Axion-like particle imprint in cosmological very-high-energy sources

Discoveries of very high energy (VHE) photons from distant blazars suggest that, after correction by extragalactic background light (EBL) absorption, there is a flatness or even a turn-up in their spectra at the highest energies that cannot be easily explained by the standard framework. Here, it is shown that a possible solution to this problem is achieved by assuming the existence of axion-like particles (ALPs) with masses ~1 neV. The ALP scenario is tested making use of observations of the highest redshift blazars known in the VHE energy regime, namely 3C 279, 3C 66A, PKS 1222+216 and PG 1553+113. In all cases, better fits to the observed spectra are found when including ALPs rather than considering EBL only. Interestingly, quite similar critical energies for photon/ALP conversions are also derived, independently of the source considered.Comment: 12 pages, 2 figures, 2 tables; accepted by JCAP. Replaced to match the accepted versio

Baryons: What, When and Where?

We review the current state of empirical knowledge of the total budget of baryonic matter in the Universe as observed since the epoch of reionization. Our summary examines on three milestone redshifts since the reionization of H in the IGM, z = 3, 1, and 0, with emphasis on the endpoints. We review the observational techniques used to discover and characterize the phases of baryons. In the spirit of the meeting, the level is aimed at a diverse and non-expert audience and additional attention is given to describe how space missions expected to launch within the next decade will impact this scientific field.Comment: Proceedings Review for "Astrophysics in the Next Decade: JWST and Concurrent Facilities", ed. X. Tielens, 38 pages, 10 color figures. Revised to address comments from the communit

FUV and X-ray absorption in the Warm-Hot Intergalactic Medium

The Warm-Hot Intergalactic Medium (WHIM) arises from shock-heated gas collapsing in large-scale filaments and probably harbours a substantial fraction of the baryons in the local Universe. Absorption-line measurements in the ultraviolet (UV) and in the X-ray band currently represent the best method to study the WHIM at low redshifts. We here describe the physical properties of the WHIM and the concepts behind WHIM absorption line measurements of H I and high ions such as O VI, O VII, and O VIII in the far-ultraviolet and X-ray band. We review results of recent WHIM absorption line studies carried out with UV and X-ray satellites such as FUSE, HST, Chandra, and XMM-Newton and discuss their implications for our knowledge of the WHIM.Comment: 26 pages, 9 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 3; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

An ATCA radio-continuum study of the Small Magellanic Cloud - IV. A multifrequency analysis of the N 66 region

Traditional identification of supernova remnants (SNRs) include the use of radio spectral index, optical spectral studies (including strong [S ii], [N ii], [O i], [O ii] and [O iii] lines) and X-ray co-identifications. Each of these can have significant limitations within the context of a particular SNR candidate and new identification methods are continually sought. In this paper, we explore subtraction techniques by Ye, Turtle and Kennicutt to remove thermal emission estimated from Hα flux from radio-continuum images. The remaining non-thermal emission allows the identification of SNRs embedded within these H ii regions. Subtraction images of the N 66 region in the Small Magellanic Cloud (SMC) using Hα wide-field optical CCD images from the Curtis Schmidt Telescope and the recent Australia Telescope Compact Array (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640 MHz) data are presented as an example. These show three SNRs (B0057 − 724, B0056 − 724 and B0056 − 725) separated from their surrounding H ii radio emission. 2.3-m dual-beam spectrograph long-slit spectra from selected regions within N 66 suggest the presence of an additional SNR with no radio or X-ray emission. Radio spectral index, [S ii]/Hα ratio and archived Chandra images of N 66 combine to give a more coherent picture of this region, confirming B0057 − 724 as an SNR. The N 66 nebula complex is divided into 10 components, composed separately of these SNRs and H ii regions

An ATCA radio-continuum study of the Small Magellanic Cloud - III. Supernova remnants and their environments

A total of 717 sources from the Australia Telescope Compact Array (ATCA) catalogue of the Small Magellanic Cloud (SMC) have been classified in Paper II (of this series) by Payne et al. Here, we present a statistical analysis of all 16 confirmed supernova remnants (SNRs) and five new candidate remnants. Included is a detailed discussion of the latter and three other sources that have some SNR characteristics. We have also found a new microquasar candidate (ATCA J005523−721055) in addition to ATCA J004718−723947 reported in Paper II. Source diameter comparisons suggest that SNRs as a group are of similar size in radio, optical and X-ray with surface brightness values in the range of Galactic remnants. Remnant spectral indices, α[defined as Sν∝να , with Sν (flux density) and ν (frequency)], have a mean of −0.63 [standard deviation (s.d.) = 0.43] and ROSAT X-ray hardness ratios confirm them to be soft X-ray sources compared to background objects. We could not find any meaningful correlation between SNR surface brightness and diameter; we also discuss the number–diameter relation. A Venn diagram summarizes that most SNRs emit radiation in all three of the radio, optical and X-ray domains. H ii region diameter comparisons between radio and optical sources show them to be a very diverse group that defies any simple relationship, preventing any meaningful calculation of flux density or spectral index. To better understand environments containing SNRs, we have scaled Hα images of four SMC regions and subtracted their flux from the ATCA 2.37-GHz radio image. These 2.37 GHz–Hα subtraction (or difference) images reveal some new sources with predominantly non-thermal emission, exposing SNRs confused with H ii regions