A search for Galactic transients disguised as gamma-ray bursts

A significant fraction of cosmological gamma-ray bursts (GRBs) are characterised by a fast rise and exponential decay (FRED) temporal structure. This is not a distinctive feature of this class, since it is observed in many Galactic transients and is likely descriptive of a sudden release of energy followed by a diffusion process. Possible evidence has recently been reported by Tello et al. (2012) for a Galactic contamination in the sample of FRED GRBs discovered with Swift. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue up to September 2014. We selected 181 FRED GRBs (2/3 with unknown redshift) and considered different subsamples. We tested the degree of isotropy through the dipole and the quadrupole moment distributions, both with reference to the Galaxy and in a coordinate-system-independent way, as well as with the two-point angular autocovariance function. In addition, we searched for possible indicators of a Galactic origin among the spectral and temporal properties of individual GRBs. We found marginal (~3 sigma) evidence for an excess of FREDs with unknown redshift towards the Galactic plane compared with what is expected for an isotropic distribution corrected for the non-uniform sky exposure. However, when we account for the observational bias against optical follow-up observations of low-Galactic latitude GRBs, the evidence for anisotropy decreases to ~2 sigma. In addition, we found no statistical evidence for different spectral or temporal properties from the bulk of cosmological GRBs. We found marginal evidence for the presence of a disguised Galactic population among Swift GRBs with unknown redshift. The estimated fraction is f=(19 +- 11)%, with an upper limit of 34% (90% confidence).Comment: 6 pages, 4 figures, accepted by A&

Swift-XRT 6-year monitoring of the ultraluminous X-ray source M33-X8

The long term evolution of ULX with their spectral and luminosity variations in time give important clues on the nature of ULX and on the accretion process that powers them. We report here the results of a Swift-XRT 6-year monitoring campaign of the closest example of a persistent ULX, M33 X-8, that extends to 16 years the monitoring of this source in the soft X-rays. The luminosity of this source is a few 10^39 erg/s, marking the faint end of the ULX luminosity function. We analysed the set of 15 observations collected during the Swift monitoring. We searched for differences in the spectral parameters at different observing epochs, adopting several models commonly used to fit the X-ray spectra of ULX. The source exhibits flux variations of the order of 30%. No significant spectral variations are observed along the monitoring. The average 0.5-10 keV spectrum can be well described by a thermal model, either in the form of a slim disk, or as a combination of a Comptonized corona and a standard accretion disk.Comment: 6 pages, 4 figures, 2 tables. Paper published in A&

Swift reveals the eclipsing nature of the high mass X-ray binary IGR~J16195-4945

IGR J16195-4945 is a hard X-ray source discovered by INTEGRAL during the Core Program observations performed in 2003. We analyzed the X-ray emission of this source exploiting the Swift-BAT survey data from December 2004 to March 2015, and all the available Swift-XRT pointed observations. The source is detected at a high significance level in the 123-month BAT survey data, with an average 15-150 keV flux of the source of ~1.6 mCrab. The timing analysis on the BAT data reveals with a significance higher than 6 standard deviations the presence of a modulated signal with a period of 3.945 d, that we interpret as the orbital period of the binary system. The folded light curve shows a flat profile with a narrow full eclipse lasting ~3.5% of the orbital period. We requested phase-constrained XRT observations to obtain a more detailed characterization of the eclipse in the soft X-ray range. Adopting resonable guess values for the mass and radius of the companion star, we derive a semi-major orbital axis of ~31 R_sun, equivalent to ~1.8 times the radius of the companion star. From these estimates and from the duration of the eclipse we derive an orbital inclination between 55 and 60 degrees. The broad band time-averaged XRT+BAT spectrum is well modeled with a strongly absorbed flat power law, with absorbing column N_H=7x 10^22 cm^(-2) and photon index Gamma=0.5, modified by a high energy exponential cutoff at E_cut=14 keV.Comment: 5 pages, 5 figures, 2 tables. Published on MNRA

Comparison of Multispectral and Hyperspectral UAV Imagery for Late Blight (Phytophtora infestans) detection in a potato (Solanum tuberosum) field

