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    The Swift Supergiant Fast X-ray Transients Project

    IGR J11215-5952 2007 XRT campaign Romano et al., 2007

    Come meet us at the:
    6th International Symposium on High-Energy Gamma-Ray Astronomy (P. Romano, Jul 11-15, 2016, Heidelberg)

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    Il re introverso INAF Media Press Release (Italian)
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    The Swift SFXT Project

    Supergiant Fast X-ray Transients (SFXTs) are a class of High Mass X-ray Binaries (HMXBs) associated with OB supergiants and characterized by show short flares (a few hours, as observed with INTEGRAL; Negueruela et al., 2005, ESA SP-604, 165; Sguera et al, 2006, ApJ, 646, 452). These sources display short, sporadic and bright flares reaching a peak luminosity of a few 1036 erg s-1. Their quiescent level is at about 1032 erg s-1, thus making SFXTs a class of transients with a large dynamic range (about 103--5).

    Our activity has been following several lines.

  • We studied IGR J11215-5952 which, in 2007, was the only SFXT to show periodic outbursts with a period of 329 days. We organized a campaign to collect data with Swift/XRT, XMM-Newton and INTEGRAL to follow the predicted outburst around February 9, 2007. We have been following this source since, with Swift ToO and GI (Cycle 4) observations.
  • To characterize the properties of SFXTs on timescales of months (e.g. outburst recurrence and the quiescent level), we have performed a monitoring campaign (between October 2007 and November 2009) with Swift of four SFXTs:
         IGR J16479-4514, XTE J1739-302, IGR J17544-2619, and AX J1841.0-0536.
  • We have been monitoring the activity of IGR J08408-4503, IGR J18483-0311, IGR J16418-4532 and IGR J17354-3255.
  • We obtained ToO triggers for the whole sample of SFXTs through the Swift GI (Cycle 5, Cycle 6, Cycle 8, and Cycle 9) program. We activated it for SAX J1818.6-1703 on 2009 May 6, and then for two targets initially thought to be newly discovered SFXTs: Swift J164449.31573451 (Burrows et al. 2011, Nature, 476, 421), on 2011 March 28, and for IGR J18245-2452 (Papitto et al. 2013, Nature, 501, 517), on 2013 March 30.

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    Main Results

    The periodic IGR J11215-5952 (2007-2011)

    IGR J11215-5952 displays accertained periodic flares with a period of about 165 days as Swift data have demonstrated (Romano et al. ATel 1151; Sidoli et al 2007, A&A, 476, 1307; Romano et al., 2008, ATel, 1444; Romano et al 2009, ApJ, 696, 2068). Our data sample includes:

  • The Swift monitoring during the 2007 February outburst proposed, thanks to the predictable behavior of the recurrent outbursts, to study the 5th outburst (February 9) @ P=329d (Romano et al., 2007, A&A, 469, L5). This represents the most complete monitoring campaign of an outburst from a SFXT and constitutes a unique data-set for an outburst of a SFXT, thanks to the combination of sensitivity and time coverage. The campaigns lasted 23 days for a total on-source exposure of ~73ks and allowed us to study IGR J11215-5952 from outburst onset to almost quiescence showing that it reached LX ~1036 erg s-1 displaying a typical dynamic range ~103 and a hard spectrum. These observations demonstrated that the accretion phase during the bright flare lasts longer than previously thought: a few days instead of hours, with only the brightest phase lasting less than one day, and being characterized by a large variability with several flares lasting from a few minutes to a few hours (Romano et al., 2007, A&A, 469, L5). Simultaneous ToO observations with XMM (23 ks) and INTEGRAL (101 pointing of 2900s each) were performed (Sidoli, et al., 2007, A&A, 476, 1307). These allowed us to propose an alternative hypothesis for the outburst mechanism in SFXTs, linked to the possible presence of a second wind component, in the form of an equatorial disc from the supergiant donor.
  • The Swift monitoring during 2007 July to study the following 'apastron' @P/2, with a ToO for 1.5ks-2.5ks/day from June 5 to July 31, 17 ks on-source time. A new, unexpected outburst was observed (Romano et al. ATel 1151). The spectral fits and energetics are fully consistent with the ones observed during the periastron outburst (Sidoli, et al., 2007, A&A, 476, 1307).
  • The Swift 2008 March ToO observations to study the P/4. No emission observed down to a 3sigma upper limit of 9.1x10-13 erg cm-2 s-1 (photon index=1 and NH=1x1022 cm-2). We could exclude that the period is 165/2 days (Romano et al., 2008, ATel, 1444).
  • The Swift 2008 June monitoring. The properties of the rise to this new outburst and the comparison with the previous outbursts allow us to suggest that the true orbital period of IGR J11215-5952 is very likely 164.6 days, and that the orbit is eccentric, with the different outbursts produced at the periastron passage, when the neutron star crosses the inclined equatorial wind from the supergiant companion. (Figure 1, Romano et al., 2009, ApJ, 696, 2068).
  • IGR J11215-5952 2007 XRT/XMM/IMTEGRAL Figure 1. XRT light curves of the 2007 February 9 `periastron' outburst (light gray filled circles), with superimposed the 2007 July 24 `apastron' outburst (dark gray empty diamonds), and the 2008 June 16 `apastron' outburst (orange empty squares), folded with a period of 164.6 days. The time is relative to the peak of the 2007 February 9 outburst. Adapted from Romano et al., 2009, ApJ, 696, 2068.

