Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku

Abstract

We present results of an observation with Suzaku of the dipping, periodic bursting low mass X-ray binary XB 1323-619. Using the energy band 0.8-70 keV, we show that the source spectrum is well-described as the emission of an extended accretion disk corona, plus a small contribution of blackbody emission from the neutron star. The dip spectrum is well-fitted by the progressive covering model in which the extended ADC is progressively overlapped by the absorbing bulge of low ionization state in the outer accretion disk and that dipping is basically due to photoelectric absorption in the bulge. An energy-independent decrease of flux at high energies (20-70 keV) is shown to be consistent with the level of Thomson scattering expected in the bulge. An absorption feature at 6.67 keV (Fe XXV) is detected in the non-dip spectrum and other possible weak features. In dipping, absorption lines of medium and highly ionized states are seen suggestive of absorption in the ADC but there is no evidence that the lines are stronger than in non-dip. We show that the luminosity of the source has changed substantially since the Exosat observation of 1985, increasing in luminosity between 1985 and 2003, then in 2003-2007 falling to the initial low value. X-ray bursting has again become periodic, which it ceased to be in its highest luminosity state, and we find that the X-ray bursts exhibit both the fast decay and later slow decay characteristic of the rp burning process. We present arguments against the recent proposal that the decrease of continuum flux in the dipping LMXB in general can be explained as absorption in an ionized absorber rather than in the bulge in the outer disk generally accepted to be the site of absorption.

New orbital ephemerides for the dipping source 4U 1323-619: constraining the distance to the source

“…(10) and (9), we infer A v = 14.2 mag and N H ∼ 3.1 × 10 22 cm −2 . This value of N H is fully in agreement with that obtained by Bałucińska-Church et al (2009).…”

“…Bałucińska-Church et al (2009) using Suzaku data, Boirin et al (2005) infer N H = (3.5 +0.1 −0.2 ) × 10 22 cm −2 using XMM-Newton data and Parmar et al (1989) obtained N H = (4.0 ± 0.3) × 10 22 cm −2 using EXOSAT data. Smale (1995) suggested that the large extinction to the source also explains why the optical counterpart has not been detected yet.…”

Infrared identification of 4U1323-619 revisited

“…We obtained simple estimations of optical luminosity of the accretion disc illuminated by the central isotropic spherical X‐ray source with a 0.1–10 keV luminosity L X ∼ 5.2 × 10 36 erg s −1 (Boirin et al 2005) assuming for convenience that its radius is ∼ few × 10 R g which means that it is relatively small comparing to the accretion disc size. While it is known that the source has varied in luminosity over a 20 yr period (Bałucińska‐Church et al 2009) and an interpolation to the date of the IR observations gives ≃2.5 times smaller luminosity, we leave mentioned L X value because the bolometric luminosity is expected to be the same factor higher. Given a system period P = 2.94 h and assuming a compact object to be a neutron star with M X = 1.4M ⊙ , one can estimate the mass of the secondary component from the mass–radius relation for main‐sequence stars and the constraint that the star fills its Roche lobe of size (Eggleton 1983) where q = M opt / M X and a is the semimajor axis of the binary.…”

“…Also, we assumed in the beginning the compact nature of the central X‐ray source while it is now known that the corona in low‐mass X‐ray binaries is extended (Church & Bałucińska‐Church 2004; Bałucińska‐Church et al 2009; Schulz et al 2009). In 4U1323‐619, the corona has a radius of ≃30 000 km for adopted L X .…”

Broad-band spectral analysis of Aql X-1

“…Suzaku observations cover a broad energy range of 0.7–70 keV with good spectral resolution. This capability of Suzaku allows us to distinguish between several modelling approaches for NS LMXBs: Eastern Model like model, as in the case of the Z‐source LMC X‐2 (Agrawal & Misra 2009); Western Model like model, as in the case of the periodic burster XB 1323−619 (Bałucińska‐Church et al 2009); and scattered‐in dust emission plus direct NS and/or disc blackbody (BB) emission, as for the eclipsing LMXB SAX J1745.6−2901 (Hyodo et al 2009).…”