ChandraACIS-I particle background: an analytical model

Abstract

Aims. Imaging and spectroscopy of X-ray extended sources require a proper characterisation of a spatially unresolved background signal. This background includes sky and instrumental components, each of which are characterised by its proper spatial and spectral behaviour. While the X-ray sky background has been extensively studied in previous work, here we analyse and model the instrumental background of the ACIS-I detector on board the Chandra X-ray observatory in very faint mode. Methods. Caused by interaction of highly energetic particles with the detector, the ACIS-I instrumental background is spectrally characterised by the superimposition of several fluorescence emission lines onto a continuum. To isolate its flux from any sky component, we fitted an analytical model of the continuum to observations performed in very faint mode with the detector in the stowed position shielded from the sky, and gathered over the eight-year period starting in 2001. The remaining emission lines were fitted to blank-sky observations of the same period. We found 11 emission lines. Analysing the spatial variation of the amplitude, energy and width of these lines has further allowed us to infer that three lines of these are presumably due to an energy correction artefact produced in the frame store. Results. We provide an analytical model that predicts the instrumental background with a precision of 2% in the continuum and 5% in the lines. We use this model to measure the flux of the unresolved cosmic X-ray background in the Chandra deep field south. We obtain a flux of 10.

Constraints on the presence of a 3.5 keV dark matter emission line fromChandraobservations of the Galactic centre

“…However, even the extreme case of removing one or the other from the base model leads to very similar flux constraints, and we take the sum of the two to be robustly determined. The best fit normalisations of the instrumental lines are of the same order of magnitude as those given in the literature for blank sky spectra (Bartalucci et al 2014;Chandra X-ray Center 2014).…”

“…Rather we added the line emission visible in observations taken with the telescope in the stowed position ("lid on") 6 to the model later. These are denoted "instrumental lines" in Table 3 (Chandra X-ray Center 2014; Bartalucci et al 2014). Figure 1 clearly shows pockets of extended diffuse emission, on scales smaller than expected for dark matter emission from the halo.…”

“…If we fit the 2.0−9.0 keV interval for the absorption we obtain a value of 5.83 +0.037 −0.029 × 10 22 atoms cm −2 , consistent with the fixed value. The line-free model is pre-fitted to intervals that appear line-free (3.4−3.6 keV and 4.3−5.2 keV), before we add the known instrumental lines from Bartalucci et al (2014) and atomic emission lines (all listed in Table 3). …”

Relic—Shock Connection in Abell 115

“…Three background components were modeled in the fits: the Galactic background (taken into account with two thermal plasmas with temperatures 0.14 and 0.25 keV), the cosmic X-ray background (taken into account assuming an absorbed power-law with photon index 1.4) and the ACIS-I particle background (modelled following [4]). Since the low X-ray count rate, Cash statistics and an ICM metallicity fixed at 0.3 Z were adopted in the fits.…”

“…where Ψ = p min Q(p)E dp p 0 Q(p)E dp (4) accounts for the ratio of the energy flux injected in "all" electrons and those visible in the radio band (ν ≥ ν 0 ), p 0 is the momentum of the relativistic electrons emitting the synchrotron frequency ν 0 in a magnetic field B and B cmb = 3.25(1 + z) 2 µG accounts for inverse Compton scattering of cosmic microwave background photons. In a DSA regime, e.g., [26], the particle injection spectrum is Q(p) ∝ p −δ inj , with slope related to the shock Mach number via…”