V. Zabalza

Astropy: A community Python package for astronomy

We present the first public version (v0.2) of the open-source and community-developed Python package, Astropy. This package provides core astronomy-related functionality to the community, including support for domain-specific file formats such as flexible image transport system (FITS) files, Virtual Observatory (VO) tables, and common ASCII table formats, unit and physical quantity conversions, physical constants specific to astronomy, celestial coordinate and time transformations, world coordinate system (WCS) support, generalized containers for representing gridded as well as tabular data, and a framework for cosmological transformations and conversions. Significant functionality is under active development, such as a model fitting framework, VO client and server tools, and aperture and point spread function (PSF) photometry tools. The core development team is actively making additions and enhancements to the current code base, and we encourage anyone interested to participate in the development of future Astropy versions.

Unraveling the high-energy emission components of gamma-ray binaries

The high and very-high energy spectrum of gamma-ray binaries has become a challenge for all theoretical explanations since the detection of powerful, persistent GeV emission from LS 5039 and LS I +61 303 by Fermi/LAT. The spectral cutoff at a few GeV indicates that the GeV component and the fainter, hard TeV emission above 100 GeV are not directly related. We explore the possible origins of these two emission components in the framework of a young, non-accreting pulsar orbiting the massive star, and initiating the non-thermal emission through the interaction of the stellar and pulsar winds. The pulsar/stellar wind interaction in a compact orbit binary gives rise to two potential locations for particle acceleration: the shocks at the head-on collision of the winds and the termination shock caused by Coriolis forces on scales larger than the binary separation. We explore the suitability of these two locations to host the GeV and TeV emitters, respectively, through the study of their non-thermal emission along the orbit. We focus on the application of this model to LS 5039 given its well determined stellar wind with respect to other gamma-ray binaries. The application of the proposed model to LS 5039 indicates that these two potential emitter locations provide the necessary conditions for reproduction of the two-component high-energy gamma-ray spectrum of LS 5039. In addition, the ambient postshock conditions required at each of the locations are consistent with recent hydrodynamical simulations. The scenario based on the interaction of the stellar and pulsar winds is compatible with the GeV and TeV emission observed from gamma-ray binaries with unknown compact objects, such as LS 5039 and LS I +61 303.

On the origin of correlated X-ray/VHE emission from LS I +61 303

Context. The MAGIC collaboration recently reported correlated X-ray and very high-energy (VHE) gamma-ray emission from the gamma-ray binary LS I +61 303 during ∼60% of one orbit. These observations suggest that the emission in these two bands has its origin in a single particle population. Aims. We aim at improving our understanding of the source behaviour by explaining the simultaneous X-ray and VHE data through a radiation model. Methods. We use a model based on a one zone population of relativistic leptonic particles at the position of the compact object and assume dominant adiabatic losses. The adiabatic cooling time scale is inferred from the X-ray fluxes. Results. The model can reproduce the spectra and light curves in the X-ray and VHE bands. Adiabatic losses could be the key ingredient to explain the X-ray and partially the VHE light curves. From the best-fit result we obtain a magnetic field of B � 0. 2G , a minimum luminosity budget of ∼2 × 10 35 erg s −1 and a relatively high acceleration efficiency. In addition, our results seem to confirm that the GeV emission detected by Fermi does not come from the same parent particle population as the X-ray and VHE emission. Moreover, the Fermi spectrum poses a constraint on the hardness of the particle spectrum at lower energies. In the context of our scenario, more sensitive observations would allow us to constrain the inclination angle, which could determine the nature of the compact object.

Binaries with the eyes of CTA

The binary systems that have been detected in gamma rays have proven very useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. Binary systems, either detected or potential gamma-ray emitters, can be grouped in different subclasses depending on the nature of the binary components or the origin of the particle acceleration: the interaction of the winds of either a pulsar and a massive star or two massive stars; accretion onto a compact object and jet formation; and interaction of a relativistic outflow with the external medium. We evaluate the potentialities of an instrument like the Cherenkov telescope array (CTA) to study the non-thermal physics of gamma-ray binaries, which requires the observation of high-energy phenomena at different time and spatial scales. We analyze the capability of CTA, under different configurations, to probe the spectral, temporal and spatial behavior of gamma-ray binaries in the context of the known or expected physics of these sources. CTA will be able to probe with high spectral, temporal and spatial resolution the physical processes behind the gamma-ray emission in binaries, significantly increasing as well the number of known sources. This will allow the derivation of information on the particle acceleration and emission sites qualitatively better than what is currently available.

