arXiv subjects

Marco Roncadelli

Publications and source records attributed to Marco Roncadelli.

At least 19 recordsLinked to original sources

arXiv2026

Where within the 3C 84 jet are $\gamma$-rays produced?

The location of $\gamma$-ray creation and emission within extra-galactic jets is a matter of active debate. One particularly well-suited source to pinpoint the location is the nearby, bright radio galaxy 3C 84, harbouring a powerful jet. Here we investigate the origin of $\gamma$-rays measured during a recent $\gamma$-ray flare, by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is overall almost unpolarised, we find that close-in-time to the $\gamma$-ray flare peak regions at parsec-scale distances from the central engine shows a fractional linear polarisation increase. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the $\gamma$-ray flare, and combined with insights from concurrent X-ray polarisation measurements, the $\gamma$-rays being created in this region is a physically motivated scenario, in a process consistent with synchrotron self-Compton.

astro-ph.HE↗

arXiv2025

Detection of Compton scattering in the jet of 3C 84

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $\gamma$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $\gamma$-rays. This is the longest observation for a radio-loud active galaxy that allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of $\rm\Pi_X=4.2\pm1.3\%$ ($\sim3.2\sigma$ confidence) at an X-ray electric vector polarization angle of $\rm \psi_X=163^{\circ}\pm9^{\circ}$, that is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons -- specifically Synchrotron Self-Compton -- that takes places downstream, away from the supermassive black hole.

astro-ph.HE↗

arXiv2025

Lorentz-Violating Scenarios for the Highest-Energy Photons from GRB 221009A

A photon at ${\cal E} \simeq 251 \, \rm TeV$ from GRB 221009A was detected by the Carpet collaboration in 2022 using a partial data set. Very recently, Carpet has completed its full data analysis reporting further support for its previous photon now at ${\cal E} = 300^{+ 43}_{- 38} \, {\rm TeV}$. Within standard propagation models, this observation is in strong tension with conventional expectations since such a photon is absorbed by the CMB. Further, we show that this detection is strongly disfavored within the explored scenarios involving axion-like particles (ALPs) alone. Instead, we find that the considered photon is compatible with specific Lorentz invariant violation (LIV) frameworks with the LIV scale obeying in the linear case ${\cal E}_{{\rm LIV}, 1} < 1.22_{-0.22}^{+0.19} \times 10^{21} \, {\rm GeV}$ at $95 \%$ CL and in the quadratic case ${\cal E}_{{\rm LIV}, 2} < 2.03_{-0.22}^{+0.17} \times 10^{13} \, {\rm GeV}$ at $95 \%$ CL. Finally, we outline scenarios where standard photon-ALP oscillations are combined with LIV-induced modifications of photon propagation, which provide a consistent interpretation of the observations of GRB 221009A including the highest energy photons detected by the LHAASO and Carpet collaborations.

astro-ph.HE↗

arXiv2025

Gamma Ray Burst GRB 221009A: two distinct hints at once at new physics

The brightest ever observed gamma ray burst GRB 221009A at redshift $z = 0.151$ was detected on October 9, 2022. Its highest energy photons have been recorded by the LHAASO collaboration up to above $12 \, \rm TeV$, and one of the at ${\cal E} = 251 \, \rm TeV$ by the Carpet-2 collaboration. Very recently, the Carpet-3 collaboration has completed the data analysis, showing that the evidence of the $251 \, {\rm TeV}$ photon is quite robust. Still, according to conventional physics photons with ${\cal E} \gtrsim 10 \, \rm TeV$ cannot be observed owing to the absorption by the extragalactic background light (EBL). Previously it has been demonstrated that an axion-like particle (ALP) with allowed parameters ensures the observability of the LHAASO photons. Here we show that the Lorentz invariance violation allows the ${\cal E} = 251 \, {\rm TeV}$ (now around 300 TeV) Carpet photon to be detected.