Mestrado em Engenharia Agronómica. Universidade de Lisboa, Instituto Superior de AgronomiaLate Blight (LB) is of high concern in the potato crop production. The disease, caused by the oomycete Phytophtora infestans, is responsible for causing huge impacts on the global production. A prompt and specific control, besides being most likely to succeed, enables strategies of small-scale site-specific management which, when combined with a correct detection and good decision model, are expected to reduce the pesticide use. The detection of diseased plants with multispectral and hyperspectral UAV imagery has a great potential in improving LB management and control. In this context, the Portuguese company HPDRONES, within the PG-PSA project, wants to investigate the use of hyperspectral solutions for LB early detection. A drone embedded with a hyperspectral camera has been flighted across an experimental plot in the Ribatejo region. The present work provide an exemplification of a model for the detection of LB using UAV imagery in an open field of potato, involving the use of drones for the image collection and of machine learning algorithms to implement the data analysis. A supervised classification method is proposed. The result is a hyperspectral classifier able to perform an accurate LB detection. On the basis of the limitation of the hyperspectral technologies, a sensitivity approach is proposed, in which spatial and spectral degraded data are used to evaluate the classification’s success under different conditions. The results suggest that the use of drones with multispectral sensors or using different operational parameters (i.e. flight altitude) do not affect the efficiency of the LB detection.O míldio é uma das principais doenças que afeta a produção de batata. A doença, causada pelo oomiceta Phytophtora infestans, é responsável por ter grandes impactos na produção mundial desta cultura. Um controle rápido e específico, para além de ter maior probabilidade de sucesso, permite estratégias de controle smallscale e site-specific que, quando combinadas com uma detecção correta e um modelo de decisão adequado, conseguem reduzir sensivelmente o uso de pesticidas. A detecção de plantas doentes através de imagens multiespectrais e hiperespectrais de drone tem um grande potencial para o controle de míldio. Neste contexto, a empresa portuguesa HPDRONES, no âmbito do projeto PG-PSA, pretende investigar a utilização de soluções hiperespectrais para a deteção precoce de míldio. Para o efeito, foi usado um drone com uma câmara hiperespectral para a captura das imagens aéreas de uma parcela experimental localizada na região do Ribatejo. O presente trabalho oferece uma exemplificação de um modelo de detecção de míldio usando imagens coletadas em um campo aberto de batata, envolvendo o uso de drones para a coleta de imagens e de algoritmos de aprendizagem para implementar a análise de dados. Propõe-se um método de classificação supervisionado. O resultado é um classificador hiperespectral capaz de detectar a presença de míldio em campo. Com base nas limitações das tecnologias hiperespectrais, propõe-se uma análise de sensibilidade, através da qual os dados espaciais e espectrais degradados são usados para avaliar o sucesso da classificação sob diferentes condições. Os resultados sugerem que o uso de drones com sensores multiespectrais ou usar diferentes parâmetros operacionais (ou seja, altitude de voo) não afeta a eficiência da detecção de míldio.N/

Temporal features of LS I +61∘^{\circ}303 in hard X-rays from the Swift/BAT survey data

We study the long-term spectral and timing behaviour of LS I +61$^{\circ}$303 in hard X-rays (15--150 keV) using $\sim$10 years of survey data from the $Swift$ Burst Alert Telescope (BAT) monitor. We focus on the detection of long periodicities known to be present in this source in multiple wavelengths. We clearly detect three periods: the shorter one at 26.48 days is compatible with the orbital period of the system; the second, longer, periodicity at 26.93 days, is detected for the first time in X-rays and its value is consistent with an analogous temporal feature recently detected in the radio and in the gamma-ray waveband, and we associate it with a modulation caused by a precessing jet in this system. Finally, we find also evidence of the long-term periodicity at $\sim$1667 d, that results compatible with a beat frequency of the two close, and shorter, periodicities. We discuss our results in the context of the multi-band behaviour of the physical processes of this source.Comment: 5 pages, 8 figures. Published in MNRA

The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries

A growing number of previously hidden Galactic X-ray sources are now detected with recent surveys performed by the Integral and Swift satellites. Most of these new sources eluded past surveys due to their large local X-ray extinction and consequent low soft X-ray flux. The Swift-BAT performs daily monitoring of the sky in an energy band (15-150 keV) which is only marginally affected by X-ray extinction, thus allowing for the search of long periodicities in the light curve and identification of the nature of the X-ray sources. We performed a period search using the folding technique in the Swift-BAT light curves of three Integral sources: IGR J05007-7047, IGR J13186-6257 and IGR J17354-3255. Their periodograms show significant peaks at 30.77$\pm$0.01 d, 19.994$\pm$0.01 d and 8.448$\pm$0.002 d, respectively. We estimate the significance of these features from the chi squared distribution of all the trials, finding a probability less than 1.5$\times10^{-4}$ that the detections occurred due to chance. We complement our analysis with the study of their broadband X-ray emission. We identify the periodicities with the orbital periods of the sources. The periods are typical for the wind accretors X-ray binaries and we support this identification showing that also their energy spectra are compatible with an X-ray spectral emission characteristic of high-mass X-ray binaries. The spectrum of IGR J05007-704 that resides in the Large Magellanic Cloud, does not show any intrinsic local absorption, whereas the spectra of the Galactic sources IGR J17354-3255 and IGR J13186-6257 may be affected by a local absorber. The folded light curve for IGR J13186-6257 suggests a possible Be companion star.Comment: 10 pages, 14 figures. Accepted for publication in A&