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Register a new userDetectability of Pair Echos from Gamma-Ray Bursts and Intergalactic Magnetic Fields
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High-energy emission from gamma-ray bursts (GRBs) can give rise to pair echos, i.e. delayed inverse Compton emission from secondary $e^{pm}$ pairs produced in $gamma-gamma$ interactions with intergalactic background radiation. We investigate the detectability of such emission with modern-day gamma-ray telescopes. The spectra and light curves are calculated for a wide range of parameters, applying the formalism recently developed by Ichiki et al. The flux depends strongly on the unknown magnitude and coherence length of intergalactic magnetic fields, and we delineate the range of field strength and redshift that allow detectable echos. Relevant uncertainties such as the high-energy cutoff of the primary gamma-ray spectrum and the intensity of the cosmic infrared background are addressed. GLAST and MAGIC may be able to detect pair echo emission from GRBs with redshift $lesssim 1$ if the primary spectra extend to $sim 10 ~ {rm TeV}$.
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We investigate the delayed, secondary GeV-TeV emission of gamma-ray bursts and its potential to probe the nature of intergalactic magnetic fields. Geometrical effects are properly taken into account for the time delay between primary high energy photons and secondary inverse Compton photons from electron-positron pairs, which are produced in $gamma$-$gamma$ interactions with background radiation fields and deflected by intervening magnetic fields. The time-dependent spectra of the delayed emission are evaluated for a wide range of magnetic field strengths and redshifts. The typical flux and delay time of secondary photons from bursts at $z sim 1$ are respectively $sim 10^{-8}$ GeV cm$^{-2}$ s$^{-1}$ and $sim 10^4$ s if the field strengths are $sim 10^{-18}$ G, as might be the case in intergalactic void regions. We find crucial differences between the cases of coherent and tangled magnetic fields, as well as dependences on the field coherence length.
Several high-frequency peaked BL Lac objects such as Mrk 501 are strong TeV emitters. However, a significant fraction of the TeV gamma rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding electron-positron pairs. Hence, the observed TeV spectrum must be steeper than the intrinsic one. Using the recently derived intrinsic $gamma$-ray spectrum of Mrk 501 during its 1997 high state, we study the inverse-Compton scattering of cosmic microwave photons by the resulting electron-positron pairs, which implies the existence of a hitherto undiscovered GeV emission. The typical duration of the GeV emission is determined by the flaring activity time and the energy-dependent magnetic deflection time. We numerically calculate the scattered photon spectrum for different intergalactic magnetic field (IGMF) strengths, and find a spectral turnover and flare duration at GeV energies which are dependent on the field strength. We also estimate the scattered photon flux in the quiescent state of Mrk 501. The GeV flux levels predicted are consistent with existing EGRET upper limits, and should be detectable above the synchrotron -- self Compton (SSC) component with the {em Gamma-Ray Large Area Space Telescope} ({em GLAST}) for IGMFs $lesssim 10^{-16}$ G, as expected in voids. Such detections would provide constraints on the strength of weak IGMFs.
Intergalactic magnetic fields (IGMF) can cause the appearance of halos around the gamma-ray images of distant objects because an electromagnetic cascade initiated by a high-energy gamma-ray interaction with the photon background is broadened by magnetic deflections. We report evidence of such gamma-ray halos in the stacked images of the 170 brightest active galactic nuclei (AGN) in the 11-month source catalog of the Fermi Gamma-Ray Space Telescope. Excess over point spread function in the surface brightness profile is statistically significant at 3.5sigma (99.95% confidence level), for the nearby, hard population of AGN. The halo size and brightness are consistent with IGMF, B_{IGMF} ~ 10^{-15} G. The knowledge of IGMF will facilitate the future gamma-ray and charged-particle astronomy. Furthermore, since IGMF are likely to originate from the primordial seed fields created shortly after the Big Bang, this potentially opens a new window on the origin of cosmological magnetic fields, inflation, and the phase transitions in the early Universe.
More than a dozen blazars are known to be emitters of multi-TeV gamma rays, often with strong and rapid flaring activity. By interacting with photons of the cosmic microwave and infrared backgrounds, these gamma rays inevitably produce electron-positron pairs, which in turn radiate secondary inverse Compton gamma rays in the GeV-TeV range with a characteristic time delay that depends on the properties of the intergalactic magnetic field (IGMF). For sufficiently weak IGMF, such pair echo emission may be detectable by the Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on the IGMF. We perform detailed calculations of the time-dependent spectra of pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking proper account of the echo geometry and other crucial effects. In some cases, the presence of a weak but non-zero IGMF may enhance the detectability of echos. We discuss the quantitative constraints that can be imposed on the IGMF from GLAST observations, including the case of non-detections.
We study the generation of intergalactic magnetic fields in two models for first-order phase transitions in the early Universe that have been studied previously in connection with the generation of gravitational waves (GWs): the Standard Model supplemented by an $|H|^6$ operator (SM+$H^6$) and a classically scale-invariant model with an extra gauged U(1) $B - L$ symmetry (SM$_{B-L}$). We consider contributions to magnetic fields generated by bubble collisions and by turbulence in the primordial plasma, and we consider the hypotheses that helicity is seeded in the gauge field or kinetically. We study the conditions under which the intergalactic magnetic fields generated may be larger than the lower bounds from blazar observations, and correlate them with the observability of GWs and possible collider signatures. In the SM+$H^6$ model bubble collisions alone cannot yield large enough magnetic fields, whereas turbulence may do so. In the SM$_{B-L}$ model bubble collisions and turbulence may both yield magnetic fields above the blazar bound unless the B$-$L gauge boson is very heavy. In both models there may be observable GW and collider signatures if sufficiently large magnetic fields are generated.
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