Radio continuum and near-infrared study of the MGRO J2019+37 region

Context. MGRO J2019+37 is an unidentified extended source of very high energy gamm a-rays originally reported by the Milagro Collaboration as the brightest TeV source in the Cygnus region. Its extended emission could be powered by either a single or several sources. The GeV pulsar AGL J2020.5+3653, discovered by AGILE and associated with PSR J2021+3651, could contribute to the emission from MGRO J2019+37. Aims. Our aim is to identify radio and near-infrared sources in the field of the extended TeV source MGRO J2019+37, and study potential counterparts to explain its emission. Methods. We surveyed a region of about 6 square degrees with the Giant Metrewave Radio Telescope (GMRT) at the frequency 610 MHz. We also observed the central square degree of this survey in the near-infrared Ks-band using the 3.5 m telescope in Calar Alto. Archival X-ray observations of some specific fiel ds are included. VLBI observations of an interesting radio source were performed. We explored possible scenarios to produce the multi-TeV emission from MGRO J2019+37 and studied which of the sources could be the main particle accelerator. Results. We present a catalogue of 362 radio sources detected with the GMRT in the field of MGRO J2019+37, and the results of a cross-correlation of this catalog with one obtained at n ear-infrared wavelengths, which contains∼ 3× 10 5 sources, as well as with available X-ray observations of the region. Some peculiar sources inside the∼1 ◦ uncertainty region of the TeV emission from MGRO J2019+37 are discussed in detail, including the pulsar PSR J2021+3651 and its pulsar wind nebula PWN G75.2+0.1, two new radio-jet sources, the Hii region Sh 2-104 containing two star clusters, and the radio source NVSS J202032+363158. We also find that the hadronic scenario is the most likely in case of a s ingle accelerator, and discuss the possible contribution f rom the sources mentioned above. Conclusions. Although the radio and GeV pulsar PSR J2021+3651 / AGL J2020.5+3653 and its associated pulsar wind nebula PWN G75.2+0.1 can contribute to the emission from MGRO J2019+37, extrapolation of the GeV spectrum does not explain the detected multi-TeV flux. Other sources discussed here could contribute to the emission of the Milagro source.

Search for radio pulsations in LS I +61 303

Context. LS I +61 303 is a member of the select group of gamma-ray binaries: galactic binary systems that contain a massive star and a compact object, show a changing milliarcsecond morphology and a similar broad spectral energy distribution (SED) that peaks at MeV-TeV energies and is modulated by the orbital motion. The nature of the compact object is unclear in LS I +61 303, LS 5039 and HESS J0632+057, whereas PSR B1259‐63 harbours a 47.74 ms radio pulsar. Aims. A scenario in which a young pulsar wind interacts with the stellar wind has been proposed to explain the very high energy (VHE, E> 100 GeV) gamma-ray emission detected from LS I+61 303, although no pulses have been reported from this system at any wavelength. We aim to find evidence of the pulsar nature of the compact object. Methods. We performed phased array observations with the Giant Metrewave Radio Telescope (GMRT) at 1280 MHz centred at phase 0.54. Simultaneous data from the multi-bit phased array (PA) back-end with a sampling time of tsamp= 128µs and from the polarimeter (PMT) back-end with tsamp = 256µs where taken. Results. No pulses have been found in the data set, with a minimum detectable mean flux density of ∼ 0.38 mJy at 8-� level for the pulsed emission from a putative pulsar with period P>2 ms and duty cycle D=10% in the direction of LS I +61 303. Conclusions. The detection of posible radio pulsations will require deep and sensitive observations at frequencies∼ 0.5‐5 GHz and orbital phases 0.6−0.7. However, it may be unfeasible to detect pulses if the putative pulsar is not beamed at the Earth or if there is a strong absorption within the binary system.

