NuSTAROBSERVATIONS OF THE BULLET CLUSTER: CONSTRAINTS ON INVERSE COMPTON EMISSION

Abstract

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.

THE DISTRIBUTION OF RADIOACTIVE 44Ti IN CASSIOPEIA A

“…As the flux from the solar flares only affects the spectrum below 10 keV (Wik et al 2014) and the amount of time that is affected by solar flares is small compared to the duration of the observations, we do not apply any filtering to the data. We do not use the data for sequence IDs 40021001004, 40021002010, and 40021003002 because these short observations were significantly offset from the target pointing position.…”

“…We generate simulated background spectra using nuskybgd (Wik et al 2014) following the procedure described in Grefenstette et al (2014). This results in 22 sets of data files (11 epochs ×2 telescopes) for each region.…”

The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

“…(for details, see Wik et al 2014). In particular, the latter at 20 keV < is strongly affected by stray light from unfocused CXB photons reaching the detectors through the open design of the observatory (called "aperture background").…”

“…We therefore optimize the spectral extraction radius to maximize the signal-to-noise ratio (S/N) and, within the Poissonian uncertainties, the number of collected net counts. To do this, we started with the level 2 data products and simulated background maps where the latter were created using the software NUSKYBGD (Wik et al 2014) as described in C15 and M15. The simulated background maps reproduce the "aperture background" across the FoV and the normalization of the total background in each observation.…”

Energy distribution of relativistic electrons in the young supernova remnant G1.9+0.3

“…The background regions were selected in a way to minimise the contamination caused by the PSF wings as well as from the stray light. The excess in the south of the FPMA image is the stray light from X-rays that hit the detector without impinging on the optics (Wik et al 2014). We use the same source regions for Chandra observations.…”