Juan R. Sanchez-Sutil

XMM-Newton detection of the supernova remnant G304.6 + 0.1 (Kes 17)

Aims. We report the first detailed X-ray study of the supernova remn ant (SNR) G304.6+0.1, achieved with the XMM-Newton mission. Methods. The powerful imaging capability of XMM-Newton was used to study the X-ray characteristics of the remnant at different energy ranges. The X-ray morphology and spectral properties were analyzed. In addittion, radio and mid-infrared data obtained with the Molonglo Observatory Synthesis Telescope and the Spitzer Space Telescope were used to study the association with the detected X-ray emission and to understand the structure of the SNR at differents wavelengths. Results. The SNR shows an extended and arc-like internal structure in the X-ray band with out a compact point-like source inside the remnant. We find a high column density of NH in the range 2.5-3.5� 10 22 cm −2 , which supports a relatively distant location ( d� 9.7 kpc). The X-ray spectrum exhibits at least three emission lines, indicating that the X-ray emission has a thin thermal plasma origin, although a non-thermal contribution cannot be discarded. The spectra of three different regions (north, center and south) are well represented by a combination of a non-equilibrium ionization (PSHOCK) and a power-law (PL) model. The mid-infrared observations show a bright filamentary structure along the n orth-south direction coincident with the NW radio shell. This suggests that Kes 17 is propagating in a non-uniform environment with high density and that the shock front is interacting with several adjacent massive molecular clouds. The good correspondence of radio and mid-infrared emissions suggests that the filamentary fe atures are caused by shock compression. The X-ray characteristics and well-known radio parameters indicate that G304.6+0.1 is a middle-aged SNR (2.8-6.4)� 10 4 yr old and a new member of the recently proposed group of mixed-morphology SNRs.

ARE T TAURI STARS GAMMA-RAY EMITTERS?

T Tauri stars are young, low-mass, pre-main-sequence stars surrounded by an accretion disk. These objects present strong magnetic activity and powerful magnetic reconnection events. Strong shocks are likely associated with fast reconnection in the stellar magnetosphere. Such shocks can accelerate particles up to relativistic energies. We aim at developing a simple model to calculate the radiation produced by non-thermal relativistic particles in the environment of T Tauri stars. We want to establish whether this emission is detectable at high energies with the available or forthcoming γ-ray telescopes. We assume that particles (protons and electrons) pre-accelerated in reconnection events are accelerated at shocks through the Fermi mechanism and we study the high-energy emission produced by the dominant radiative processes. We calculate the spectral energy distribution of T Tauri stars up to high energies and we compare the integrated flux obtained with that from a specific Fermi source, 1FGL J1625.8–2429c, that we tentatively associate with this kind of young stellar object. We suggest that under reasonable general conditions nearby T Tauri stars might be detected at high energies and be responsible for some unidentified Fermi sources on the Galactic plane.

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.

A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources

Aims. We search for ultra luminous X-ray source (ULXs) radio counterparts located in nearby galaxies in order to constrain their physical nature. Methods. Our work is based on a systematic cross-identification of the most recent and extensive available ULX catalogues and archival radio data. Results. A catalogue of 70 positional coincidences is reported. Most of them are located within the galaxy nucleus. Among them, we find 11 new cases of non-nuclear ULX sources with possibly associated radio emission.Aims. We search for ultra luminous X-ray source (ULXs) radio counterparts located in nearby galaxies in order to constrain their physical nature. Methods. Our work is based on a systematic cross-identification of the most recent and extensive available ULX catalogues and archival radio data. Results. A catalogue of 70 positional coincidences is reported. Most of them are located within the galaxy nucleus. Among them, we find 11 new cases of non-nuclear ULX sources with possibly associated radio emission.