astro-ph.HE↗

arXiv2024

Hint at an axion-like particle from GRB 221009A

The detection by the LHAASO Collaboration of the gamma-ray burst GRB 221009A at redshift $z = 0.151$ with energies up to $(13-18) \, \rm TeV$ challenges conventional physics. Photons emitted with energies above $10 \, \rm TeV$ at this redshift can hardly be observed on Earth due to their interaction with the extragalactic background light (EBL). We show that indeed the LHAASO Collaboration should not have observed photons with energies above $10 \, \rm TeV$ if the state-of-the-art EBL model by Saldana-Lopez et al. is taken into account. A problem therefore arises: the Universe should be more transparent than currently believed. We also show that the issue is solved if we introduce the interaction of photons with axion-like particles (ALPs). ALPs are predicted by String Theory, are among the best candidates for dark matter and can produce spectral and polarization effects on astrophysical sources in the presence of external magnetic fields. In particular, for GRB 221009A, photon-ALP oscillations occur within the crossed magnetized media, i.e. the host galaxy, the extragalactic space, the Milky Way, partially reducing the EBL absorption to a level that explains the LHAASO detection of GRB 221009A and its observed spectrum without the need of contrived choices of parameter values, which are instead compulsory within proposed emission models within conventional physics. This fact regarding GRB 221009A represents a strong hint at the ALP existence, which adds to two other indications coming from blazars, a class of active galactic nuclei.

astro-ph.HE↗

arXiv2024

GRB multi-TeV detection: Beyond standard physics?

The recent detection by LHAASO up to 18 TeV of the gamma ray burst GRB 221009A at redshift $z = 0.151$ challenges standard physics because of the strong absorption due to the extragalactic background light (EBL) for photons with energies above 10 TeV. Emission models partially avoiding EBL absorption proposed to explain such an event are unsatisfactory since they require peculiar and contrived assumptions. By introducing in magnetized media the interaction of photons with axion-like particles (ALPs) - which are a generic prediction of most theories extending the standard model of particle physics towards a more satisfying theory - the detection of GRB 221009A can be naturally explained, thereby providing a strong hint at ALP existence.

astro-ph.HE↗

arXiv2023

ALP induced polarization effects on photons from blazars

Axion-like particles (ALPs), which are very light neutral spin zero elusive particles primarily interacting with two photons and predicted by superstring and superbrane theories, have come to help by solving two distinct problems about blazars (a type of active galactic nuclei), thus providing two hints at the existence of ALPs themselves. In the presence of an external magnetic field, ALPs produce: (i) photon-ALP oscillations, (ii) the change of the polarization state of photons. The former effect has many consequences in the astrophysical contest, such as the modification of the transparency of the Universe and the alteration of the astrophysical spectra. We address here the latter effect by analyzing how the photon degree of linear polarization and the polarization angle get modified by photon-ALP interaction in the case where photons are generated at the jet base of some BL Lacs (a blazar class): OJ 287, BL Lacertae, Markarian 501 and 1ES 0229+200, by considering both a leptonic and hadronic emission mechanism. We show that OJ 287 and BL Lacertae are good observational targets for ALP studies both in the X-ray band with IXPE (already operative) and with the proposed eXTP, XL-Calibur, NGXP and XPP missions and in the high-energy range with the COSI, e-ASTROGAM and AMEGO missions, while 1ES 0229+200 represents a strong candidate in the X-ray band only. Since these blazars show a very high final photon degree of linear polarization, which cannot be explained by conventional physics, such a possible detection would represent an additional hint at the ALP existence. Instead, Markarian 501 does not appear as a good target for these studies. We conclude that all these observatories can give us additional fundamental information about ALP physics.

