Axion-like Particles Implications for High-Energy Astrophysics
Abstract
1. Introduction
- In another region of the parameter plane —which can overlap with the previous one—ALPs give rise to very interesting astrophysical effects (for a very incomplete list of references, see [39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129]. In particular, we shall see that ALPs can be indirectly detected by the new generation of gamma-ray observatories, such as CTA (Cherenkov Telescope Array) [130], HAWC (High-Altitude Water Cherenkov Observatory) [131], GAMMA-400 (High-Altitude Water Cherenkov Observatory) [132], LHAASO (High-Altitude Water Cherenkov Observatory) [133], TAIGA-HiSCORE (Hundred Square km Cosmic Origin Explorer) [134] and HERD (High Energy cosmic-Radiation Detection) [135].
- The last reason is that the region of the parameter plane relevant for astrophysical effects can be probed—and ALPs can be directly detected—in the laboratory experiment called shining through the wall within the next few years thanks to the upgrade of ALPS (Any Light Particle Search) II at DESY [136] and by the STAX experiment [137]. Alternatively, these ALPs can be observed by the planned IAXO (International Axion Observatory) observatory [138], as well as with other strategies developed by Avignone and collaborators [139,140,141]. Moreover, if the bulk of the dark matter is made of ALPs they can also be detected by the planned experiment ABRACADABRA (A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus) [142].
2. The Standard Lore in Particle Physics
2.1. General Framework
2.2. Axion as a Prototype
2.3. Emergence of ALPs
- Only photon-ALP interaction terms are taken into account. Nothing prevents ALPs from coupling to other SM particles, but for our purposes they will henceforth be discarded. Observe that such an ALP coupling to two photons is just supposed to exist without further worrying about its origin.
- The parameters and are to be regarded as unrelated for ALPs, and it is merely assumed that and .
3. General Properties of ALPs
3.1. Beam Propagation Equation
3.2. A Simplified Case
3.3. A More General Case
3.4. Complications
3.5. Unpolarized Beam
3.6. Parameter Bounds
* * * |
- for from PKS 2155-304 [83].
- for from NGC 1275 [97].
- for from M87 [103].
- for from PKS 2155-304 [111].
- – for from Mrk 421 [120].
- for from Mrk 421 [121].
- for from NGC 1275 (center of Perseus cluster) [123].
- Payez et al. 2015 say that ALPs are emitted simultaneously with neutrinos, and this is repeated by everybody. Concerning our ALPs which are supposed to interact only with two photons, this is not true. Since they do not interact either with matter nor with radiation (see Appendix B), they escape as soon as they are produced, while neutrinos remain trapped. Thus, this weakens the supernova bound.
- Because the value of is rather uncertain—basically depending on where the strong-mixing regime sets in—we will consider throughout this Review and . Hence, we can easily take no contradiction exists even apart from the previous remark.
4. Astrophysical Context
4.1. Blazars
- One is called leptonic mechanism (syncrotron-self Compton). Basically, in the presence of the magnetic fields inside the AGN jet, relativistic electrons emit synchrotron radiation and the produced photons are boosted to much higher energies by inverse Compton scattering off the parent electrons (in some cases also external photons from the disc participate in this process). The resulting emitted spectral energy distribution (SED) — is the specific apparent luminosity—has two humps: the synchrotron one—somewhere from the IR band to the X-ray band—while the inverse Compton one lies in the -ray band [177,178,179,180].
- The other mechanism is named hadronic mechanism (proton-proton scattering). As far as the synchrotron emission is concerned the situation is the same as before, but the gamma hump is produced by hadronic collisions. The resulting immediately decays as , while the produce neutrinos and antineutrinos [181,182]. Thus, the detection of these neutrinos can discriminate between the two mechanisms. In 2017 the IceCube neutrino telescope has detected one neutrino coming from the flaring blazar TXS 0506+056, thereby demonstrating that the hadronic mechanism does work [183].
4.2. Conventional Photon Propagation
4.3. Extragalactic Background Light (EBL)
4.4. Extragalactic Magnetic Field
4.5. Galaxy Clusters
5. Propagation of ALPs in Extragalactic Space—1
5.1. Strategy
* * * |
5.2. Propagation over a Single Domain
5.3. Calculation of the Photon Survival Probability in the Presence of Photon-ALP Oscillations
6. VHE BL Lac Spectral Anomaly
- We focus our attention on flaring blazars, which show episodic time variability with their luminosity increasing by more than a factor of two, on the time span from a few hours to a few days: the reason is both their enhanced luminosity—which entails in turn their detectability [233,234]—and our desire to consider a homogeneous sample of BL Lacs.
- As we shall see, our analysis requires the knowledge of the redshift, the observed spectrum and the energy range wherein every blazar is observed. This information is available only for some of the observed flaring sources.
- In order to get rid of evolutionary effects inside blazars we restrict our attention to those with .