An X-ray study of the SNR G344.7-0.1 and the central object CXOU J170357.8-414302

Aims. We report results of an X-ray study of the supernova remnant (SNR) G344.7-0.1 and the point-like X-ray source located at the geometrical center of the SNR radio structure. Methods. The morphology and spectral properties of the remnant and the central X-ray point-like source were studied using data from the XMM-Newton and Chandra satellites. Archival radio data and infrared Spitzer observations at 8 and 24 μ mw ere used to compare and study its multi-band properties at different wavelengths. Results. The XMM-Newton and Chandra observations reveal that the overall X-ray emission of G344.7-0.1 is extended and correlates very well with regions of bright radio and infrared emission. The X-ray spectrum is dominated by prominent atomic emission lines. These characteristics suggest that the X-ray emission originated in a thin thermal plasma, whose radiation is represented well by a plane-parallel shock plasma model (PSHOCK). Our study favors the scenario in which G344.7-0.1 is a 6 × 10 3 year old SNR expanding in a medium with a high density gradient and is most likely encountering a molecular cloud on the western side. In addition, we report the discovery of a soft point-like X-ray source located at the geometrical center of the radio SNR structure. The object presents some characteristics of the so-called compact central objects (CCO). However, its neutral hydrogen absorption column (NH) is inconsistent with that of the SNR. Coincident with the position of the source, we found infrared and optical objects with typical early-K star characteristics. The X-ray source may be a foreground star or the CCO associated with the SNR. If this latter possibility were confirmed, the point-like source would be the farthest CCO detected so far and the eighth member of the new population of isolated and weakly magnetized neutron stars.

Identification of the optical and near-infrared counterpart of GRS 1758-258

Context. Understood to be a microquasar in the Galactic center region, GRS 1758-258 has not yet been unambiguously identified to have an optical/near-infrared counterpart, mainly because of the high absorption and the historic lack of suitable astrometric stars, which led to the use of secondary astrometric solutions. Although it is considered with 1E 1740.7-2942 as the prototypical microquasar in the Galactic center region, the Galactic origin of both sources has not yet been confirmed. Aims. We attempt to improve previous astrometry to identify a candidate counterpart to GRS 1758-258. We present observations with the Gran Telescopio de Canarias (GTC), in which we try to detect any powerful emission lines that would infer an extragalactic origin of this source. Methods. We use modern star catalogues to reanalyze archival images of the GRS 1758-258 field in the optical and near-infrared wavelengths, and compute a new astrometric solution. We also reanalyzed archival radio data of GRS 1758-258 to determine a new and more accurate radio position. Results. Our improved astrometric solution for the GRS 1758-258 field represents a significant advancement on previous works and allows us to identify a single optical/near-infrared source, which we propose as the counterpart of GRS 1758-258. The GTC spectrum of this source is however of low signal-to-noise ratio and does not rule out a Galactic origin. Hence, new spectral observations are required to confirm or discard a Galactic nature.

The star forming region Monoceros R2 as a gamma-ray source

Context. After the release of the gamma-ray source catalog produced by the Fermi satellite during its first two years of operation, a significant fraction of sources still remain unassociated a t lower energies. In addition to well-known high-energy emitters (pulsars, blazars, supernova remnants, etc.), theoretical expectat ions predict new classes of gamma-ray sources. In particular, gamma-ray emission could be associated with some of the early phases of stellar evolution, but this interesting possibility is sti ll poorly understood. Aims. The aim of this paper is to assess the possibility of the Fermi gamma-ray source 2FGL J0607.5−0618c being associated with the massive star forming region Monoceros R2. Methods. A multi-wavelength analysis of the Monoceros R2 region is carried out using archival data at radio, infrared, X-ray, and gamma-ray wavelengths. The resulting observational properties are used to estimate the physical parameters needed to test the different physical scenarios. Results. We confirm the 2FGL J0607.5−0618c detection with improved confidence over the Fermi two-year catalog. We find that a combined effect of the multiple young stellar objects in Monoceros R2 is a viable picture for the nature of the source.

