Two years of monitoring Supergiant Fast X-ray Transients with Swift

We present two years of intense Swift monitoring of three SFXTs, IGR J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007). Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed power laws with by hard photon indices (G~1-2). Their outburst broad-band (0.3-150 keV) spectra can be fit well with models typically used to describe the X-ray emission from accreting NSs in HMXBs. We assess how long each source spends in each state using a systematic monitoring with a sensitive instrument. These sources spend 3-5% of the total in bright outbursts. The most probable flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19, 39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively. We present a complete list of BAT on-board detections further confirming the continued activity of these sources. This demonstrates that true quiescence is a rare state, and that these transients accrete matter throughout their life at different rates. X-ray variability is observed at all timescales and intensities we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves variations up to one order of magnitude that can occur down to timescales as short as ~1ks, and whichcan be explained by the accretion of single clumps composing the donor wind with masses M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table

Inverse Compton Emission from Galactic Supernova Remnants: Effect of the Interstellar Radiation Field

The evidence for particle acceleration in supernova shells comes from electrons whose synchrotron emission is observed in radio and X-rays. Recent observations by the HESS instrument reveal that supernova remnants also emit TeV gamma-rays; long awaited experimental evidence that supernova remnants can accelerate cosmic rays up to the ``knee'' energies. Still, uncertainty exists whether these gamma-rays are produced by electrons via inverse Compton scattering or by protons via neutral pion decay. The multi-wavelength spectra of supernova remnants can be fitted with both mechanisms, although a preference is often given to neutral pion decay due to the spectral shape at very high energies. A recent study of the interstellar radiation field indicates that its energy density, especially in the inner Galaxy, is higher than previously thought. In this paper we evaluate the effect of the interstellar radiation field on the inverse Compton emission of electrons accelerated in a supernova remnant located at different distances from the Galactic Centre. We show that contribution of optical and infra-red photons to the inverse Compton emission may exceed the contribution of cosmic microwave background and in some cases broaden the resulted gamma-ray spectrum. Additionally, we show that if a supernova remnant is located close to the Galactic Centre its gamma-ray spectrum will exhibit a ``universal'' cutoff at very high energies due to the Klein-Nishina effect and not due to the cut-off of the electron spectrum. As an example, we apply our calculations to the supernova remnants RX J1713.7-3946 and G0.9+0.1 recently observed by HESS.Comment: 4 pages, 4 figures. Uses emulateapj.cls. Accepted by ApJ

Decaying Sterile Neutrinos as a Heating Source in the Milky Way Center

Recent Chandra and Newton observations indicate that there are two-temperature components ($T \sim$ 8 keV, 0.8 keV) of the diffuse x-rays emitted from deep inside the center of Milky Way. We show that this can be explained by the existence of sterile neutrinos, which decay to emit photons that can be bound-free absorbed by the isothermal hot gas particles in the center of Milky Way. This model can account for the two-temperature components naturally as well as the energy needed to maintain the $\sim$ 8 keV temperature in the hot gas. The predicted sterile neutrino mass is between 16-18 keV.Comment: Accepted by MNRAS with minor correction

Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period

IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray Transients. This system is one of the two members of this class, together with IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are known. We report 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 28d, 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), 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 10^{18}g to 5x 10^{21}g.Comment: Accepted for publication in MNRAS. 7 pages, 7 figures, 2 table

Monitoring Supergiant Fast X-ray Transients with Swift. Results from the first year

Swift has allowed the possibility to give Supergiant Fast X-ray Transients (SFXTs), the new class of High Mass X-ray Binaries discovered by INTEGRAL, non serendipitous attention throughout all phases of their life. 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. Spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred meters, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We also present the UVOT data of these sources. (Abridged)Comment: Accepted for publication in MNRAS. 20 pages, 9 figures, 8 table

XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient

IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been ascribed to the combined effect of the intense magnetic field and rotation of the compact object hosted in them and/or the presence of dense structures ("clumps") in the wind of their supergiant companion. IGR J18410-0535 was observed for 45 ks by XMM-Newton as part of a program designed to study the quiescent emission of supergiant fast X-ray transients and clarify the origin of their peculiar X-ray variability. We carried out an in-depth spectral and timing analysis of these XMM-Newton data. IGR J18410-0535 underwent a bright X-ray flare that started about 5 ks after the beginning of the observation and lasted for \sim15 ks. Thanks to the capabilities of the instruments on-board XMM-Newton, the whole event could be followed in great detail. The results of our analysis provide strong convincing evidence that the flare was produced by the accretion of matter from a massive clump onto the compact object hosted in this system. By assuming that the clump is spherical and moves at the same velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl \simeq1.4\times10^(22) g and Rcl \simeq8\times10^(11) cm. These are in qualitative agreement with values expected from theoretical calculations. We found no evidence of pulsations at \sim4.7 s after investigating coherent modulations in the range 3.5 ms-100 s. A reanalysis of the archival ASCA and Swift data of IGR J18410-0535, for which these pulsations were previously detected, revealed that they were likely to be due to a statistical fluctuation and an instrumental effect, respectively.Comment: Accepted for publication on A&A. V2: Inserted correct version of Fig.1

X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature

SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was the longest and brightest. We have analyzed X-ray data sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data cover unabsorbed 2--10 keV fluxes between 0.1 and 5.3 X 10^-9 erg/s/cm^2. The equivalent luminosity range is 6 X 10^35 to 2 X 10^37 erg/s. The 0.3--10 keV spectrum is well described by a combination of a multi-temperature disk blackbody, a hot Comptonization component and a narrow Fe-K emission line at 6.5 to 6.8 keV with an equivalent width of up to 285 eV. The hydrogen column density in the line of sight is (8.8+/-0.5) X 10^22 cm^-2. The most conspicuous spectral changes in this model are represented by variations of the temperature and radius of the inner edge of the accretion disk, and a jump of the equivalent width of the Fe-K line in one observation. Furthermore, 45 type-I X-ray bursts were unambiguously detected between 1998 and 2001 which all occurred during or close to outbursts. We derive a distance of 7.5+/-1.3 kpc which is consistent with previous determinations. Our failure to detect bursts for prolonged periods outside outbursts provides indirect evidence that the source returns to quiescence between outbursts and is a true transient.Comment: accepted by A&A, minor adjustments incorporated from copy editing proces

Flexural and fracture behaviour of a cement-based material reinforced with GO nanoplates

In the present research work, the mechanical properties of a cement-based material reinforced with Graphene Oxide (GO) nanoplates are experimentally investigated. In particular, a detail experimental campaign, consisting of three-point bending tests on both unnotched and edge-notched specimens, is performed in order to determine flexural strength and fracture toughness. More precisely, the flexural strength is computed as a function of the experimental values of the peak load according to UNI EN Recommendation, whereas the fracture toughness is analytically determined according to the Modified Two-Parameter Mode