Ke4 decays and Wigner cusp

The NA48/2 experiment at the CERN SPS collected in 2003 and 2004 large samples of the decays K+- -> pi+ pi- e+- nu (Ke4+-), K+- -> pi0 pi0 e+- nu (Ke400) and K+- -> pi0 pi0 pi+-. From the Ke4+- form factors and from the cusp in the M00^2 distribution of the K+- -> pi0 pi0 pi+- events, the pi-pi scattering lengths a00 and a20 could be extracted. This measurement is a fundamental test of Chiral Perturbation Theory (ChPT). The branching fraction and form factors of the Ke400 decay were precisely measured, using a much larger data sample than in previous experiments. An improved measurement of the slope parameters for the decay K+- -> pi0 pi0 pi+- showed evidence for a non-zero value of k.Comment: Contribution to the proceedings of HQL06, Munich, October 16th-20th 200

Measurements of Ke4K_{e4} and K±→π0π0π±K^{\pm} \to \pi^{0} \pi^{0} \pi^{\pm} decays

The NA48/2 experiment at the CERN SPS collected in 2003 and 2004 large samples of the decays $K^{\pm} \to \pi^{+} \pi^{-} e^{\pm} \nu_{e} (K_{e4}^{+-})$, $K^{\pm} \to \pi^{0} \pi^{0} e^{\pm} \nu_{e}(K^{00}_{e4})$ and $K^{\pm} \to \pi^{0}\pi^{0} \pi^{\pm}$. From the $K_{e4}^{+-}$ form factors and from the cusp in the $M_{00}^{2}$ distribution of the $K^{\pm} \to \pi^{0} \pi^{0} \pi^{\pm}$ events, the $\pi\pi$ scattering lengths $a_{0}^{0}$ and $a^{2}_{0}$ could be extracted. This measurement is a fundamental test of Chiral Perturbation Theory ($\chi PT$). The branching fraction and form factors of the $K_{e4}^{00}$ decay were precisely measured, using a much larger data sample than in previous experiments

Unveiling GRB hard X-ray afterglow emission with Simbol-X

Despite the enormous progress occurred in the last 10 years, the Gamma-Ray Bursts (GRB) phenomenon is still far to be fully understood. One of the most important open issues that have still to be settled is the afterglow emission above 10 keV, which is almost completely unexplored. This is due to the lack of sensitive enough detectors operating in this energy band. The only detection, by the BeppoSAX/PDS instrument (15-200 keV), of hard X-ray emission from a GRB (the very bright GRB 990123), combined with optical and radio observations, seriously challenged the standard scenario in which the dominant mechanism is synchrotron radiation produced in the shock of a ultra-relativistic fireball with the ISM, showing the need of a substantial revision of present models. In this respect, thanks to its unprecedented sensitivity in the 10-80 keV energy band, Simbol-X, through follow-up observations of bright GRBs detected and localized by GRB dedicated experiments that will fly in the >2010 time frame, will provide an important breakthrough in the GRB field.Comment: 4 pages, 2 figures. Paper presented at "Simbol-X: the hard X-ray universe in focus", held in Bologna, Italy, on 14-16 May 2007. To be published in Memorie della Societa' Astronomica Italian

Swift/XRT counterparts to unassociated Fermi high-energy LAT sources

We report the results from our analysis of a large set of archival data acquired with the X-ray telescope (XRT) onboard Swift, covering the sky region surrounding objects from the first Fermi Large Area Telescope (LAT) catalogue of high-energy sources (1FHL), which still lack an association. Of the 23 regions analysed, ten did not show any evidence of X-ray emission, but 13 were characterised by the presence of one or more objects emitting in the 0.3-10 keV band. Only in a couple of cases is the X-ray counterpart located outside the Fermi positional uncertainty, while in all other cases the associations found are compatible with the high-energy error ellipses. All counterparts we found have been studied in detail by means of a multi-waveband approach to evaluate their nature or class; in most cases, we have been able to propose a likely or possible association except for one Fermi source whose nature remains doubtful at the moment. The majority of the likely associations are extragalactic in nature, most probably blazars of the BL Lac type.Comment: 13 pages, 5 figures, 4 tables. Accepted for publication in A&

Using X-ray catalogues to find counterparts to unassociated high-energy Fermi/LAT sources

The first Fermi Large Area Telescope (LAT) catalogue of sources (1FHL) emitting at high energies (above 10 GeV) reports the details of 514 objects detected in the first three years of the Fermi mission. Of these, 71 were reported as unidentified in the 1FHL catalogue, although six are likely to be associated with a supernova remnant (SNR), a Pulsar Wind Nebula (PWN) or a combination of both, thereby leaving a list of 65 still unassociated objects. Herein, we report a preliminary analysis on this sample of objects concentrating on nine 1FHL sources, which were found to have a clear optical extragalactic classification. They are all blazar, eight BL Lac and one flat spectrum radio quasar, typically at redshift greater than 0.1.Comment: Proceedings of "Swift: 10 Years of Discovery", December 2-5 2014, Rome, Italy, in Proceedings of Science (SWIFT 10

