Radio Observations of the Double-relic Galaxy Cluster Abell 1240

Journal Article · Wed May 02 00:00:00 EDT 2018 · Monthly Notices of the Royal Astronomical Society

DOI:https://doi.org/10.1093/mnras/sty1123· OSTI ID:1631523

Hoang, D. N. ^[1]; Shimwell, T. W. ^[2]; van Were, R. J. ^[1]; Intema, H. T. ^[1]; Andrade-Santos, F. ^[3]; Akamatsu, H. ^[4]; Bonafede, A. ^[5]; Brunetti, G. ^[6]; Dawson, W. A. ^[7]; Golovich, N. R. ^[8]; Best, P. N. ^[9]; Botteon, A. ^[10]; Bruggen, M. ^[11]; Cassano, R. ^[6]; de Gasperin, F. ^[1]; Hoeft, M. ^[12]; Stroe, A. ^[13]; White, G. J. ^[14]

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1. Leiden Observatory (Netherlands)
2. Leiden Observatory (Netherlands); Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo (Netherlands)
3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
4. SRON Netherlands Inst. for Space Research, Utrecht (Netherlands)
5. IRA INAF, Bologna (Italy); Univ. of Hamburg (Germany)
6. IRA INAF, Bologna (Italy)
7. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
8. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Univ. of California, Davis, CA (United States)
9. Univ. of Edinburgh, Scotland (United Kingdom). The Royal Observatory
10. IRA INAF, Bologna (Italy); Univ. of Bologna (Italy)
11. Univ. of Hamburg (Germany)
12. Thuringer Landessternwarte (Germany)
13. European Southern Observatory, Garching (Germany)
14. The Open Univ., Milton Keynes (United Kingdom); Science and Technology Facilities Council (STFC), Oxford (United Kingdom). Rutherford Appleton Lab. (RAL)

We present LOFAR 120 - 168 MHz images of the merging galaxy cluster Abell 1240 that hosts double radio relics. In combination with the GMRT 595-629 MHz and VLA 2- 4 GHz data, we characterised the spectral and polarimetric properties of the radio emission. The spectral indices for the relics steepen from their outer edges towards the cluster centre and the electric field vectors are approximately perpendicular to the major axes of the relics. The results are consistent with the picture that these relics trace large-scale shocks propagating outwards during the merger. Assuming diffusive shock acceleration (DSA), we obtain shock Mach numbers of M = 2.4 and 2.3 for the northern and southern shocks, respectively. For M ≲ 3 shocks, a pre-existing population of mildly relativistic electrons is required to explain the brightness of the relics due to the high (> 10 per cent) particle acceleration efficiency required. However, for M ≳ 4 shocks the required efficiency is ≥ 1% and ≥ 0.5%, respectively, which is low enough for shock acceleration directly from the thermal pool. We used the fractional polarization to constrain the viewing angle to > 53±3° and > 39±5° for the northern and southern shocks, respectively. We found no evidence for diffuse emission in the cluster central region. If the halo spans the entire region between the relics (~ 1.8 Mpc) our upper limit on the power is P1.4 GHz = (1.4 ± 0.6) × 10²³ W Hz^-1 which is approximately equal to the anticipated flux from a cluster of this mass. However, if the halo is smaller than this, our constraints on the power imply that the halo is underluminous

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1631523
• Report Number(s): LLNL-JRNL--774220; 966887
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Vol. 478; ISSN 0035-8711
• Publisher: Royal Astronomical Society
• Country of Publication: United States
• Language: English

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79 ASTRONOMY AND ASTROPHYSICS