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NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission

Wik, Daniel R. and Hornstrup, A. and Molendi, S. and Madejski, G. and Harrison, F. A. and Zoglauer, A. and Grefenstette, B. W. and Gastaldello, F. and Madsen, K. K. and Westergaard, N. J. and Ferreira, D. D. M. and Kitaguchi, T. and Pedersen, K. and Boggs, S. E. and Christensen, F. E. and Craig, W. W. and Hailey, C. J. and Stern, D. and Zhang, W. W. (2014) NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission. Astrophysical Journal, 792 (1). Art. No. 48. ISSN 0004-637X. doi:10.1088/0004-637X/792/1/48.

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The search for diffuse non-thermal inverse Compton (IC) emission from galaxy clusters at hard X-ray energies has been undertaken with many instruments, with most detections being either of low significance or controversial. Because all prior telescopes sensitive at E > 10 keV do not focus light and have degree-scale fields of view, their backgrounds are both high and difficult to characterize. The associated uncertainties result in lower sensitivity to IC emission and a greater chance of false detection. In this work, we present 266 ks NuSTAR observations of the Bullet cluster, which is detected in the energy range 3-30 keV. NuSTAR's unprecedented hard X-ray focusing capability largely eliminates confusion between diffuse IC and point sources; however, at the highest energies, the background still dominates and must be well understood. To this end, we have developed a complete background model constructed of physically inspired components constrained by extragalactic survey field observations, the specific parameters of which are derived locally from data in non-source regions of target observations. Applying the background model to the Bullet cluster data, we find that the spectrum is well—but not perfectly—described as an isothermal plasma with kT = 14.2 ± 0.2 keV. To slightly improve the fit, a second temperature component is added, which appears to account for lower temperature emission from the cool core, pushing the primary component to kT ~ 15.3 keV. We see no convincing need to invoke an IC component to describe the spectrum of the Bullet cluster, and instead argue that it is dominated at all energies by emission from purely thermal gas. The conservatively derived 90% upper limit on the IC flux of 1.1 × 10^(–12) erg s^(–1) cm^(–2) (50-100 keV), implying a lower limit on B ≳ 0.2 μG, is barely consistent with detected fluxes previously reported. In addition to discussing the possible origin of this discrepancy, we remark on the potential implications of this analysis for the prospects for detecting IC in galaxy clusters in the future.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Wik, Daniel R.0000-0001-8952-676X
Harrison, F. A.0000-0003-2992-8024
Grefenstette, B. W.0000-0002-1984-2932
Madsen, K. K.0000-0003-1252-4891
Boggs, S. E.0000-0001-9567-4224
Christensen, F. E.0000-0001-5679-1946
Stern, D.0000-0003-2686-9241
Zhang, W. W.0000-0002-1426-9698
Additional Information:© 2014 American Astronomical Society. Received 2014 March 11; accepted 2014 July 17; published 2014 August 13. This research was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with the National Aeronautics and Space Administration (NASA) and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). The authors wish to thank Maxim Markevitch for providing a 0.5 Ms Chandra image of the Bullet cluster to confirm the cluster region lacks bright point sources and the referee for helpful comments that improved the paper.
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
NASA Postdoctoral ProgramUNSPECIFIED
Subject Keywords:galaxies: clusters: general; galaxies: clusters: individual (Bullet cluster); intergalactic medium; magnetic fields; radiation mechanisms: non-thermal; X-rays: galaxies: clusters
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2014-37
Issue or Number:1
Record Number:CaltechAUTHORS:20140925-100352945
Persistent URL:
Official Citation:NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission Daniel R. Wik et al. 2014 ApJ 792 48
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:50024
Deposited By: Ruth Sustaita
Deposited On:25 Sep 2014 17:26
Last Modified:10 Nov 2021 18:51

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