CXOU J005047.9-731817: a 292-s X-ray binary pulsar in the Small Magellanic Cloud

We report on the discovery of a transient X-ray pulsars, located in the Small Magellanic Cloud, with a pulse period of 292 s. A series of Chandra pointings fortuitously recorded in 2010 April-May the occurrence of a two-weeks-long outburst, during which the source luminosity increased by a factor of about 100, reaching a peak of ~1E36 erg/s (for a distance of 61 kpc). Complex-shape and energy-dependent pulsations were detected close to the outburst peak and during the very first part of its decay phase. During the outburst, the phase-averaged spectrum of the pulsar was well described by an absorbed power law with photon index ~0.6, but large variations as a function of phase were present. The source was also detected by Chandra several times (during 2002, 2003, 2006, and 2010) at a quiescent level of ~1E34 erg/s. In 2012 we performed an infrared photometric follow-up of the R ~ 15 mag optical counterpart with the ESO/VLT and a spectroscopic observation by means of the CTIO telescope. The optical spectra suggest a late-Oe or early-Be V-III luminosity-class star, though a more evolved companion cannot be ruled out by our data (we can exclude a luminosity class I and a spectral type later than B2). Finally, we show that the outburst main parameters (duration and peak luminosity) can be accounted for by interpreting the source transient activity as a type I outburst in a Be X-ray binary.Comment: MNRAS, in press; 8 pages, 7 figures, 4 tables; Figure 1 in reduced qualit

GRB Afterglows from Anisotropic Jets

Some progenitor models of gamma-ray bursts (GRBs) (e.g., collapsars) may produce anisotropic jets in which the energy per unit solid angle is a power-law function of the angle ($\propto\theta^{-k}$). We calculate light curves and spectra for GRB afterglows when such jets expand either in the interstellar medium or in the wind medium. In particular, we take into account two kinds of wind: one ($n\propto r^{-3/2}$) possibly from a typical red supergiant star and another ($n\propto r^{-2}$) possibly from a Wolf-Rayet star. We find that in each type of medium, one break appears in the late-time afterglow light curve for small $k$ but becomes weaker and smoother as $k$ increases. When $k\ge 2$, the break seems to disappear but the afterglow decays rapidly. Thus, one expects that the emission from expanding, highly anisotropic jets provides a plausible explanation for some rapidly fading afteglows whose light curves have no break. We also present good fits to the optical afterglow light curve of GRB 991208. Finally, we argue that this burst might arise from a highly anisotropic jet expanding in the wind ($n\propto r^{-3/2}$) from a red supergiant to interpret the observed radio-to-optical-band afterglow data (spectrum and light curve).Comment: 12 pages + 10 figures, accepted by Ap

X-ray, optical and infrared investigation of the candidate Supergiant Fast X-ray Transient IGR J18462-0223

We report on a broad-band X-ray study (0.5-60 keV) of the poorly known candidate Supergiant Fast X-ray Transient (SFXT) IGR J18462-0223, and on optical and near-infrared (NIR) followup observations of field objects. The out-of-outburst X-ray state has been investigated for the first time with archival INTEGRAL/IBIS, ASCA, Chandra and Swift/XRT observations. This allowed us to place stringent 3 sigma upper limits on the soft (0.5-10 keV) and hard (18-60 keV) X-ray emission of 2.9x10^-13 erg cm^-2 s^-1 and 8x10^-12 erg cm^-2 s^-1, respectively; the source was also detected during an intermediate soft X-ray state with flux equal to 1.6x10^-11 erg cm^-2 s^-1 (0.5-10 keV). In addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray flares (18-60 keV) having a duration in the range 1-12 hours: the flaring behavior was also investigated in soft X-rays (3-10 keV) with archival INTEGRAL/JEM-X observations. The duty cycle (1.2%) and the dynamic ranges (> 1,380 and > 190 in the energy bands 0.5-10 keV and 18-60 keV, respectively) were measured for the first time. Archival UKIDSS JHK NIR data, together with our deep R-band imaging of the field, unveiled a single, very red object inside the intersection of the Swift/XRT and XMM-Newton error circles: this source has optical/NIR photometric properties compatible with a very heavily absorbed blue supergiant located at about 11 kpc, thus being a strong candidate counterpart for IGR J18462-0223. NIR spectroscopy is advised to confirm the association. Finally, a hint of a possible orbital period was found at about 2.13 days. If confirmed by further studies, this would make IGR J18462-0223 the SFXT with the shortest orbital period among the currently known systems.Comment: accepted for publication in A&A, 9 pages, 7 figures, 2 table

IGR J11215-5952: a hard X-ray transient displaying recurrent outbursts

The hard X-ray source IGRJ11215-5952 has been discovered with INTEGRAL during a short outburst in 2005 and proposed as a new member of the class of supergiant fast X-ray transients. We analysed INTEGRAL public observations of the source field in order to search for previous outbursts from this transient, not reported in literature.Our results are based on a systematic re-analysis of INTEGRAL archival observations, using the latest analysis software and instrument calibrations. We report the discovery of two previously unnoticed outbursts, spaced by intervals of ~330 days, that occurred in July 2003 and May 2004. The 5-100keV spectrum is well described by a cut-off power law, with a photon index of 0.5, and a cut-off energy ~15-20keV, typical of High Mass X-ray Binaries hosting a neutron star. A 5-100keV luminosity of 3E36 erg/s has been derived (assuming 6.2kpc, the distance of the likely optical counterpart). The 5-100keV spectral properties, the recurrent nature of the outbursts,together with the reduced error region containing the blue supergiant star HD306414,support the hypothesis that IGRJ11215-5952 is a member of the class of the Supergiant Fast X-ray Transients.Comment: 4 pages, 4 figures, accepted for publication in Astronomy and Astrophysics Letter