit value of the inner disk radius as found from the Fe line width, which was about 30 R g , that means about 60 km. However, in that case, the lack of broad-band coverage and the limited statistics, gave a large uncertainty on the inner disk radius, whose 90% c.l. range was from 6 to 90 R g . Considering the large uncertainty on this measurement we cannot state that the result was in contrast with more recent results (Egron et al. 2013, and this work). Note also that the best-fit blackbody radius reported by D'Aì et al. (2010)  inferred from the smeared reflection, is found between 6 and 20 R g (Di Salvo et al. 2001).
This result is in agreement with the results of Young et al. (2001) who fitted ASCA, Ginga and EXOSAT data of Cygnus X-1 in both soft and hard spectral states to a model of an ionized accretion disk, whose spectrum is blurred by relativistic effects. They found that relativistic blurring provided a much better fit to the low/hard state and that data of both states were consistent with an ionized thin accretion disk with a reflected fraction of unity extending to the innermost stable circular orbit around the BH (see, however, Barrio et al. 2003, for a different interpretation). Up to date, one of the strongest evidence of a truncated disk, based on Fe line profile measurements, in a BH hard state is that of GX 339-4 (Tomsick et al. 2009). In that case, from Suzaku and RXTE spectra, it was found that R in was a factor > 27 higher than in the bright state when the luminosity was about at 0.14% of the corresponding Eddington limit.
More recently Reis et al. (2010) have analyzed a sample of stellar mass BHs, including Cygnus X-1, in the low-hard state, down to luminosities of ∼ 10 −3 L Edd , finding no clear evidence of a truncation of the inner disk at radii larger than 10 R g . Furthermore, the thermal-disk continuum yields colour temperatures consistent with the relation L ∝ T 4 , implying that the emitting surface is consistent with being constant with luminosity. A similar relation, L ∝ T 3.2 , seems also to hold in the case of 4U 1705-44 at least for the soft state (Lin et al. 2010). The authors suggest that the deviation may be caused by a luminosity-dependent spectral hardening factor. On the other hand, the relatively low reflection amplitude (Ω/2π ≃ 0.34) we find in the hard state of 4U 1705-44 fitting its spectrum to self-consistent reflection models, such as rfxconv, is compatible with a spherical geometry with the hot (spherical) corona inside an outer accretion disk. In this case, the small inner radius of the disk we find (R in ∼ 17 R g , corresponding to approximately 30 − 35 km for a 1.4 M ⊙ NS) would indicate a very compact hot corona filling the central part of the accretion disk, which in the case of a NS may be identified with a boundary layer between the inner accretion disk and the NS surface. This might represent an important difference between BH and NS systems, since for a BH, in the absence of a boundary layer, the inner disk should extend down to the last stable orbit. Note, however, that alternative explanations for this weak reflection cannot be ruled out. This can be caused by a highly ionized inner disk (e.g. Ross et al. 1999) or mildly relativistic outflow of the hot corona away from the disk (Beloborodov 1999). In the latter case, it was shown that reflection fractions as low as ∼ 0.3 can be obtained in the low-hard state without invoking a truncated disk.
In summary, we have analyzed a deep (