Candidate counterparts to the soft gamma-ray flare in the direction of LS I +61 303

Context. A short duration burst reminiscent of a soft gamma-ray repeater/anomalous X-ray pulsar behaviour was detected in the direction of LS I +61 303 by the Swift satellite. While the association with this well known gamma-ray binary is likely, a different origin cannot be excluded. Aims. We explore the error box of this unexpected flaring event and establish the radio, near-infrared and X-ray sources in our search for any peculiar alternative counterpart. Methods. We carried out a combined analysis of archive Very Large Array radio data of LS I +61 303 sensitive to both compact and extended emission. We also reanalysed previous near infrared observations with the 3.5 m telescope of the Centro Astronomico Hispano Aleman and X-ray observations with the Chandra satellite. Results. Our deep radio maps of the LS I +61 303 environment represent a significant advancement on previous work and 16 compact radio sources in the LS I +61 303 vicinity are detected. For some detections, we also identify near infrared and X-ray counterparts. Extended emission features in the field are also detected and confirmed. The possible connection of some of these sources with the observed flaring event is considered. Based on these data, we are unable to claim a clear association between the Swift-BAT flare and any of the sources reported here. However, this study represents the most sophisticated attempt to determine possible alternative counterparts other than LS I +61 303.

CONFIRMING THE ORBITAL X-RAY VARIABILITY OF LS 5039 THROUGH CHANDRA OBSERVATIONS

We present an analysis of two Chandra observations of LS 5039 performed in 2004 in two different orbital phases during the same orbital cycle. Our results show a clear flux variability, confirming a trend of increasing flux with orbital phase in the range 0.05 ≲ ϕ ≲ 0.7 as has been found in XMM observations carried out in 2005 during the same orbital cycle. We suggest that the X-ray variations are linked to orbital changes of the intrinsic properties of the emitter, which should have implications for possible emission models to explain the present multiwavelength knowledge of the source.

H.E.S.S. observations of PSR B1259-63 during its 2014 periastron passage

C. Romoli∗ a, P. Bordasb, C. Mariaudc, T. Murachd , F. Aharoniana b, M. de Nauroisc, G. Puhlhofere, U. Schwanked , B. van Soelen f , I. Sushchg and V. Zabalzah for the H.E.S.S. Collaboration a Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland b Max-Planck-Institut fur Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany c Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France d Institut fur Physik, Humboldt-Universitat zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany e Institut fur Astronomie und Astrophysik, Universitat Tubingen, Sand 1, D 72076 Tubingen, Germany f Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa g Centre for Space Research, North-West University, Potchefstroom 2520, South Africa h Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom E-mail: romolic@cp.dias.ie

Deep GMRT radio observations and a multi-wavelength study of the region around HESS J1858+020

Context. There are a number of very high energy sources in the Galaxy that remain unidentified. Multi-wavelength and variability studies, and catalogue searches, are powerful tools to identify the physical counterpart, given the uncertainty in the source location and extension. Aims. This work carries out a thorough multi-wavelength study of the unidentified, very high energy source HESS J1858+020 and its environs. Methods. We have performed Giant Metrewave Radio Telescope observations at 610 MHz and 1.4 GHz to obtain a deep, low-frequency radio image of the region surrounding HESS J1858+020. We analysed archival radio, infrared, and X-ray data as well. This observational information, combined with molecular data, catalogue sources, and a nearby Fermi gamma-ray detection of unidentified origin, are combined to explore possible counterparts to the very high energy source. Results. We provide with a deep radio image of a supernova remnant that might be related to the GeV and TeV emission in the region. We confirm the presence of an H ii region next to the supernova remnant and coincident with molecular emission. A potential region of star formation is also identified. We identify several radio and X-ray sources in the surroundings. Some of these sources are known planetary nebulae, whereas others may be non-thermal extended emitters and embedded young stellar objects. Three old, background Galactic pulsars also neighbour HESS J1858+020 along the line of sight. Conclusions. The region surrounding HESS J1858+020 is rich in molecular structures and non-thermal objects that may potentially be linked to this unidentified very high energy source. In particular, a supernova remnant interacting with nearby molecular clouds may be a good candidate, but a star forming region, or a non-thermal radio source of yet unclear nature, may also be behind the gamma-ray source. The neighbouring pulsars, despite being old and distant, cannot be discarded as candidates. Further observational studies are needed, however, to narrow the search for a counterpart to the HESS source.