  • The Swift 2009 October observations. We observed the sources for a total of about 5 ks on 2009 October 23 (Esposito et al., 2009, ATel 2257; Romano et al., 2011, POS, Texas 2010, 117) and 2009 October 24.
  • The Swift 2011 March observations. We observed the sources for a total of about 2.3 ks on 2011 March 1 and 2 (Romano et al., 2011, ATel 3200; Romano et al., 2011, Procs. Fermi Symposium).
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    Monitoring 4 SFXTs with Swift (October 2007 - November 2009)

    During the first year of monitoring we collected a total of 330 Swift observations as part of our program, for a total net XRT exposure of about 363 ks accumulated on IGR J16479-4514, XTE J1739-302, IGR J17544-2619, AX J1841.0-0536.

  • We detect a low level X-ray activity in all four SFXTs which demonstrates that these transient sources accrete matter even outside their outbursts (Sidoli et al. 2008, ApJ 687, 1230, Paper I). This fainter X-ray activity is composed of many flares with a large flux variability, on timescales of thousands of seconds. The light curve variability is also evident on larger timescales of days, weeks and months, with a dynamic range of more than one order of magnitude in all four SFXTs. The X-ray spectra are typically hard, with an average 2-10 keV luminosity during this monitoring of about 1033-34 erg s-1. Our monitoring demonstrates that these transients spend most of the time accreting matter, although at a much lower level (about 100-1000 times lower) than during the bright flares, and that the ``true quiescence'' is probably a very rare state.
  • We observed the first broad-band simultaneous spectra (0.3-150 keV) of three SFXTs: IGR J16479-4514 (Figure 2; Romano et al. 2008, ApJ, 680, L137, Paper II) the prototypes XTE J1739-302 and IGR J17544-2619 (Sidoli et al., 2009, ApJ, 690, 120, Paper III; Sidoli et al., 2009, MNRAS, 397, 1528, Paper IV). We have shown that they can be fit with models traditionally adopted to fit accreting neutron stars (absorbed cutoff power laws), even in the objects where proof of the presence of a neutron star (from a spin period) is not available. There are considerable differences in the behaviour of the absorbing column among the examined cases.
  • Swift has allowed the possibility to give SFXTs non serendipitous attention throughout all phases of their life. In Romano et al. (2009, MNRAS, 399, 2021, paper V) we present our results based on the first year of intense Swift monitoring of four SFXTs, IGR J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the first assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is 17, 28, 39, 55% (5% uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE J1739-302, and IGR J17544-2619, respectively, so that true quiescence is a rare state. This demonstrates that these transients accrete matter throughout their life at different rates. AX J1841.0-0536 is the only source which has not undergone a bright outburst during our campaign. Although individual sources behave somewhat differently, common X-ray characteristics of this class are emerging such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of 4 orders of magnitude has been observed. We performed out-of-outburst intensity-based spectroscopy. We also present the UVOT data of these sources.
  • XRT and BAT spectra of IGR J16479-4514
    Figure 2. Spectroscopy of the 2008 March 19 outburst of IGR J16479-4514. Top: data from the second BAT observation (black empty circles) and simultaneous XRT/WT data (red filled circles) fit with an absorbed power law with a high energy cutoff. Bottom: the residuals of the fit in units of standard deviations. Adapted from Romano et al. 2008, ApJ, 680, L137.
    SFXT light curve comparison
    Figure 3. Light curves of the outbursts of SFXTs followed by Swift/XRT referred to their respective triggers. We show the 2005 outburst of IGR J16479-4514 (Sidoli et al. 2008, ApJ 687, 1230), which is more complete than the one observed in 2008 (Romano et al. 2008, ApJ, 680, L137). The IGR J11215-5952 light curve has an arbitrary start time, since the source did not trigger the BAT (the observations were obtained as a ToO; Romano et al., 2007, A&AL, 469, L5 ). The data on XTE J1739-302 and IGR J17544-2619 are presented in Sidoli et al., 2009, ApJ, 690, 120. The bottom plot shows the outburst from XTE J1739-302 that occurred on 2008 August 13. Note that where no data are plotted, no data were collected. For clarity, we drew dashed vertical lines to mark each day (up to one week) since the trigger. From Sidoli et al. 2008, PoS (Integral08) 084.