astro-ph.HE↗

arXiv2022

Assessment of ALP scenarios for GRB 221009A

About one month after the revolutionary discovery of the Gamma Ray Burst (GRB) GRB 221009A and intense theoretical efforts to explain its detection, time seems to us ripe to make an assessment of the axion-like particle (ALP) based scenarios, since it is a common belief that conventional physics would have prevented such a detection. We overcome the almost complete lack of information -- so far only astronomical telegrams have been released -- by relying as much as possible upon the analogy with the emission from the GRB 190114C detected by the MAGIC collaboration in 2019, since it was the highest energy GRB detected before and for a time lapse similar to that over which GRB 221009A has been observed.

astro-ph.HE↗

arXiv2022

Observability of the very-high-energy emission from GRB 221009A

The LHAASO Collaboration detected the gamma ray burst GRB 221009A at energies above $500 \, {\rm GeV}$ with a tail extending up to $18 \, \rm TeV$, whose spectral analysis has presently been performed up to $7 \, \rm TeV$ for the lower energy instrument LHAASO-WCDA only, with no indication of a cutoff. Soon thereafter, Carpet-2 at Baksan Neutrino Observatory reported the observation of an air shower consistent with being caused by a photon of energy $251 \, {\rm TeV}$ from the same GRB. Given the source redshift $z=0.151$, the expected attenuation due to the extragalactic background light is very severe so that these detections have proven very hard to explain. In this Letter, we show that the existence of axion-like-particles (ALPs) with mass $m_a \simeq (10^{-11}-10^{-7}) \, {\rm eV}$ and two-photon coupling $g_{a \gamma \gamma} \simeq (3-5) \times 10^{-12} \, {\rm GeV}^{- 1}$ strongly reduce the optical depth of TeV photons, thus explaining the observations. Our ALPs meet all available constraints, are consistent with two previous hints at their existence and are good candidates for cold dark matter. Moreover, we show that Lorentz Invariance Violation (LIV) can explain the Carpet-2 result but not the LHAASO observations.

astro-ph.HE↗

arXiv2022

Axion-like Particles Implications for High-Energy Astrophysics

We offer a pedagogical introduction to axion-like particles (ALPs) as far as their relevance for high-energy astrophysics is concerned, from a few MeV to 1000 TeV. This review is self-contained, in such a way to be understandable even to non-specialists. Among other things, we discuss two strong hints at a specific ALP that emerge from two very different astrophysical situations. More technical matters are contained in three Appendices.

hep-ph↗

arXiv2022

ALP induced polarization effects on photons from galaxy clusters

Many extensions of the Standard Model of particle physics and in particular superstring and superbrane theories predict the existence of axion-like particles (ALPs). ALPs are very elusive, extremely light and interact primarily with photons. In the presence of an external magnetic field two effects show up: (i) photon-ALP oscillations and (ii) a change of the photon polarization state. The astrophysical context represents the best opportunity to get indirect evidence for the ALP existence thanks to various effects that the photon-ALP interaction produces in the sky. Great attention has been paid so far to photon-ALP oscillations, since they modify the transparency of the crossed media at very high energies and so the final spectra of faraway sources exhibit a flux excess and a characteristic oscillatory behavior. Two hints at the ALP existence have hitherto been discovered. But less interest has been attracted by the modification of the photon polarization. In this paper we address it in the X-ray and in the high energy (HE) bands. Specifically, we analyze the photon degree of linear polarization and the polarization angle induced by the photon-ALP interaction for photons generated in the central region of two galaxy clusters: Perseus and Coma. We find a substantial departure from conventional physics in both considered bands. We conclude that the ALP-induced polarization effects are more likely detectable with the proposed missions like COSI (approved to launch), e-ASTROGAM and AMEGO in the HE range. Still, possible ALP-induced effects on photon polarization could also be detected by IXPE (already operative) and by the proposed eXTP, XL-Calibur, NGXP and XPP in the X-ray band.

astro-ph.HE↗

arXiv2021

New strong constraints on the central behaviour of spherical galactic models -- No NFW cusp

