Ultra-low frequency LOFAR spectral indices of cluster radio halos
T. Pasini, F. de Gasperin, M. Brüggen, R. Cassano, A. Botteon, G. Brunetti, H. W. Edler, R. J. van Weeren, V. Cuciti, T. Shimwell. G. Di Gennaro, M. Gaspari, M.… 展开作者
T. Pasini, F. de Gasperin, M. Brüggen, R. Cassano, A. Botteon, G. Brunetti, H. W. Edler, R. J. van Weeren, V. Cuciti, T. Shimwell. G. Di Gennaro, M. Gaspari, M. Hardcastle, H. J. A. Rottgering, C. Tasse
A fraction of galaxy clusters harbor diffuse radio sources known as radio halos. The currently adopted scenario for their formation is based on second-order Fermi re-acceleration of seed electrons that is driven by merger-driven turbulence in the intra-cluster medium. This mechanism is expected to be inefficient, which implies that a significant fraction of halos should have very steep ($α< -1.5$) energy spectra. We start investigating the potential and current limitations of the combination of the two surveys conducted by LOFAR, LoTSS (144 MHz) and LoLSS (54 MHz), to probe the origin of radio halos. We follow up the 20 radio halos detected in the DR1 of LoTSS, which covers the HETDEX field, with the LoLSS survey, and we study their spectral properties between 54 and 144 MHz. After the removal of compact sources, 9 halos were excluded due to unreliable halo flux density measurements at 54 MHz. Our main finding is that 7 out of 11 ($\sim$ 64%) exhibit an ultra-steep spectrum ($α< -1.5$), which is a key prediction of turbulent re-acceleration models. We also note a tentative trend for more massive systems to host flatter halos, although the currently poor statistics does not allow for a deeper analysis. Our sample suffers from low angular resolution at 54 MHz, which limits the accuracy of the compact-sources subtraction. Nevertheless, this study is the first step towards providing compelling evidence for the existence of a large fraction of radio halos with very steep spectrum, which is a fundamental prediction of turbulent re-acceleration models. In this regard, the forthcoming second data release of LoLSS, along with the integration of LOFAR international stations and the instrumental upgrade to LOFAR2.0, will improve both the statistics and the low-frequency angular resolution, allowing to conclusively determine the origin of radio halos in galaxy clusters.
The topology of the magnetic field in Abell 2255 out to its virial radius. Results from the LOFAR Galaxy Cluster Ultra-Deep Field
INAF - IRA（意大利国家天体物理研究所-博洛涅分院）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）；Institute for Advanced Study（普林斯顿高等研究院）；Dipartimento di Fisica e Astronomia, Università di Bologna（博洛尼亚大学物理与天文学系）；Center for Astrophysics Harvard & Smithsonian（哈佛&史密森尼天体物理中心）；Department of Astronomy, University of Wisconsin-Madison（威斯康星大学麦迪逊分校天文学系）；ASTRON, the Netherlands Institute for Radio Astronomy（荷兰射电天文研究所）；Hamburger Sternwarte, Universität Hamburg（汉堡大学汉堡天文台）；INAF - IASF Milano（意大利国家天体物理研究所-米兰天文物理研究所）
Investigating the radio emission in the Perseus cluster with LOFAR sub-80 MHz LBA
INAF – Istituto di Radioastromia（意大利国家天体物理研究所射电天文学研究所）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）；Université Laval（拉瓦尔大学）；Dunlap Institute for Astronomy and Astrophysics, University of Toronto（多伦多大学邓普天文学与天体物理学研究所）；Università di Bologna（博洛尼亚大学）
Revealing the intricacies of radio galaxies and filaments in the merging galaxy cluster Abell 2255. II. Properties of filaments using multi-frequency radio data
Università di Bologna（博洛尼亚大学）；INAF - Istituto di Radioastronomia di Bologna（意大利国家天体物理研究所波洛尼亚射电天文台）；Leiden University（莱顿大学）；ASTRON, The Netherlands Institute for Radio Astronomy（荷兰射电天文研究所）；University of Minnesota（明尼苏达大学）；Universität Hamburg（汉堡大学）；Durham University（杜伦大学）
Two-component large-scale radio emission in Abell 2244
INAF - Istituto di Radioastronomia（意大利国家天体物理研究所射电天文研究所）；Dipartimento di Fisica ed Astronomia, Università di Bologna（博洛尼亚大学物理与天文系）；INAF - IASF Milano（意大利国家天体物理研究所米兰天文物理研究所）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）
