A Hard Tail in the X‐Ray Broadband Spectrum of Circinus X‐1 at the Periastron: A Peculiar Z Source

Abstract

We report on the spectral analysis of the peculiar source Cir X-1 observed by the BeppoSAX satellite when the X-ray source was near the periastron. A Ñare lasting D6 ] 103 s is present at the beginning of the observation. The luminosity during the persistent emission is 1 ] 1038 ergs s~1, while during the Ñare it is 2 ] 1038 ergs s~1. We produced broadband (0.1È100 keV) energy spectra during the Ñare and the persistent emission. At low energies the continuum is well Ðtted by a model consisting of Comptonization of soft photons, with a temperature of D0.4 keV, by electrons at a temperature of D1 keV. After the Ñare, a power-law component with photon index D3 is dominant at energies higher than 10 keV. This component contributes D4% of the total luminosity. During the Ñare its addition is not statistically signiÐcant. An absorption edge at D8.4 keV, with optical depth D1, corresponding to the K edge of Fe XXIIIÈFe XXV, and an iron emission line at 6.7 keV are also present. The iron-line energy is in agreement with the ionization level inferred from the absorption edge. The hydrogen column deduced from the absorption edge is D1024 cm~2, 2 orders of magnitude larger than the low-energy absorption measured in this source. We calculated the radius of the region originating the Comptonized seed photons, km. We propose a scenario where (the Wien radius) is the inner disk radius, a R W D 150 R W highly ionized torus surrounds the accretion disk, and a magnetosphere is present up to The R W . absorption edge and the emission line could originate in the photoionized torus, while the Comptonized component originates in an inner region of the disk.

Study of the Circinus X‐1 Broadband Spectrum at Orbital Phases Close to the Apoastron

“…The light curve and the hardness ratio do not show large variations during the observation. From Figures 1 and 2 it is clear that the spectrum, in the range 1È10 keV, is di †erent from that observed at the periastron (Iaria et al 2001a) ; in fact, the spectrum at the apoastron is harder in both the soft and hard colors and the intensity is lower than that at the periastron. The spectrum of Cir X-1 during this observation is very similar to that of other Z sources (see Di Salvo et al 2000 ;Di Salvo et al 2001).…”

“…Then, in agreement with the model proposed by Iaria et al (2001a) for the spectrum of Cir X-1 at the periastron, we added a power law to Ðt the high energy excess, an absorption edge at D8.4 keV, and a Gaussian emission line at D6.7 keV (with the Gaussian p Ðxed at 0.25 keV), keeping Ðxed the PDS normalization factor. The addition of each of these components is statistically signiÐcant and in this way the Ðt improves signiÐcantly, giving s2/dof \ 117/94.…”

“…As customary, in the spectral-Ðtting procedure we allowed for Table 1. a di †erent normalization of the PDS spectrum relative to the MECS spectrum, always checking that derived values were in the standard range for that instrument.5 The energy ranges used for the spectral analysis are 1.8È10 keV for the MECS and 15È200 keV for the PDS. Because of the lack of low-energy data, we Ðxed the value of the average photoelectric absorption by cold matter at 1.7 ] 1022 cm~2 (see, e.g., Iaria et al 2001aIaria et al , 2001b. We started Ðtting the data to the Comptonization model Comptt (Titarchuk 1994).…”

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

“…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). …”

The complex enviroment around Cir X-1

“…The continuum X-ray emission consists of a thick Comptonized component, which, for relatively narrow band coverage, caimot be distinguished from a simpler sum of two thermal components, and a variable hard-tail above 10 keV [1]. The spectrum shows a phase-dependent evolution, whose major characteristic, when the source is near the periastron passage is the appearence of a large column density of neutral matter occulting the continuum X-ray emission.…”