- It seems a good thing to deal with sources that are as similar as possible. Therefore, we consider only intermediate-frequency peaked (IBL) and high-frequency peaked (HBL) flaring BL Lacs with observed energy .
6.1. Conventional Physics
6.2. ALPs Enter the Game
6.3. A New Scenario for Flaring BL Lacs
7. New Explanation of VHE Emission from FSRQs
7.1. Detection of PKS 1222+216 in the VHE Range
7.2. Observations and Setup
7.3. An ALP Model for PKS 1222+216
7.4. Photon-ALP Oscillations Inside the BLR
7.5. Photon-ALP Oscillations in the Large Scale Jet
7.6. Photon-ALP Oscillations in the Host Galaxy
7.7. Photon-ALP Oscillations in Extragalactic Space
7.8. Overall Photon-ALP Oscillations
7.9. Results
- The explanation of why MAGIC data have been observed [236].
- .
- .
- .
8. Propagation of ALPs in Extragalactic Space—2
8.1. Preliminary Remarks
- —This is the low-energy weak-mixing regime, wherein the terms dominate. Correspondingly, we haveHowever, since we will not consider this case throughout the paper.
- —This is the intermediate-energy or strong-mixing regime in which the term dominates. Accordingly, we obtainClearly, and are independent of all the energy-dependent terms, and becomes maximal: observe that enters and nowhere else.
- —This is the high-energy weak-mixing regime, which is in a sense a sort of reversed low-energy weak-mixing regime, where however the term dominates over . Correspondingly, we getManifestly, decreases with increasing E and exhibits oscillations in E: this means that the individual realizations of the beam propagation are also oscillating functions of E. Moreover—since —as E increases the photon-ALP oscillations become unobservable at some point.
8.2. Domain-like Smooth-Edges (DLSME) Model
8.3. Propagation over a Single Domain
8.4. Solution of the Beam Propagation Equation
8.5. Results
9. A Full Scenario
- Markarian 501 at .
- The extreme BL Lac 1ES 0229+200 at .
- A simulated source like BL Lac 1ES 0229+200 but at
9.1. Propagation in the BL Lac Jet
9.2. Propagation in the Host Galaxy
9.3. Propagation in the Extragalactic Space
9.4. Propagation in the Milky Way
9.5. Overall Photon Survival Probability
9.6. Blazar Spectra
- Markarian 501—This source is a high-frequency peaked blazar (HBL) at redshift . We use the observational data points from HEGRA [257] in a condition where Markarian 501 was observed in a high emission state, which allows us to have a very good quality spectrum up to ∼30 TeV. This fact is important for testing our model, since at such high energies it starts to make predictions which depart from conventional physics. In Figure 25 we report its observed SED when conventional physics alone is considered, and when oscillations are at work. In order to obtain the SED we take , and in Equation (146).
- 1ES 0229+200—This is a BL Lac at redshift . This is the prototype of the so-called `extreme HBL’ (EHBL) [258,259], which exhibit a rather hard VHE observed spectrum up to at least 10 TeV. This fact is particularly interesting since the observed data points at such high energies allow to distinguish between the models based on conventional physics and those containing oscillations. Future observations with the CTA that can eventually reach energies up to 100 TeV can provide a definitive answer. In Figure 26 we plot its observed SED both when only conventional physics is taken into account and in the case in which also oscillations are present. The SED is obtained by taking in Equation (146) in the case of conventional physics, and when also oscillations are considered, while in either case we choose and . Note that is in agreement with the one derived for the Fermi/LAT spectrum in the recent analysis of [259].
- Extreme BL Lac at —BL Lacs have been observed also at redshift , and so we assume the existence of an EHBL at redshift . For this blazar we take a SED similar to the one of 1ES 0229+200, namely , and in Equation (146) for both cases (conventional physics alone, presence of oscillations). We consider two possibilities: (1) such BL Lac is observed in the sky along the direction of the galactic pole: in Figure 27 we plot its observed SED for both cases of presence/absence of photon-ALP interaction; (2) in Figure 28 we exhibit the corresponding observed SED for the same BL Lac instead observed in the sky along the direction of the galactic plane for both cases of presence/absence of photon-ALP interaction.
9.7. Results
* * * |
- The jet parameters (, ) are affected by uncertainties, and the amount of produced ALPs in this region clearly reflects this fact. Nevertheless, we have checked that the final spectra qualitatively possess the above-mentioned features regardless of the choice of the jet parameters, provided of course that they are realistic.
- Even if we consider very low values of the extragalactic magnetic field—namely —the considered model predicts the above-mentioned features even if partially reduced, in particular concerning the amplitude of the energy oscillations. However, the peak in the spectra at remains unaffected at high redshift.
- The electromagnetic cascade proposed to mimic oscillation effects in blazar spectra [266] can work only for , which is indeed quite close to the lower limits [208,209,210]. Still, for the charged particles produced in the cascade are deflected by and the resulting additional photon flux turns out to be very likely irrelevant (for more details, see e.g., [267]). This argument also applies to the possible additional pairs produced in the process .
