FAST RADIO BURSTS AS PROBES of MAGNETIC FIELDS in the INTERGALACTIC MEDIUM

We examine the proposal that the dispersion measures (DMs) and Faraday rotation measures (RMs) of extragalactic linearly polarized fast radio bursts (FRBs) can be used to probe the intergalactic magnetic field (IGMF) in filaments of galaxies. The DM through the cosmic web is dominated by contributions from the warmhot intergalactic medium (WHIM) in filaments and from the gas in voids. On the other hand, RM is induced mostly by the hot medium in galaxy clusters, and only a fraction of it is produced in the WHIM. We show that if one excludes FRBs whose sightlines pass through galaxy clusters, the line of sight (LOS) strength of the IGMF in filaments, B , is approximately C (???1 + z???/fDM)(RM/DM), where C is a known constant. Here, the redshift of the FRB is not required to be known; fDM is the fraction of total DM due to the WHIM, while ???1 + z??? is the redshift of interevening gas weighted by the WHIM gas density, both of which can be evaluated for a given cosmology model solely from the DM of an FRB. Using data on structure formation simulations and a model IGMF, we show that C (???1 + z???/fDM)(RM/DM) closely reproduces the density-weighted LOS strength of the IGMF in filaments of the large-scale structure.clos

X-ray Sources and their Optical Counterparts in the Globular Cluster M4

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.Comment: 11 pages, 6 figures, accepted for publication in ApJ. Figure 1 and 5 are of reduced qualit

Chandra Monitoring of the Candidate Anomalous X-ray Pulsar AX J1845.0-0258

The population of clearly identified anomalous X-ray pulsars has recently grown to seven, however, one candidate anomalous X-ray pulsar (AXP) still eludes re-confirmation. Here, we present a set of seven Chandra ACIS-S observations of the transient pulsar AX J1845.0-0258, obtained during 2003. Our observations reveal a faint X-ray point source within the ASCA error circle of AX J1845.0-0258's discovery, which we designate CXOU J184454.6-025653 and tentatively identify as the quiescent AXP. Its spectrum is well described by an absorbed single-component blackbody (kT~2.0 keV) or power law (Gamma~1.0) that is steady in flux on timescales of at least months, but fainter than AX J1845.0-0258 was during its 1993 period of X-ray enhancement by at least a factor of 13. Compared to the outburst spectrum of AX J1845.0-0258, CXOU J184454.6-025653 is considerably harder: if truly the counterpart, then its spectral behaviour is contrary to that seen in the established transient AXP XTE J1810-197, which softened from kT~0.67 keV to ~0.18 keV in quiescence. This unexpected result prompts us to examine the possibility that we have observed an unrelated source, and we discuss the implications for AXPs, and magnetars in general.Comment: 4 pages, 3 figures. To be published in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface" (April 24-28, 2006, London, UK), eds. D. Page, R. Turolla, & S. Zan

The Origin and Evolution of Cosmic Magnetism

Magnetism is one of the four fundamental forces. However, the origin of magnetic fields in stars, galaxies and clusters is an open problem in astrophysics and fundamental physics. When and how were the first fields generated? Are present-day magnetic fields a result of dynamo action, or do they represent persistent primordial magnetism? What role do magnetic fields play in turbulence, cosmic ray acceleration and galaxy formation? Here we demonstrate how the Square Kilometer Array (SKA) can deliver new data which will directly address these currently unanswered issues. Much of what we present is based on an all-sky survey of rotation measures, in which Faraday rotation towards >10^7 background sources will provide a dense grid for probing magnetism in the Milky Way, in nearby galaxies, and in distant galaxies, clusters and protogalaxies. Using these data, we can map out the evolution of magnetised structures from redshifts z>3 to the present, can distinguish between different origins for seed magnetic fields in galaxies, and can develop a detailed model of the magnetic field geometry of the intergalactic medium and of the overall Universe. With the unprecedented capabilities of the SKA, the window to the Magnetic Universe can finally be opened.Comment: 11 pages, 7 embedded EPS figures, uses espcrc2.sty. Minor typos fixed. To appear in "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam

Observations of magnetic fields in the Milky Way and in nearby galaxies with a Square Kilometre Array

The role of magnetic fields in the dynamical evolution of galaxies and of the interstellar medium (ISM) is not well understood, mainly because such fields are difficult to directly observe. Radio astronomy provides the best tools to measure magnetic fields: synchrotron radiation traces fields illuminated by cosmic-ray electrons, while Faraday rotation and Zeeman splitting allow us to detect fields in all kinds of astronomical plasmas, from lowest to highest densities. Here we describe how fundamental new advances in studying magnetic fields, both in our own Milky Way and in other nearby galaxies, can be made through observations with the proposed Square Kilometre Array. Underpinning much of what we propose is an all-sky survey of Faraday rotation, in which we will accumulate tens of millions of rotation measure measurements toward background radio sources. This will provide a unique database for studying magnetic fields in individual Galactic supernova remnants and HII regions, for characterizing the overall magnetic geometry of our Galaxy's disk and halo, and for understanding the structure and evolution of magnetic fields in galaxies. Also of considerable interest will be the mapping of diffuse polarized emission from the Milky Way in many narrow bands over a wide frequency range. This will allow us to carry out Faraday tomography of the Galaxy, yielding a high-resolution three-dimensional picture of the magnetic field within a few kpc of the Sun, and allowing us to understand its coupling to the other components of the ISM. Finally, direct synchrotron imaging of a large number of nearby galaxies, combined with Faraday rotation data, will allow us to determine the magnetic field structure in these sources, and to test both the dynamo and primordial field theories for field origin and amplification.Comment: 17 pages, 8 figures. To appear in "Science with the Square Kilometer Array", eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam); Typos corrected 26/10/200

