Axionic dark radiation and the Milky Way’s magnetic field

Fairbairn, Malcolm

doi:10.1103/physrevd.89.064020

Cited by 17 publications

(17 citation statements)

References 59 publications

(83 reference statements)

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“…The maximum signal is predicted at an X-ray energy of 0.2 keV. This work has in turn given rise to a paper (Fairbairn 2014) which further explores the connection between CAB and CXB, asking:…”

Section: ~01 Counts/s/kevmentioning

confidence: 89%

Potential solar axion signatures in X-ray observations with the XMM–Newton observatory

Fraser

Read

Sembay

et al. 2014

Monthly Notices of the Royal Astronomical Society

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The soft X-ray flux produced by solar axions in the Earth's magnetic field is evaluated in the context of ESA's XMM-Newton observatory. Recent calculations of the scattering of axion-conversion X-rays suggest that the sunward magnetosphere could be an observable source of 0.2-10 keV photons. For XMM-Newton, any conversion X-ray intensity will be seasonally modulated by virtue of the changing visibility of the sunward magnetic field region. A simple model of the geomagnetic field is combined with the ephemeris of XMM-Newton to predict the seasonal variation of the conversion X-ray intensity. This model is compared with stacked XMM-Newton blank sky datasets from which point sources have been systematically removed. Remarkably, a seasonally varying X-ray background signal is observed. The EPIC count rates are in the ratio of their X-ray grasps, indicating a non-instrumental, external photon origin, with significances of 11σ (pn), 4σ (MOS1) and 5σ (MOS2). After examining the distribution of the constituent observations spatially, temporally and in terms of the accepted representation of the cosmic X-ray background, we conclude that this variable signal is consistent with the conversion of solar axions in the Earth's magnetic field, assuming the resultant photons are not strictly forward-directed, and enter the field-of-view of XMM-Newton. The spectrum is consistent with a solar axion spectrum dominated by bremsstrahlung-and Compton-like processes, distinct from a Primakoff spectrum, i.e. axion-electron coupling dominates over axion-photon coupling and the peak of the axion spectrum is below 1 keV. A value of 2.2x10 -22 GeV -1 is derived for the product of the axion-photon and axion-electron coupling constants, for an axion mass in the µeV range. Comparisons, e.g., with limits derived from white dwarf cooling may not be applicable, as these refer to axions in the ~0.01 eV range. Preliminary results are given of a search for axion-conversion X-ray lines, in particular the predicted narrow features due to silicon, sulphur and iron in the solar core, and the 14.4 keV transition line from 57 Fe.Keywords : axion, astroparticle physics, X-ray diffuse background, XMM-Newton * Corresponding Author : e-mail amr30@le.ac.uk 2 IntroductionThe direct detection of dark matter has preoccupied Physics for over thirty years. Of the current candidate dark matter particles, axions -using the term indiscriminately to encompass the several families of weakly-interacting, light, neutral, spin-zero bosons -may be observable as a result of their mixing with photons in an external electromagnetic field, either astrophysical or laboratory-based (Asztalos et al. 2006;Raffelt et al. 2007). GECOSAX 1.Primakoff spectrumExtending the work of Di Lella and Zioutas (2003), Davoudiasl and Huber (hereafter DH) (2006, 2008) proposed viewing the axion-emitting solar core through the solid Earth with an X-ray telescope. Solar axions converting via the inverse Primakoff effect (Sikivie, 1983) in the nightside planetary magnetic field between the E...

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Section: ~01 Counts/s/kevmentioning

confidence: 89%

Potential solar axion signatures in X-ray observations with the XMM–Newton observatory

Fraser

Read

Sembay

et al. 2014

Monthly Notices of the Royal Astronomical Society

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“…We have seen that axions are a generic prediction of string / M-theory and as such light axions can lead to strong constraints in light of bounds on N ef f . The consequences have been explored in both the case of M-theory [97], Type IIB Large Volume string compactifications [21,163,[170][171][172], and more generally [162,168,[173][174][175]. If these axions remain light (i.e., if the shift symmetry holds to a good approximation after moduli stabilization), then they could constitute dark radiation.…”