We first stress that any spherically symmetric galactic model whose integrated mass profile $M (r) \to 0$ as $r \to 0$ is physically consistent close to the centre only provided that the circular velocity $v_c (r) \to 0$ and the gravitational field $g (r) \to 0$ as $r \to 0$. Next, we apply such a statement to a broad class of five-parameter spherical galactic models, which includes most of those used in astrophysics and cosmology. In particular, we discover that the Jaffe and Hernquist models can only be trusted for $r \gtrsim 0.2 \, R_e$, while the NFW model cannot describe the central region either of regular galaxy clusters or of pure dark matter halos, thereby failing to predict any central cusp.

astro-ph.CO↗

arXiv2021

Precision cosmology made more precise

So far, the standard attitude to solve the Friedmann equations in the simultaneous presence of radiation $R$, matter $M$ and cosmological constant $Λ$ is to find solutions $R_R (t)$, $R_M (t)$ and $R_Λ (t)$ separately for each individual component alone, and next to join them together, thereby obtaining a piecewise solution $R_{\rm pw} (t)$. We instead find the exact and analytic solution $R (t)$ of the same equations in flat space. Moreover, we quantify the error made when $R_{\rm pw} (t)$ is used in place of $R (t)$.

astro-ph.CO↗

arXiv2020

Hint at an axion-like particle from the redshift dependence of blazar spectra

We consider the largest observed sample including all intermediate-frequency peaked (IBL) and high-frequency peaked (HBL) flaring blazars above 100 GeV up to redshift $z = 0.6$. We show that the best-fit regression line of the emitted spectral indices $Γ_{\rm em} (z)$ is a concave parabola decreasing as $z$ increases, thereby implying a statistical correlation between the $\{Γ_{\rm em} (z) \}$ distribution and $z$. This result contradicts our expectation that such a distribution should be $z$-independent. We argue that the above correlation does not arise from any selection bias. We show that our expectation naturally emerges provided that axion-like particles (ALPs) are put into the game. Moreover, ALPs can also explain why flat spectrum radio quasars emit up to 400 GeV, in sharp contradiction with conventional physics. So, the combination of the two very different but consistent results -- taken at face value -- leads to a hint at an ALP with mass $m = {\cal O} (10^{-10} \, {\rm eV})$ and two-photon coupling in the range $2.94 \times 10^{- 12} \, {\rm GeV}^{- 1} < g_{a γγ} < 0.66 \times 10^{- 10} \, {\rm GeV}^{- 1}$. As a bonus, the Universe would become considerably more transparent above energies $E \gtrsim 1 \, {\rm TeV}$ than dictated by conventional physics. Our prediction can be checked not only by the new generation of observatories like CTA, HAWC, GAMMA-400, LHAASO, TAIGA-HiSCORE and HERD, but also thanks to the planned laboratory experiments ALPS II (upgraded), STAX, IAXO and with other techniques now being developed by Avignone and collaborators.

astro-ph.HE↗

arXiv2019

Estimating $γγ$ absorption for UHE photons with lepton and hadron production

Surprisingly, the contribution to the cosmic opacity to UHE ($E>10^{18} \, \rm eV$) $γ$-rays has been systematically computed so far for the $γγ\to e^+e^-$ and $γγ\to e^+ e^- e^+ e^-$ processes alone. We go a step further by systematically evaluating the additional opacity brought about by other leptons and hadrons. We find that the dominant channels are those leading to the production of $μ^{\pm}$ and hadrons (mainly $π^{\pm}$, $π^0$, $K^{\pm}$, $K^0$, $η$). For nearly the GZK radius, the photon survival probability becomes smaller by about a factor of two with respect to current estimates.