  • During the second year we monitored IGR J16479-4514, XTE J1739-302, IGR J17544-2619, and collected a 228 more Swift observations for a total net XRT exposure of about 243 ks. In Romano et al. (2011, MNRAS, 410, 1825, paper VI) we present our results. We confirm our findings on the inactivity duty cycle, and by considering our monitoring as a casual sampling of the X-ray light curves, we infer that the time these sources spend in bright outbursts is between 3 and 5% of the total. The most probable X-ray flux for these sources is about 1-2x10-11 erg cm-2 s-1 (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 1033 to a few 1034 erg s-1 (two orders of magnitude lower than the bright outbursts). Variability in the X-ray flux is observed at all timescales and intensity ranges we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves flux variations up to one order of magnitude that can occur down to timescales as short as 1 ks, and which can be naturally explained by the accretion of single clumps composing the donor wind with masses of 0.3-2x1019g.
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    Other Monitorings

    Our most recent results using Swift data on outbursting SFXTs also include:

  • The unique multiple flaring activity in IGR J08408-4503 (Romano et al., 2009, MNRAS, 392, 45).
    The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of about 10-9 erg cm-2 s-1 (2-10 keV), followed by two equally bright flares within 75 ks. The spectral characteristics of the flares differ dramatically, with most of the difference, as derived via time-resolved spectroscopy, being due to absorbing column variations. We observe a gradual decrease in the NH, derived with a fit using absorbed power-law model, as time passes. We interpret these NH variations as due to an ionization effect produced by the first flare, resulting in a significant decrease in the measured column density towards the source. Light curve of IGR J08408-4503
    Figure 4. XRT 0.3-10 keV light curve of the 2008 July 5 outburst of IGR J08408-4503. Adapted from Romano et al. 2009, MNRAS, 392, 45.
  • Coverage of the outburst of SAX J1818.6-1703, obtained as part of Swift GI (Cycle 5) program (Romano et al., 2009, ATel 2044 , Sidoli et al., 2009, MNRAS, 400, 258).
    We performed the first broad-band X-ray study of SAX J1818.6-1703 in outburst (2009 May 6). The spectrum can be deconvolved well with models usually adopted to describe the emission from HMXB X-ray pulsars, and is characterized by a very high absorption, a flat power law (photon index 0.1-0.5) and a cut-off at about 7-12 keV. Alternatively, the emission can be described with a Comptonized emission from a cold and optically thick corona, with an electron temperature kTe = 5-7 keV, a hot seed photon temperature, kT0, of 1.3-1.4 keV and an optical depth for the Comptonizing plasma, of about 10. The 1-100 keV luminosity at the peak of the flare is 3x1036 erg s-1 (assuming the optical counterpart distance of 2.5 kpc). These properties of SAX J1818.6-1703 resemble those of the prototype of the SFXT class, XTE J1739-302. The monitoring with Swift/XRT reveals an outburst duration of about 5 d, similar to other members of the class of SFXTs, confirming SAX J1818.6-1703 as a member of this class.
  • Monitoring of IGR J18483-0311 for a whole orbital period (Romano et al., 2010, MNRAS, 401, 1564).
    IGR J18483-0311 is one of the few SFXTs where both the orbital (18.52 d) and spin period (21 s) are known. Our paper reports on the first complete monitoring of the X-ray activity along an entire orbital period of a Supergiant Fast X-ray Transient. These Swift observations, lasting 28 days, cover more than one entire orbital phase consecutively. They are a unique data-set, which allows us to constrain the different mechanisms proposed to explain the nature of this new class of X-ray transients. We applied the new clumpy wind model for blue supergiants developed by Ducci et al. (2009, 398, 2152), to the observed X-ray light curve. Assuming an eccentricity of e=0.4, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from 1018g to 5x1021g.
