Swift/XRT monitoring of the supergiant fast X-ray transient IGR J17354-3255 L. Ducci1,2, P. Romano3, P. Esposito4, E. Bozzo2, H. A. Krimm5,6, S. Vercellone2, V. Mangano7, J. A. Kennea7 1 Institut für Astronomie und Astrophysik, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany; 2ISDC, Data Center for Astrophysics of the University of Geneva, Chemin d’Ecogia, 16 1290 Versoix Switzerland; 3INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica - Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy; 4INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via E. Bassini 15, I-20133 Milano, Italy; 5NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA; 6Universities Space Research Association, Columbia, MD, USA; 7Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA Results: Identification of the soft X-ray counterpart of IGR J17354–3255 0.12 0.25 0.52 1 10.0 2.1 05.0 4.2 17:35:00.0 8.4 17 0.1 SWIFT J173518.7−325428 Swift/XRT Swift/BAT Counts s−1 0.05 15.0 10−12 10−4 Counts s−1 1 0.017 20.0 10−11 10−13 0.01 0 25.0 Count rate IGR J17354-3255 30.0 35.0 40.0 Flux 1 0.1 Count rate SRC1 10−10 IGR J17354−3255 0.01 54:00.0 -32:56:00.0 55:00.0 SRC2 = SWIFT J173518.7-325428 0.5 0 IBIS/ISGRI 0 Figure 2: Swift/XRT observation (0.3–10 keV). 56126 56128 56130 56132 56134 56136 MJD The large circle is the INTEGRAL 90% error box. 0 0.5 1 1.5 The crosses mark the two XRT sources in the Figure 3: Swift/XRT 0.3–10 keV lightcurves of Phase field of IGR J17354–3255 (src1 within the the two X-ray sources detected during the 2012 Figure 4: Lightcurves of IGR J17354–3255 INTEGRAL error circle, src2 outside of it). July monitoring program. Top: folded at Porb = 8.4474 d. Swift J173527.7–325555 (src1, identified with the soft X-ray counterpart of IGR J17354–3255). 2 Results: The Swift/XRT lightcurve Properties of IGR J17354–3255 The soft X-ray lightcurve (Figure 5) shows: ◮ It shows a hard X-ray flaring activity (mean flare −10 The observed orbital modulation in the folded lightcurves of Swift J173527.7–325555 and IGR J17354–3255 (which has a dip corresponding to the minimum observed in the folded BAT and INTEGRAL lightcurves) allows a definitive identification of the soft X-ray counterpart of IGR J17354–3255. This identification was previously mainly based on positional association. 57:00.0 We report on the first monitoring of the supergiant fast X-ray transient IGR J17354−3255 with the soft X-ray instrument Swift/XRT. The Swift observations span 1.2 orbital periods (P = 8.4474 d) for a total exposure of about 24 ks. The study of the flux variability of the sources in the XRT field of view allowed us to unambiguously identify the soft X-ray counterpart of IGR J17354−3255. The 0.3 − 10 keV XRT light curve shows a moderate orbital modulation and a dip. We compared the observed X-ray light curve with those calculated with a model based on the Bondi-Hoyle-Lyttleton accretion theory, for different wind parameters, eccentricities and spectral type of the donor star. We found that the X-ray orbital modulation produced by a neutron star in an eccentric orbit cannot explain the presence of the dip. We showed that an eclipse or the onset of a gated mechanism are the most likely explanations for the dip. 58:00.0 Abstract −2 −1 18 − 60 keV flux of ∼ 4 × 10 erg cm s ) and an average flux of ∼ 1.4 × 10−11 erg cm−2 s−1 (20 − 40 keV). ◮ Porb = 8.4474 ± 0.0017 d (D’Aì et al. 2011, A&A 529, ◮ a moderate orbital modulation; ◮ flaring activity on short time scales (hundreds of seconds); ◮ a dip centered at