astro-ph.HE↗

arXiv2019

Extragalactic photon--axion-like particle oscillations up to 1000 TeV

Axion-like particles (ALPs) are attracting increasing interest since, among other things, they are a prediction of many extensions of the standard model of elementary particles physics and in particular of superstrings and superbranes. Remarkably, depending on the set of their parameter space, they strongly increase the photon transparency in the very-high energy band. The recent discovery of photon dispersion on the CMB requires a substantial modification of the previous picture: this is indeed the goal of the present paper. We compute the photon survival probability from a blazar to us exactly, and we plot it versus the observed energy for 7 simulated blazars at different $z$ and 4 values of a model parameter. Our predictions can be tested by the new generation of $γ$-ray observatories like CTA, HAWC, GAMMA-400, LHAASO, TAIGA-HiSCORE and HERD. Finally, for our guessed values of $m_a$ and $g_{γγa}$ our ALP can be detected in the upgrade of ALPS II at DESY, the planned experiments IAXO, STAX and ABRACADABRA as well as with other techniques.

astro-ph.HE↗

arXiv2019

Behavior of axion-like particles in smoothed out domain-like magnetic fields

Basically, in certain circumstances axion-like particles (ALPs) substantially enhance the photon survival probability $P_{γ\to γ} ({\cal E})$ of a beam emitted by a far-away source through the mechanism of photon-ALP oscillation (${\cal E}$ denotes the energy). But in order for this to work, an external magnetic field ${\bf B}$ must be present. In several cases ${\bf B}$ is modeled as a domain-like network with `sharp edges': all domains have the same size $L_{\rm dom}$ (set by the ${\bf B}$ coherence length) and the same strength B, but the direction of ${\bf B}$ changes randomly and abruptly from one domain to the next. It is obviously a highly mathematical idealization wherein the components of ${\bf B}$ are discontinuous across the edges (whence the name sharp edges). It is therefore highly desirable to go a step further, and to find out what happens when the edges are smoothed out, namely when the abrupt change of ${\bf B}$ is replaced by a smooth one. Moreover, this step becomes compelling when the photon-ALP oscillation length $l_{\rm osc}$ turns out to be comparable to -- or smaller than -- $L_{\rm dom}$, because in this case the photon survival probability $P_{γ\to γ} ({\cal E})$ critically depends on the domain shape. In the present paper we propose a smoothed out version of the previous domain-like structure of ${\bf B}$ which incorporates the above changes, and we work out its implications. Even in the present case we are able to solve analytically and exactly the photon/ALP beam propagation equation inside a single smoothed-out domain, thereby evaluating the corresponding $P_{γ\to γ} ({\cal E})$ exactly. Our results is of particular importance in view of the new generation of gamma-ray detectors, since in such a situation $l_{\rm osc} \lesssim L_{\rm dom}$ occurs just above the TeV scale.

astro-ph.HE↗

arXiv2019

Blazar VHE spectral alterations induced by photon-ALP oscillations

Prompted by the increasing interest of axion-like particles (ALPs) for very-high-energy (VHE) astrophysics, we have considered a full scenario for the propagation of a VHE photon/ALP beam emitted by a BL Lac and reaching us in the light of the most up-to-date astrophysical information and for energies up to above $100 \, \rm TeV$. During its trip, the beam -- generated in a small region of a BL Lac jet -- crosses a variety of magnetic structures in very different astronomical environments: the BL Lac jet, the host elliptical galaxy, the extragalactic space and the Milky Way. We have taken an effort to model all these magnetic fields in the most realistic fashion and using a new model developed by us concerning the extragalactic magnetic field. Assuming an intrinsic spectrum with a power law exponentially truncated at a fixed cut-off energy, we have evaluated the resulting observed spectra of Markarian 501, the extreme BL Lac 1ES 0229+200 and a similar source located at $z = 0.6$ up to above $100 \, \rm TeV$. We obtain interesting results: the model with photon-ALP oscillations possesses features (spectral energy oscillatory behaviour and photon excess above $20 \, \rm TeV$) which can be tested by $γ$-ray observatories like CTA, HAWC, GAMMA 400, LHAASO, TAIGA-HiSCORE and HERD. In addition, our ALP can be detected in dedicated laboratory experiments like the upgrade of ALPS II at DESY, the planned IAXO and STAX experiments, as well as with other techniques developed by Avignone and collaborators.

astro-ph.HE↗