    Light curve of IGR J18483-0311
    Figure 5. Swift/XRT 0.2-10 keV light curve of IGR J18483-0311 during our monitoring program, background-subtracted and corrected for pile-up, PSF losses, and vignetting. Downward-pointing arrows are 3sigma upper limits. The upper limit with the wide symbol centered on MJD 55007.9 (about 0.004 counts/s) is obtained accumulating the 4 observations (individual upper limits) between MJD 55006.5 and 55010.2. Different colours mark different observations, with a colour scheme that generally mimics the phase (top axis) with a P = 18.52 days (Sguera et al. 2007). The inset zooms on observation 024. From Romano et al. 2010, MNRAS, 401, 1564.

  • Coverage of the outburst of AX J1841.0-0536 on 2010 Jun 5 ( Romano et al., 2011, MNRAS, 412, L30, Paper VII). Swift observed an outburst on 2010 June 5, and followed it with XRT for 11 days. The X-ray light curve shows an initial flare followed by a decay and subsequent increase, as often seen in other SFXTs, and a dynamical range of about 1600. Our observations allow us to analyse the simultaneous broad-band (0.3-100 keV) spectrum of this source, for the first time down to 0.3 keV, which can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high-mass X-ray binaries, and is characterized by a high absorption (NH ~ 2x1022 cm-2), a flat power law (photon index 0.2), and a high energy cutoff.

  • Monitoring of IGR J16418-4532 along the orbital period (Romano et al., 2011, ATel 3174, Romano et al., 2012, MNRAS, 419, 2695).
    Our Swift observations of this candidate supergiant fast X-ray transient, with known orbital and spin period (3.7d and 1250s), span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations we can address the nature of this transient. By applying the clumpy wind model for blue supergiants (Ducci et al. 2009, 398, 2152) to the observed X-ray light curve, and assuming a circular orbit, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from 5x1016g to 21g. Our data suggest, based on the X-ray behaviour, that this is an intermediate SFXT.

  • We have developed an algorithm solving the radiative transfer equation in the Fokker-Planck approximation when both thermal and bulk Comptonization take place. (Farinelli et al., 2012, A&A, 538, A67).
    The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steady-state equilibrium. Our XSPEC model will be publically available and distributed as a contributed model to the official XSPEC web page. The code is written using C-language, and can be easily installed following the standard procedure reported both in the official XSPEC manual and in the brief cookbook which will be delivered together with the source code. The free parameters of the model are the blackbody seed photon temperature kTbb, the electron temperature kTe, the vertical optical depth of the accretion column tau, the radius of the accretion column r0 (in units of the NS Schwarzschild radius), and the albedo A of the NS surface. Additional parameters for the case of accretion with matter velocity increasing progressively with decreasing altitude are the index eta, which corresponds to a free-fall case when it is set equal to 0.5, and the terminal velocity beta0 at the NS surface.
    The COMPGMAG model has been applied to the most recent outbursts (2011 February 22 and March 24, respectively) of XTE J1739-302 and IGR J17544-2619, the prototypes of the SFXT class (Farinelli et al., 2012, MNRAS, 424, 2854). We discuss the possible accretion scenarios derived by the different models, and we also emphasize the fact that the electron density derived from the Comptonization models, in the regions where the X-ray spectrum presumably forms, is lower than that estimated using the continuity equation at the magnetospheric radius and the source X-ray luminosity, and we give some possible explanations.