φ ∼ 0.7 which lasts ∆φ ∼ 0.2 − 0.24. ◮ SFXTs are high mass X-ray binaries (HMXBs) with O or B supergiant stars which display short X-ray outbursts with peak luminosities of 1036 − 1037 erg s−1 and a dynamic range of 3−5 orders of magnitude. See more details in the poster “The Swift SFXTs Project” of P. Romano et al. Flux (0.3−10 keV) proposed that IGR J17354–3255 is a supergiant fast X-ray transient (SFXT). ◮ The donor star is a O8.5-9Iab star (Coleiro et al. 2013, 10−13 10−12 10−11 10−10 30; Sguera et al. 2011, MNRAS 417, 573). ◮ Based on the fast flaring activity, Sguera et al. (2011) 0 0.5 1 Phase arXiv:1310.0451). 1.5 2 Figure 5: Swift/XRT 0.3–10 keV flux light curve of IGR J17354–3255, folded at Porb = 8.4474 d, which includes the data collected in 2012 (black) and 2008-2009 (grey). Downward-pointing arrows are 3σ upper limits. The red arrow at ∼ 7 × 10−14 erg cm−2 s−1 at phase 0.66 is the 3-σ upper limit obtained from a 19 ks exposure on 2011 March 6 with XMM-Newton (Bozzo et al. 2012). Reduction and data analysis ◮ We analyzed the Swift/XRT data collected in 2012 July. The ToO monitoring program lasted 11 days for a total exposure time of ∼ 24 ks. Results: An investigation of the nature of the dip 2009. We investigated the nature of the dip by comparing the X-ray lightcurve with the prediction of the Bondi-Hoyle-Lyttleton (BHL) accretion theory, assuming both spherical and nonspherical symmetry of the outflow from the donor star, by varying the mass and radius of the OB supergiant star, and for different values of the mass loss rate, terminal velocity, and orbital eccentricity. IGR J17354−3255 2009 ◮ We found that the dip cannot be explained only with 2012 ◮ We propose that an eclipse or the onset of a gated mechanism is the most likely explanation for the observed lightcurve. 10 15 20 25 30 35 40 45 0 2 5 Energy (keV) ◮ For each solution we also plotted the mean (black). The data fit with an absorbed power-law model. NH Γ Fluxa Lb χ2ν /dof (1022 cm−2) 2009 7.8+2.9 1.7+0.6 −2.2 −0.5 2.0 1.7 1.04/18 2012 11+3 1.7+0.5 −2 −0.4 1.0 0.9 1.17/42 a Unabs. 2–10 keV fluxes (10−11 erg cm−2 s−1). b 2–10 keV luminosities in units of 1035 erg s−1, at 8.5 kpc. 30000 solutions that reproduce the observed X-ray luminosities obtained assuming the accretion from a spherically symmetric wind of a supergiant star. 25000 20000 N 1 Figure 1: Spectrum of the 2009 observation (grey) and 2012 campaign Ṁ (10−7 M⊙ yr−1 ) Figure 6: 2D histogram showing the solutions that reproduce the out-of-dip luminosities. Different colors refer to different numbers of occurrences. ◮ The 2D histogram of Figure 6 shows the BHL −2 χ 2000 1800 1600 1400 1200 1000 800 600 400 200 0 1800 1700 1600 1500 1400 1300 1200 1100 1000 the X-ray orbital modulation (i.e. high eccentricity). 2 Counts s−1 keV−1 10−3 2×10−3 5×10−3 0.01 0.02 Results: Spectral analysis v∞ (km s−1 ) ◮ We also re-analyzed the Swift observations of 2008 and luminosity (averaged over the orbit) in the histogram of Figure 7. The allowed eccentricities (obtained by comparing the observed X-ray luminosities with those calculated with the BHL model) are 0 . e . 0.64. 15000 10000 5000 0 34.6 34.8 35 35.2 35.4 35.6 35.8 36 36.2 36.4 36.6 log10 L̄x (erg s−1 ) Figure 7: Histograms of the mean luminosity. Created with LATEXbeamerposter http://www-i6.informatik.rwth-aachen.de/~dreuw/latexbeamerposter.php See more details in: Ducci et al. 2013, A&A, 556, 72 Project Web page: http://www.ifc.inaf.it/sfxt/
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