Section: Dark Radiation and N Ef Fmentioning

confidence: 99%

Cosmological moduli and the post-inflationary universe: A critical review

Kane

Sinha

Watson

2015

Int. J. Mod. Phys. D

169

222

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We critically review the role of cosmological moduli in determining the post-inflationary history of the universe. Moduli are ubiquitous in string and M-theory constructions of beyond the Standard Model physics, where they parametrize the geometry of the compactification manifold. For those with masses determined by supersymmetry breaking this leads to their eventual decay slightly before Big Bang Nucleosynthesis (without spoiling its predictions). This results in a matter dominated phase shortly after inflation ends, which can influence baryon and dark matter genesis, as well as observations of the Cosmic Microwave Background and the growth of large-scale structure. Given progress within fundamental theory, and guidance from dark matter and collider experiments, nonthermal histories have emerged as a robust and theoretically well-motivated alternative to a strictly thermal one. We review this approach to the early universe and discuss both the theoretical challenges and the observational implications. * 5 The case of non-thermal histories with multiple moduli was considered in [12]. Generically, string / M-theory theories have many moduli with various masses, and the lightest is typically the relevant one. 6 One can argue rather generally for the positivity of cn based on causality, unitarity, and demanding that the model have a UV completion (although a few known counter-examples exist in gravitational systems). We refer the reader to [14] for details. On the other hand, the choice of c 1 positive (leading to a tachyonic mass) is chosen to realize the general expectation that the high energy and low energy minima of the field would not generically be expected to coincide in a gravitational and/or time dependent background [15]. Ideally one would like to calculate this in an explicit, UV complete model. 7 This is a generic expectation in string theories where new thresholds before the Planck scale are common place. Examples include both the compactification (Kaluza-Klein) scale M kk and string scale Ms where M M pl is required for consistency of the effective theory [16]. 9 The relation between moduli and gravitino mass in this case was first noted in [23]. 10 It is noteworthy that even defining the level of fine-tuning can be an issue, see e.g. [28]. 11 We note that already in 2006 this mass range for the lightest scalars and gravitino was noted in [29] long before the Higgs discovery and LHC bounds favoring heavy scalar superpartners.

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“…In the presence of a magnetic field this induces 'oscillations' of axions into photons in a process analogous to neutrino oscillations [11,86]. The observational consequences of this conversion of the CAB have been considered in [1,87,88]. Axions may also play a role by scattering off ambient particles in the thermal plasma, which was considered in [8].…”

Section: A Cosmic Axion Backgroundmentioning

confidence: 99%

Soft X-ray excess in the Coma cluster from a Cosmic Axion Background

Angus

Conlon

Marsh

et al. 2014

J. Cosmol. Astropart. Phys.

134

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We show that the soft X-ray excess in the Coma cluster can be explained by a cosmic background of relativistic axions converting into photons in the cluster magnetic field. We provide a detailed self-contained review of the cluster soft X-ray excess, the proposed astrophysical explanations and the problems they face, and explain how a 0.1 − 1 keV axion background naturally arises at reheating in many string theory models of the early universe. We study the morphology of the soft excess by numerically propagating axions through stochastic, multi-scale magnetic field models that are consistent with observations of Faraday rotation measures from Coma. By comparing to ROSAT observations of the 0.2 − 0.4 keV soft excess, we find that the overall excess luminosity is easily reproduced for g aγγ ∼ 2 × 10 −13 GeV −1 . The resulting morphology is highly sensitive to the magnetic field power spectrum. For Gaussian magnetic field models, the observed soft excess morphology prefers magnetic field spectra with most power in coherence lengths on O(3 kpc) scales over those with most power on O(12 kpc) scales. Within this scenario, we bound the mean energy of the axion background to 50 eV E a 250 eV, the axion mass to m a 10 −12 eV, and derive a lower bound on the axion-photon coupling g aγγ 0.5/∆N eff 1.4 × 10 −13 GeV −1 .In the enlightening case of sufficiently high axion energies or small ambient electron densities, the conversion probability for a fixed domain is given bywhere B ⊥ denotes the magnetic field component transverse to the axion velocity and L denotes the corresponding coherence length [11]. This conversion allows the potential detection of a CAB through axion-photon conversion.Galaxy clusters support magnetic fields that are modest in magnitude (B ≈ µG) but are extended over megaparsec distances and have kiloparsec coherence scales, allowing observationally significant axion-photon conversion probabilities. In [1], a crude single-domain model with a fixed magnitude and coherence length for the magnetic field was used to estimate the axion-photon coupling M that would be required to reproduce the soft excess in Coma from a CAB, finding M ≈ 10 13 GeV.In this paper we continue the study of axion-photon conversion in the Coma cluster using a far more detailed model of the Coma magnetic field. This model was constructed in [12] to fit rotation measure (RM) observations of seven polarised light sources, using the Coma cluster as a Faraday screen. The model describes the central Mpc 3 of Coma (see also [13] for a magnetic field model describing the region 1.5 Mpc to the southwest of the cluster centre). We review the observational evidence for cluster magnetic fields in section 4 and describe the model for the Coma magnetic field in detail in section 4.2. Using this stochastic model, we construct a numerical simulation of the magnetic field in the central region of the cluster, propagate axions through it and quantitatively study the resulting predictions for the soft excess morphology. This paper is organised as follo...

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Axionic dark radiation and the Milky Way’s magnetic field

Cited by 17 publications

References 59 publications

Potential solar axion signatures in X-ray observations with the XMM–Newton observatory

Potential solar axion signatures in X-ray observations with the XMM–Newton observatory

Cosmological moduli and the post-inflationary universe: A critical review

Soft X-ray excess in the Coma cluster from a Cosmic Axion Background

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