Relevance of jet magnetic field structure for blazar axionlike particle searches

Many theories beyond the Standard Model of particle physics predict the existence of axionlike particles (ALPs) that mix with photons in the presence of a magnetic field. One prominent indirect method of searching for ALPs is to look for irregularities in blazar gamma-ray spectra caused by ALP-photon mixing in astrophysical magnetic fields. This requires the modeling of magnetic fields between Earth and the blazar. So far, only very simple models for the magnetic field in the blazar jet have been used. Here, we investigate the effects of more complicated jet magnetic field configurations on these spectral irregularities by imposing a magnetic field structure model onto the jet model proposed by Potter & Cotter. We simulate gamma-ray spectra of Mrk 501 with ALPs and fit them to ALP-less spectra, scanning the ALP and B-field configuration parameter space, and show that the jet can be an important mixing region, able to probe new ALP parameter space around $m_a\sim 1–1000\mathrm{neV}$ and $g_{a\gamma }\gtrsim 5\times {10}^{-12}{\mathrm{GeV}}^{-1}$. However, reasonable (i.e., consistent with observation) changes of the magnetic field structure can have a large effect on the mixing. For jets in highly magnetized clusters, mixing in the cluster can overpower mixing in the jet. This means that the current constraints using mixing in the Perseus cluster are still valid.

Figure 1

ALP ($m_a$, $g_{a\gamma }$) parameter space with current and future predicted experimental exclusions. In particular, exclusions from CAST, IAXO, and Fermi observation of NGC 1275 and SMM SN1987A exclusions are shown (see text for details). The region where the axion could solve the CP problem is shown in yellow. Plot from Ref. [9].

Figure 2

Diagram showing the PC jet model: a magnetically dominated parabolic base that transitions to a conical ballistic jet once the magnetic field and particles come into energy equipartition. Gamma-ray emission is strongly dominated by the transition region (see Fig. 6.)

Figure 3

High energy peaks of Mrk 501 spectra, showing ALP-photon mixing induced oscillations for different values of ALP mass ($m_a$) and coupling ($g_{a\gamma }$). Blue (dotted) lines are the fit to data from the PC model. Red (solid) lines show the spectrum when mixing in the jet, IGMF, and Milky Way is included. Green (dashed) lines show the spectrum when only mixing in the jet is included.

Figure 4

Values of $F$—sum of least-squares fit between ALP and ALP-less spectrum—calculated over ALP mass-coupling parameter space. Dashed line shows current exclusions (see Sec. 1).

Figure 5

$F\text{−}{F}_{\text{jet}}$ over ALP mass-coupling parameter space. $F$ is the fit including mixing in the jet, IGMF, and the Milky Way. $F_{\text{jet}}$ is the fit only including mixing in the jet. Dashed line shows current exclusions (see Sec. 1).

Figure 6

One realization of the jet magnetic field ($\alpha =1$, $f=0.7$), showing the overall field strength (blue dashed) and the transverse magnetic field strength (orange, calculated from $r_{\mathrm{em}}$ onward) used for calculating ALP-photon conversion, against $r$, the distance down the jet. $\nu F_{\nu }$ from the PC model is also shown, showing that a very localized gamma-ray emission region is reasonable.

Figure 7

$\mathrm{\Delta }{F}_{\text{jet}}/F_{\max }$ over ALP mass-coupling parameter space. $\mathrm{\Delta }{F}_{\text{jet}}$ is the change in $F_{\text{jet}}$ when the jet magnetic field parameters are varied within the limits shown in Table 1. $F_{\max }$ is the maximum value of $F_{\text{jet}}$ obtained during this variation. Dashed line shows current exclusions (see Sec. 1).

Figure 8

$F_{\text{jet}}$ vs magnetic field parameters for $m_a=1\mathrm{neV}$, $g_{a\gamma }=2\times {10}^{-12}{\mathrm{GeV}}^{-1}$ (left column) and $m_a=100\mathrm{neV}$ and $g_{a\gamma }=7\times {10}^{-12}{\mathrm{GeV}}^{-1}$ (right column). Shaded regions show values of $F_{\text{jet}}$ that can be obtained by varying the other magnetic field parameters.

Figure 9

$F_{\text{jet}}$ for four different values of $B_{r_{\mathrm{em}}}$. Below $B_{r_{\mathrm{em}}}\sim 0.05\mathrm{G}$, the jet becomes unimportant.

Figure 10

Redshift dependence of $F$ for different values of $m_a$ at coupling strength $g_{a\gamma }=1.5\times {10}^{-11}{\mathrm{GeV}}^{-1}$. Figure 5 shows that the jet starts to dominate the fits at around $m_a\gtrsim 10\mathrm{neV}$, which is where $F$ ceases to change with $z$. Redshifts of blazars Mrk 501 (black) and 1ES $0229+200$ (purple) are shown for reference; these are the two blazars discussed in the context of CTA ALP searches in Ref. [23].

Figure 11

$F_{\text{cluster}}$ for cool core cluster parameters as used in Ref. [15], with $B_{\mathrm{rms}}=10\mu \mathrm{G}$. Dashed line shows current exclusions (see Sec. 1).

Figure 12

$F_{cj}\text{−}{F}_{\text{jet}}$ for cool core cluster parameters as used in Ref. [15], with $B_{rms}=10\mu \mathrm{G}$. $F_{cj}$ is the value of $F$ when the beam is propagated through the jet and the cluster, and $F_{\text{jet}}$ when it is propagated through just the jet. Dashed line shows current exclusions (see Sec. 1).