- For the infrared radiation from dust present inside the Milky Way could play a moderate role in absorbing photons [268]. But this effect is irrelevant for us and can be safely discarded. Basically, the resulting absorption is substantial only inside the Galactic plane and a few degrees above and below it, hence only ALPs converted to photons in the Galactic plane close to the outer border of the Milky Way disk fully undergo such an effect. Actually, two points should be be stressed. (1) It goes without saying that when the line of sight to the blazar lies outside the galactic plane the considered effect is totally irrelevant. (2) Even for the photon/ALP beam entering the Milky Way along the Galactic plane the oscillations reduce photon absorption, thereby considerably weakening dust absorption.
10. Polarization Effects
10.1. ALP Effects on Photon Polarization
- When photon absorption is negligible and photons (without initial ALPs) are emitted with initial degree of linear polarization , the two conditions and hold. If photons are emitted unpolarized () we have the two inequalities and .
- Under the previous conditions, can be viewed as the measure of the overlap between the values assumed by and . If photons are emitted unpolarized (), then and have no overlap apart from the value 1/2, at most.
10.2. ALP-Induced Polarization Effects in Galaxy Clusters
10.3. ALP-Induced Polarization Effects in Blazars
11. Conclusions and Outlook
Author Contributions
Funding
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Appendix A
Appendix B
Appendix C
Source | z | [TeV] | ||
---|---|---|---|---|
Mrk 421 | 0.031 | |||
Mrk 501 | 0.034 | |||
1ES 2344+514 | 0.044 | |||
Mrk 180 | 0.045 | |||
1ES 1959+650 | 0.048 | |||
1ES 1959+650 | 0.048 | |||
1ES 1727+502 | 0.055 | |||
PKS 1440-389 | 0.065 | |||
PKS 0548-322 | 0.069 | |||
PKS 2005-489 | 0.071 | |||
1ES 1741+196 | 0.084 | |||
SHBL J001355.9-185406 | 0.095 | |||
W Comae | 0.102 | |||
BL Lacertae | 0.069 | |||
1ES 1312-423 | 0.105 | |||
PKS 2155-304 | 0.116 | |||
B3 2247+381 | 0.1187 | |||
RGB J0710+591 | 0.125 | |||
H 1426+428 | 0.129 | |||
1ES 1215+303 | 0.13 | |||
1ES 1215+303 | 0.13 | |||
1ES 0806+524 | 0.138 | |||
1RXS J101015.9-311909 | 0.142639 | |||
1ES 1440+122 | 0.163 | |||
H 2356-309 | 0.165 | |||
RX J0648.7+1516 | 0.179 | |||
1ES 1218+304 | 0.182 | |||
1ES 1101-232 | 0.186 | |||
RBS 0413 | 0.19 | |||
1ES 1011+496 | 0.212 | |||
PKS 0301-243 | 0.2657 | |||
1ES 0414+009 | 0.287 | |||
OJ 287 | 0.306 | |||
S5 0716+714 | 0.31 | |||
TXS 0506+056 | 0.3365 | |||
3C 66A | 0.34 | |||
PKS 0447-439 | 0.343 | |||
1ES 0033+595 | 0.467 | |||
PG 1553+113 | 0.5 |
1 | Other processes discussed in [144] are totally irrelevant for the energy range considered in this paper. |
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# of Fit Parameters | |||||
---|---|---|---|---|---|
1 | 2.37 | 2.29 | 1.29 | 1.31 | 1.43 |
2 | 1.49 | 1.47 | 1.29 | 1.31 | 1.38 |
3 | 1.46 | 1.46 | 1.32 | 1.31 | 1.37 |
# of Fit Parameters | |||||
---|---|---|---|---|---|
1 | 2.37 | 2.05 | 1.25 | 1.39 | 1.43 |
2 | 1.49 | 1.44 | 1.26 | 1.37 | 1.38 |
3 | 1.46 | 1.46 | 1.28 | 1.36 | 1.37 |
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Galanti, G.; Roncadelli, M. Axion-like Particles Implications for High-Energy Astrophysics. Universe 2022, 8, 253. https://doi.org/10.3390/universe8050253
Galanti G, Roncadelli M. Axion-like Particles Implications for High-Energy Astrophysics. Universe. 2022; 8(5):253. https://doi.org/10.3390/universe8050253
Chicago/Turabian StyleGalanti, Giorgio, and Marco Roncadelli. 2022. "Axion-like Particles Implications for High-Energy Astrophysics" Universe 8, no. 5: 253. https://doi.org/10.3390/universe8050253
APA StyleGalanti, G., & Roncadelli, M. (2022). Axion-like Particles Implications for High-Energy Astrophysics. Universe, 8(5), 253. https://doi.org/10.3390/universe8050253