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Title: NuSTAR observations of the bullet cluster: constraints on inverse Compton emission

Abstract

Here, 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 themore » 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.« less

Authors:
 [1];  [2];  [3];  [4];  [5];  [6];  [5];  [3];  [5];  [2];  [2];  [7];  [2];  [6];  [2];  [8];  [9]; ORCiD logo [5];  [10]
  1. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States); Johns Hopkins Univ., Baltimore, MD (United States)
  2. Technical Univ. of Denmark, Lyngby (Denmark)
  3. IASF-Milano, Milano (Italy)
  4. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  5. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  6. Univ. of California, Berkeley, CA (United States)
  7. RIKEN Nishina Center, Saitama (Japan)
  8. Univ. of California, Berkeley, CA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  9. Columbia Univ., New York, NY (United States)
  10. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC); National Aeronautics and Space Administration (NASA)
OSTI Identifier:
1165905
Report Number(s):
SLAC-PUB-16154
Journal ID: ISSN 1538-4357; arXiv:1403.2722
Grant/Contract Number:
AC02-76SF00515
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 792; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astrophysics; ASTRO; GRQC; galaxies: clusters: general; galaxies: clusters: individual (Bullet cluster); intergalactic medium; magnetic fields; radiation mechanisms: non-thermal; X-rays: galaxies: clusters

Citation Formats

Wik, Daniel R., Hornstrup, A., Molendi, S., Madejski, G., Harrison, F. A., Zoglauer, A., Grefenstette, B. W., Gastaldello, F., Madsen, K. K., Westergaard, N. J., Ferreira, D. D. M., Kitaguchi, T., Pedersen, K., Boggs, S. E., Christensen, F. E., Craig, W. W., Hailey, C. J., Stern, D., and Zhang, W. W.. NuSTAR observations of the bullet cluster: constraints on inverse Compton emission. United States: N. p., 2014. Web. doi:10.1088/0004-637X/792/1/48.
Wik, Daniel R., Hornstrup, A., Molendi, S., Madejski, G., Harrison, F. A., Zoglauer, A., Grefenstette, B. W., Gastaldello, F., Madsen, K. K., Westergaard, N. J., Ferreira, D. D. M., Kitaguchi, T., Pedersen, K., Boggs, S. E., Christensen, F. E., Craig, W. W., Hailey, C. J., Stern, D., & Zhang, W. W.. NuSTAR observations of the bullet cluster: constraints on inverse Compton emission. United States. doi:10.1088/0004-637X/792/1/48.
Wik, Daniel R., Hornstrup, A., Molendi, S., Madejski, G., Harrison, F. A., Zoglauer, A., Grefenstette, B. W., Gastaldello, F., Madsen, K. K., Westergaard, N. J., Ferreira, D. D. M., Kitaguchi, T., Pedersen, K., Boggs, S. E., Christensen, F. E., Craig, W. W., Hailey, C. J., Stern, D., and Zhang, W. W.. Wed . "NuSTAR observations of the bullet cluster: constraints on inverse Compton emission". United States. doi:10.1088/0004-637X/792/1/48. https://www.osti.gov/servlets/purl/1165905.
@article{osti_1165905,
title = {NuSTAR observations of the bullet cluster: constraints on inverse Compton emission},
author = {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.},
abstractNote = {Here, 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.},
doi = {10.1088/0004-637X/792/1/48},
journal = {The Astrophysical Journal (Online)},
number = 1,
volume = 792,
place = {United States},
year = {Wed Aug 13 00:00:00 EDT 2014},
month = {Wed Aug 13 00:00:00 EDT 2014}
}

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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.more » 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{sup –12} erg s{sup –1} cm{sup –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.« less
  • Some dark matter candidates, e.g., sterile neutrinos, provide observable signatures in the form of mono-energetic line emission. Here, we present the first search for dark matter line emission in themore » $$3-80\;\mathrm{keV}$$ range in a pointed observation of the Bullet Cluster with NuSTAR. We do not detect any significant line emission and instead we derive upper limits (95% CL) on the flux, and interpret these constraints in the context of sterile neutrinos and more generic dark matter candidates. NuSTAR does not have the sensitivity to constrain the recently claimed line detection at $$3.5\;\mathrm{keV}$$, but improves on the constraints for energies of $$10-25\;\mathrm{keV}$$.« less
  • We report on the detection of excess hard X-ray emission from the TeV BL Lac object Mrk 421 during the historical low-flux state of the source in 2013 January. Nuclear Spectroscopic Telescope Array observations were conducted four times between MJD 56294 and MJD 56312 with a total exposure of 80.9 ks. The source flux in the 3–40 keV range was nearly constant, except for MJD 56307 when the average flux level increased by a factor of three. Throughout the exposure, the X-ray spectra of Mrk 421 were well represented by a steep power-law model with a photon index of Γmore » ≃ 3.1, although a significant excess was noted above 20 keV in the MJD 56302 data when the source was in its faintest state. Moreover, Mrk 421 was detected at more than the 4 σ level in the 40–79 keV count maps for both MJD 56307 and MJD 56302 but not during the remaining two observations. The detected excess hard X-ray emission connects smoothly with the extrapolation of the high-energy γ -ray continuum of the blazar constrained by Fermi -LAT during source quiescence. These findings indicate that while the overall X-ray spectrum of Mrk 421 is dominated by the highest-energy tail of the synchrotron continuum, the variable excess hard X-ray emission above 20 keV (on the timescale of a week) is related to the inverse Compton emission component. We discuss the resulting constraints on the variability and spectral properties of the low-energy segment of the electron energy distribution in the source.« less
  • An inverse Compton origin of the ..gamma..-ray emission of point sources reveals itself by a continuous steepening of the differential ..gamma..-ray spectrum with increasing energy due to higher order effects in the Klein-Nishina cross section. It is shown that the observed breaks in the ..gamma..-ray spectra of the Vela pulsar PSR 0833--45, the Crab pulsar NP 0532, and the Seyfert galaxy NGC 4151 can be explained quite naturally by this effect. The implied constraints on the mean energy of the target photon gases allow us to discuss the place of origin of the observed ..gamma..-rays.
  • Here, GRB 130427A occurred in a relatively nearby galaxy; its prompt emission had the largest GRB fluence ever recorded. The afterglow of GRB 130427A was bright enough for the Nuclear Spectroscopic Telescope ARray ( NuSTAR) to observe it in the 3-79 keV energy range long after its prompt emission (~1.5 and 5 days). This range, where afterglow observations were previously not possible, bridges an important spectral gap. Combined with Swift, Fermi, and ground-based optical data, NuSTAR observations unambiguously establish a single afterglow spectral component from optical to multi-GeV energies a day after the event, which is almost certainly synchrotron radiation.more » Such an origin of the late-time Fermi/Large Area Telescope >10 GeV photons requires revisions in our understanding of collisionless relativistic shock physics.« less