  • Monitoring of IGR J17354-3255 along the orbital period (Ducci et al., 2013, A&A, 556, A72).
    We performed an 11-day Swift/XRT monitoring of the field of view around IGR J17354-3255, which is positionally associated with the AGILE/GRID gamma-ray transient AGL J1734-3310. These new data (~24 ks, spanning 1.2 orbital periods, Porb = 8.4474 d) allow us to exploit the timing variability properties of the sources in the field to unambiguously identify the soft X-ray counterpart of IGR J17354-3255. The soft X-ray light curve shows a moderate orbital modulation and a dip. We investigated the nature of the dip by comparing the X-ray light curve with the prediction of the Bondi-Hoyle-Lyttleton accretion theory, assuming both spherical and nonspherical symmetry of the outflow from the donor star. We found that the dip cannot be explained with the X-ray orbital modulation. We propose that an eclipse or the onset of a gated mechanism is the most likely explanation for the observed light curve.

  • The 100-month Swift Catalogue of Supergiant Fast X-ray Transients I. BAT on-board and transient monitor flares. (Romano et al., 2014, A&A, 562, A2).
    We assembled a catalogue of over a thousand BAT flares from 11 SFXTs, down to 15-150 keV fluxes of about 6x10-10 erg cm-2 s-1 (daily timescale) and about 1.5x10-9 erg cm-2 s-1 (orbital timescale, averaging 800 s), the great majority unpublished. The catalogue spans 100 months. This population is characterized by short (a few hundred seconds) and relatively bright (in excess of 100 mCrab, 15-50 keV) events. In the hard X-ray, these flares last generally much less than a day. Clustering of hard X-ray flares can be used to indirectly measure the length of an outburst, even when the low-level emission is not detected. We construct the distributions of flares, of their significance (in terms of S/N), and of their flux as a function of orbital phase to infer the properties of these binary systems. In particular, we observe a trend of clustering of flares at some phases as Porb increases, which is consistent with a progression from tight circular or mildly eccentric orbits at short periods to wider and more eccentric orbits at longer orbital periods. Finally, we estimate the expected number of flares for a given source for our limiting flux and provide the recipe for calculating them for the limiting flux of future hard X-ray observatories.
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    Team members:

    * P. Romano
    * P. Esposito (@ API/UvA)
    * J. A. Kennea (@ PSU)
    * V. Mangano (@ PSU)
    * S. Vercellone

    Contact person at IASF-Palermo:


    Related Sites

    Swift Home Page, QL
    The Astronomer's Telegram
    BAT Transient Monitor: IGRJ16479-4514, IGRJ17391-3021,
                                              IGRJ17544-2619, IGRJ18410-0535
    BAT 58-Month Source Catalog
    J. Rodriguez's page on IGR sources
    INTEGRAL Galactic Bulge Monitoring
    The INTEGRAL Spiral Arms (ISA) Monitoring Program
    RXTE Galactic Center Observation
    A Catalogue of optically identified INTEGRAL sources


    We acknowledge financial contribution from
    ASI-INAF I/004/11/0 (Swift),
    ASI-INAF I/037/12/0 (NARO15)

    Previous grants:
    ASI-INAF I/021/12/0 (LOFT),
    ASI-INAF I/009/10/0 (Analisi Dati Alte Energie), FFO 2009,
    ASI/INAF I/088/06/0 (Studio di Astrofisica delle Alte Energie).


    Complete list of our SFXT publications.
    Other transients (Swift followup of MAXI-discovered transients).

    Last Modification: Friday, July 1 2016
    Edited by P. Romano

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