High resolution radio emission from RCW 49/Westerlund 2

Aims. The HII region RCW 49 and its ionizing cluster form an extensive, complex region that has been widely studied at infrared (IR) and optical wavelengths. The Molonglo 843 MHz and Australia Telescope Compact Array data at 1.4 and 2.4 GHz showed two shells. Recent high-resolution IR imaging revealed a complex dust structure and ongoing star formation. New high-bandwidth and high-resolution data of the RCW 49 field have been obtained to survey the radio emission at arcsec scale and investigate the small-scale features and nature of the HII region. Methods. Radio observations were collected with the new 2-GHz bandwidth receivers and the CABB correlator of the Australia Telescope Compact Array [ATCA], at 5.5 and 9.0 GHz. In addition, archival observations at 1.4 and 2.4 GHz have been re-reduced and re-analyzed in conjunction with observations in the optical, IR, X-ray, and gamma-ray regimes. Results. The new 2-GHz bandwidth data result in the most detailed radio continuum images of RCW 49 to date. The radio emission closely mimics the near-IR emission observed by Spitzer, showing pillars and filaments. The brightest continuum emission comes from the region known as the bridge. The overall flattish spectral index is typically consistent with a free-free emission mechanism. However, hints of nonthermal components are also present in the bridge. An interesting jet-like structure surrounded by a bubble feature whose nature is still unclear has been discovered close to the Westerlund 2 core. Two apparent bow shocks and a number of discrete sources have been detected as well in the surroundings of RCW 49. In addition, we also report on and discuss the possible detection of a hydrogen recombination line. Conclusions. The radio results support an association between the cm continuum and molecular emission. The detection of the radio recombination line kinematically favors a RCW 49 distance of 6–7 kpc. If the negative spectral indices measured at the bridge should be confirmed to be caused by synchrotron emission, we propose a scenario where high-energy emission could be produced. Finally, the newly discovered jet-like structure appears to be an intriguing source that deserves a detailed study by itself.

A systematic cross-search for radio/infrared counterparts of XMM-Newton sources

We present a catalog of cross-correlated radio, infrared and X-ray sources using a very restrictive selection criteria with an IDL-based code developed by us. The significance of the observed coincidences was evaluated through Monte Carlo simulations of synthetic sources following a well-tested protocol. We found 3320 coincident radio/X-ray sources with a high statistical significance characterized by the sum of error-weighted coordinate differences. For 997 of them, 2MASS counterparts were found. The percentage of chance coincidences is less than 1%. X-ray hardness ratios of well-known populations of objects were used to provide a crude representation of their X-ray spectrum and to make a preliminary diagnosis of the possible nature of unidentified X-ray sources. The results support the fact that the X-ray sky is largely dominated by Active Galactic Nuclei at high galactic latitudes (|b|≥10°). At low galactic latitudes (|b|≤10°) most of unidentified X-ray sources (∼94%) lie at |b|≤2°. This result suggests that most of the unidentified sources found toward the Milky Way plane are galactic objects. Well-known and unidentified sources were classified in different tables with their corresponding radio/infrared and X-ray properties. These tables are intended as a useful tool for researchers interested in particular identifications.

Infrared and radio study of the W43 cluster - Resolved binaries and non-thermal emission

Context. The recent detection of very high-energy (VHE) gamma-ray emission from the direction of the W43 star-forming region prompted us to investigate its stellar population in detail in an attempt to see wether or not it is possible an association. Aims. We search for the possible counterpart(s) of the gamma-ray source or any hints of them, such as non-thermal synchrotron emission as a tracer of relativistic particles often involved in plausible physical scenarios for VHE emission. Methods. We data-mined several archival databases with different degrees of success. The most significant results came from radio and near-infrared archival data. Results. The previously known Wolf-Rayet star in the W43 central cluster and another cluster member appear to be resolved into two components,suggesting a likely binary nature. In addition, extended radio emission with a clearly negative spectral index is detected in coincidence with the W43 cluster. These findings could have important implications for possible gamma-ray emitting scenarios, which we also brief ly discuss.