We analysed the IGR J16465-4507 Burst Alert Teelescope survey data collected during the first 54 months of the Swift mission. The source is in a crowded field and it is revealed through an ad hoc imaging analysis at a significance level of ~14 standard deviations. The 15-50 keV average flux is ~3E-11 erg/cm^2/s. The timing analysis reveals an orbital period of 30.243 +/- 0.035 days. The folded light curve shows the presence of a wide phase interval of minimum intensity, lasting ~20% of the orbital period. This could be explained with a full eclipse of the compact object in an extremely eccentric orbit or with the passage of the compact source through a lower density wind at the orbit apastron. The modest dynamical range observed during the BAT monitoring suggests that IGR J16465-4507 is a wind-fed system, continuously accreting from a rather homogeneous wind, and not a member of the Supergiant Fast X-ray Transient class.
Deep Dive into Detection of an orbital period in the supergiant high mass X-ray binary IGR J16465$-$4507 with Swift-BAT.
We analysed the IGR J16465-4507 Burst Alert Teelescope survey data collected during the first 54 months of the Swift mission. The source is in a crowded field and it is revealed through an ad hoc imaging analysis at a significance level of ~14 standard deviations. The 15-50 keV average flux is ~3E-11 erg/cm^2/s. The timing analysis reveals an orbital period of 30.243 +/- 0.035 days. The folded light curve shows the presence of a wide phase interval of minimum intensity, lasting ~20% of the orbital period. This could be explained with a full eclipse of the compact object in an extremely eccentric orbit or with the passage of the compact source through a lower density wind at the orbit apastron. The modest dynamical range observed during the BAT monitoring suggests that IGR J16465-4507 is a wind-fed system, continuously accreting from a rather homogeneous wind, and not a member of the Supergiant Fast X-ray Transient class.
High mass X-ray binaries (HMXBs), stellar systems composed of a compact object and an early-type massive star, are traditionally divided in two subclasses (e.g. van Paradijs 1995, and references therein), depending on the nature of the high mass primary and, consequently, the different mass-transfer and accretion mechanism. On one side are the systems with main sequence Be primaries (Be-HMXBs). They are generally wide (P orb 10 d) eccentric (eccentricity e ∼ 0.3-0.5) systems in which the primaries are not filling their Roche lobe, and accretion onto the compact object occurs from the equatorial region of the rapidly rotating Be star. Most of these systems are highly variable: in some of them recurrent outbursts are observed caused by an enhanced rate when the compact star passes close to the Be star. On the other side are the systems with an evolved OB supergiant primary (sgHMXB). Their periods are shorter (P orb 10 d) and their orbits more circular than in Be-HMXBs. They are powered either by a geometrically thin accretion disc or by the strong radiation-driven stellar wind, depending on whether the primary fills its Roche lobe or not. Their X-ray emission is bright and persistent.
Recently, this rather clear-cut picture was made more structured with the INTEGRAL observations of the Galactic plane. Two additional classes were added to the classical OB primary HMXBs: the highly absorbed persistent systems (Walter et al. 2004(Walter et al. , 2006) ) and the supergiant fast X-ray transients (SFXTs, Sguera et al. 2005;Negueruela et al. 2006;Smith 2004;in’t Zand 2005). The former are characterized by orbital and spin periods consistent with those observed in wind-accreting systems, but a much higher absorbing column density. The latter are transient sources showing a large dynamic range of 3-5 orders of magnitude with sporadic outbursts (which however are significantly shorter than those of typical Be-HMXBs), characterized by bright flares lasting up to days with peak luminosities of 10 36 -10 37 erg s -1 (Sguera et al. 2005;Romano et al. 2009;Sidoli et al. 2009).
The Burst Alert Telescope (BAT, Barthelmy et al. 2005) on board Swift (Gehrels et al. 2004) is performing a continuous coverage of the hard X-ray sky (50 to 80% of the sky every day). This allowed the detection of many of the new INTEGRAL HMXBs (e.g. Cusumano et al. 2009) and the collection of their long term light curves. In this Letter we analyse the hard X-ray data collected during the first 54 months of Swift-BAT sky monitoring using data in the region of the IGR J16465-4507. This source was discovered by INTEGRAL in 2004 (Lutovinov et al. 2004) and X-ray activity was observed with IBIS/ISGRI starting on September 6, at a flux level of 8.8 ± 0.9 mCrab (18-60 keV), followed by a flare (up to 28 mCrab) on September 7. Follow-up observations with XMM-Newton revealed pulsations at 228±6 s (Lutovinov et al. 2005) and allowed the identification of the optical counterpart with 2MASS J16463526-4507045 (Zurita Heras & Walter 2004). This was classified as a B0.5 Ib supergiant at a distance of ∼ 8 kpc (Negueruela et al. 2007) or as a O9.5 Ia supergiant at a distance of 9.5 +14.1 -5.7 kpc (Nespoli et al. 2008). The supergiant nature of the companion, combined with the The white circles are centered on the position derived by fitting the significance profile extracted along the white line with two Gaussians plus a constant; their radius corresponds to the 90% error on the position. Bottom: Significance profile extracted along the white line in the above map. The plot shows the data (stars) and the best fit model (blue line, the sum of two Gaussian profiles plus a constant value). The higher peak (red line) corresponds to IGR J16479-4514, the lower peak (green line) corresponds to IGR J16465-4507.
observed hard X-ray variability (Lutovinov et al. 2004) and the Xray spectral distribution, modeled by a hard power law with photon index 1.0±0.52 (Lutovinov et al. 2005), suggested a classification of this source as a SFXT (Negueruela et al. 2006). However, Walter & Zurita Heras (2007), based on INTEGRAL measurements, suggested that IGR J16465-4507 is likely a classical supergiant HMXB, with an average flux just below the IBIS/ISGRI sensitivity undergoing sporadic long periods of enhanced activity. This Letter is organized as follows. Section 2 describes the BAT data reduction and the imaging analysis. Section 3 reports on the timing analysis. Sect. 4 describes the analysis of the pointed soft X-ray observation with Swift-XRT. In Sect. 5 we briefly discuss our results. Errors are at 90 % confidence level, if not stated otherwise.
We analysed the BAT survey data of the first 54 months of the Swift mission. We retrieved the raw data from the HEASARC public archive 1 and processed them with a dedicated software (Segreto et al. 2010) that performs screening, mosaicking and source detection on data from coded mask instruments. The code also produces light curves for a
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