Associations of Very High Energy Gamma-Ray Sources Discovered by H.E.S.S. with Pulsar Wind Nebulae

The H.E.S.S. array of imaging Cherenkov telescopes has discovered a number of previously unknown gamma-ray sources in the very high energy (VHE) domain above 100 GeV. The good angular resolution of H.E.S.S. (~0.1 degree per event), as well as its sensitivity (a few percent of the Crab Nebula flux) and wide 5-degree field of view, allow a much better constrained search for counterparts in comparison to previous instruments. In several cases, the association of the VHE sources revealed by H.E.S.S. with pulsar wind nebulae (PWNe) is supported by a combination of positional and morphological evidence, multi-wavelength observations, and plausible PWN model parameters. These include the plerions in the composite supernova remnants G 0.9+0.1 and MSH 15-52, the recently discovered Vela X nebula, two new sources in the Kookaburra complex, and the association of HESS J1825-137 with PSR B1823-13. The properties of these better-established associations are reviewed. A number of other sources discovered by H.E.S.S. are located near high spin-down power pulsars, but the evidence for association is less complete. These possible associations are also discussed, in the context of the available multi-wavelength data and plausible PWN scenarios.Comment: 5 pages, to appear in Astrophysics and Space Science (proceedings of "The Multi-Messenger Approach to High-Energy Gamma-Ray Sources"

A multiwavelength study of the supernova remnant G296.8-0.3

We report XMM-Newton observations of the Galactic supernova remnant G296.8-0.3, together with complementary radio and infrared data. The spatial and spectral properties of the X-ray emission, detected towards G296.8-0.3, was investigated in order to explore the possible evolutionary scenarios and the physical connexion with its unusual morphology detected at radio frequencies. G296.8-0.3 displays diffuse X-ray emission correlated with the peculiar radio morphology detected in the interior of the remnant and with the shell-like radio structure observed to the northwest side of the object. The X-ray emission peaks in the soft/medium energy range (0.5-3.0 keV). The X-ray spectral analysis confirms that the column density is high (NH \sim 0.64 x 10^{22} cm^{-2}) which supports a distant location (d>9 kpc) for the SNR. Its X-ray spectrum can be well represented by a thermal (PSHOCK) model, with kT \sim 0.86 keV, an ionization timescale of 6.1 x 10^{10} cm^{-3} s, and low abundance (0.12 Z_sun). The 24 microns observations show shell-like emission correlated with part of the northwest and southeast boundaries of the SNR. In addition a point-like X-ray source is also detected close to the geometrical center of the radio SNR. The object presents some characteristics of the so-called compact central objects (CCO). Its X-ray spectrum is consistent with those found at other CCOs and the value of NH is consistent with that of G296.8-0.3, which suggests a physical connexion with the SNR.Comment: Accepted for publication in Astrophysics & Space Scienc

Chasing the identification of ASCA Galactic Objects (ChIcAGO): An X-ray survey of unidentified sources in the galactic plane. I : Source sample and initial results

We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the Fx 10-13 to 10-11 erg cm -2 s-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multiwavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3′ of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the Fx 10-13 to 10-11 erg cm -2 s-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding wind binaries, X-ray binaries, and magnetars. There is also another primary population that is still unidentified but, on the basis of its X-ray and infrared properties, likely comprises partly Galactic sources and partly active galactic nuclei.Peer reviewedSubmitted Versio

Status of Identification of VHE gamma-ray sources

With the recent advances made by Cherenkov telescopes such as H.E.S.S., the field of very high-energy (VHE) gamma-ray astronomy has recently entered a new era in which for the first time populations of Galactic sources such as e.g. Pulsar wind nebulae (PWNe) or Supernova remnants (SNRs) can be studied. However, while some of the new sources can be associated by positional coincidence as well as by consistent multi-wavelength data to a known counterpart at other wavelengths, most of the sources remain not finally identified. In the following, the population of Galactic H.E.S.S. sources will be used to demonstrate the status of the identifications, to classify them into categories according to this status and to point out outstanding problems.Comment: To appear in Astrophysics and Space Science (Proceedings of "The multimessenger approach to unidentified gamma-ray sources

Magnetar outbursts: an observational review

Transient outbursts from magnetars have shown to be a key property of their emission, and one of the main way to discover new sources of this class. From the discovery of the first transient event around 2003, we now count about a dozen of outbursts, which increased the number of these strongly magnetic neutron stars by a third in six years. Magnetar outbursts might involve their multi-band emission resulting in an increased activity from radio to hard X-ray, usually with a soft X-ray flux increasing by a factor of 10-1000 with respect to the quiescent level. A connected X-ray spectral evolution is also often observed, with a spectral softening during the outburst decay. The flux decay times vary a lot from source to source, ranging from a few weeks to several years, as also the decay law which can be exponential-like, a power-law or even multiple power-laws can be required to model the flux decrease. We review here on the latest observational results on the multi-band emission of magnetars, and summarize one by one all the transient events which could be studied to date from these sources.Comment: 34 pages, 6 figures. Chapter of the Springer Book ASSP 7395 "High-energy emission from pulsars and their systems", proceeding of the Sant Cugat Forum on Astrophysics (12-16 April 2010). Review updated to January 201