Vittoria Vecchiotti

Context. Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration reported ultrahigh-energy (UHE; > 100 TeV) gamma rays from five microquasars. In all five sources, the emission extends beyond 100 TeV, making microquasars promising candidates for Galactic PeVatrons. Aims. We investigated whether gamma rays around 100 TeV originate from hadronic interactions of accelerated cosmic rays with the ambient medium, and we estimated the contribution of these sources to the measured cosmic-ray proton spectrum around the knee. We also derived upper limits on the contribution from the seven additional microquasars in the LHAASO field of view, from which no UHE emission was detected. Methods. We considered a cosmic-ray transport scenario in which particle propagation in the source vicinity is diffusion-dominated, with a diffusion coefficient largely suppressed with respect to the average Galactic value, while, at large distances from the source, the diffusion coefficient matches the Galactic value. In addition, we assumed continuous injection over timescales of t _age = 0.1 − 1 Myr. Results. Using available measurements of the gas density in the emission region, we find that hadronic interactions alone cannot fully account for the observed emission for any of the detected sources. However, in the case of GRS 1915+105 and MAXI J1820+070, the hadronic scenario may still be valid when considering acceleration efficiency higher than 10%. We subsequently derived upper limits on the hadronic contribution to the observed gamma-ray flux. We estimate that the detected sources contribute at most ∼4% of the Galactic cosmic-ray spectrum at 1 PeV for an injection timescale of 0.1 Myr and a cosmic-ray acceleration efficiency of 10%. When adopting the maximum acceleration efficiency allowed by the gamma-ray observations, instead, the contribution of these microquasars to the Galactic cosmic-ray spectrum at PeV energies rises to 37%. Longer injection timescales (∼1 Myr) lead to contributions exceeding the observational constraints. For sources not detected at UHE, we obtain a maximum contribution of ∼17%, achieved assuming continuous injection over 1 Myr.

Compact binary millisecond pulsars (also known as spiders) allow us to probe pulsar winds in their innermost regions, between the light cylinder (radius ∼10 ⁷ cm) and the companion star (at ∼10 ¹¹ cm). Their flux is known to vary along the orbit, from radio to X-rays. During the past decade, gamma-ray orbital modulation (GOM) has been discovered in a handful of spiders, but its origin remains largely unknown. We present the results of a systematic search for GOM among 43 systems, selecting pulsed 0.1–1 GeV photons and using spin and orbital ephemeris from Fermi’s Third Pulsar Catalog. We discover GOM from three spiders—PSR J1124-3653, PSR J1946-5403, and PSR J2215+5135—and confirm four previous detections. In all seven cases so far, the GOM peaks near the pulsar’s superior conjunction. The X-ray orbital light curves are usually in antiphase, peaking when the pulsar is at inferior conjunction, but we find one case where both gamma rays and X-rays peak around superior conjunction: PSR J1946-5403. We measure the modulated fractions of the GOM and find consistent values for all seven spiders, with an average 22.0% ± 2.6%. Including eclipsing systems seen edge-on, we find no clear dependence of the modulated fraction on the orbital inclination (within ≃45°–90°). Our results challenge previous models proposed to explain GOM in spiders, based on inverse Compton and synchrotron emission close to the companion, since these predict a clear dependence on orbital inclination (stronger modulation at high inclinations). We nearly double the number of GOM detections in spiders, showing that it is more common than previously thought.

Black widow and redback systems are millisecond pulsars in compact orbits with ultra-light and low-mass companions, respectively, collectively known as “spider pulsars.” In such systems, an intrabinary shock can form between the pulsar and the companion winds, serving as a site for particle acceleration and associated nonthermal emission. Assuming that protons can be extracted from the neutron star surface and accelerated at the intrabinary shock and/or within the pulsar wind, we model the very high-energy gamma-ray and neutrino emissions (0.1–10 ³ TeV) produced through interactions with the companion wind and the companion star. We first calculate the high-energy emissions using an optimistic combination of parameters to maximize the gamma-ray and neutrino fluxes. We find that, for energetic spider pulsars with a spin-down power ≳10 ³⁵ erg s ⁻¹ and a magnetic field of ∼10 ³ G in the companion region, the gamma-ray emission could be detectable as point sources by the Cherenkov Telescope Array and the Large High Altitude Air Shower Observatory, while the neutrino emission could be detectable by the future TRIDENT detector. Finally, we build a synthetic population of these systems, compute the cumulative neutrino flux expected from spider pulsars, and compare it with the Galactic neutrino diffuse emission measured by IceCube. We find that, under realistic assumptions on the fraction of the spin-down power converted into protons, the contribution of spiders to the diffuse Galactic neutrino flux is negligible.

Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration has obtained a measurement of the gamma-ray diffuse emission in the ultra-high energy range, 10–10^3 TeV after masking the contribution of known sources. The measurement is provided in two Galactic regions and appears to be 2–3 times higher than the gamma-ray signal expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially implying that either additional emission sources exist or cosmic ray intensities have spatial variations. In this work, we calculate the hadronic gamma-ray diffuse emission outside the masks, considering a realistic gas distribution. We present a comprehensive calculation of the emission, which includes systematic uncertainties in the gas content of the Galactic disk, in the energy and spatial distribution of cosmic rays, as well as in the hadronic interaction cross-sections. Our results show that these factors mitigate the tension between data and predictions. The LHAASO data appear compatible with our baseline model in the outer Galactic region. In the inner region, the data show an excess with respect to the predictions below ∼ 50 TeV, while at higher energies they are well described by our model. We argue that two plausible explanations for enhanced gamma-ray emission — unresolved sources and CR spectral hardening in the inner Galaxy — are likely suppressed by the LHAASO masking strategy, which excludes regions where both effects are expected to be most prominent.

We perform the first direct consistency check between the recently measured proton spectrum at the knee by Large High Altitude Air Shower Observatory (LHAASO) and the collaboration’s own high-precision mapping of Galactic diffuse gamma-ray emission. By modelling the hadronic gamma-ray production using the updated cosmic-ray spectra, gas templates, and cross-section models, we show that the predicted gamma-ray flux robustly overshoots the LHAASO data in both inner and lateral Galactic regions. This persistent mismatch in both normalization and spectral shape challenges conventional scenarios linking the local cosmic-ray sea to Galactic gamma-ray emission, and calls for a revision of current cosmic ray models in the TeV–PeV sky.

We perform the first direct consistency check between the recently measured proton spectrum at the knee by LHAASO and the collaboration's own high-precision mapping of Galactic diffuse gamma-ray emission. By modeling the hadronic gamma-ray production using the updated cosmic-ray spectra, gas templates and cross-section models, we show that the predicted gamma-ray flux robustly overshoots the LHAASO data in both inner and lateral Galactic regions. This persistent mismatch in both normalization and spectral shape challenges conventional scenarios linking the local cosmic-ray sea to Galactic gamma-ray emission, and calls for a revision of current cosmic ray models in the TeV-PeV sky.

The Large High-Altitude Air Shower Observatory (LHAASO) collaboration has recently measured the ultra-high energy gamma-ray diffuse emission (10 − 10³ TeV) after masking the contribution of known sources. The observed signal appears to be 2 − 3 times higher than expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially suggesting a contribution from unresolved sources. However, estimates of the diffuse emission are affected by large uncertainties. In this work, we calculate the hadronic gamma-ray diffuse emission, accounting for uncertainties in the gas content of the Galactic disk, the energy and spatial distribution of cosmic rays, and the hadronic interaction cross-section. We show that the LHAASO data above ∼ 30 TeV are consistent with this model, not requiring (nor probing) the existence of any further contribution due to unresolved sources or cosmic ray spectral variations in the inner Galaxy.

IceCube collaboration reported the first high-significance observation of the neutrino emission from the Galactic disk. The observed signal can be due to diffuse emission produced by cosmic rays interacting with interstellar gas but can also arise from a population of sources. In this paper, we evaluate both the diffuse and source contribution by taking advantage of gamma-ray observations and/or theoretical considerations. By comparing our expectations with IceCube measurements, we constrain the fraction of Galactic TeV gamma-ray sources (resolved and unresolved) with hadronic nature. In order to be compatible with the IceCube results, this fraction should be small, or the source proton energy cutoff should be well below the cosmic-ray proton knee. In particular, for a cutoff energy equal to 500 TeV, the fraction of hadronic sources should be less than ∼40% corresponding to a cumulative source flux Φ _ν,s ≤ 2.6 × 10⁻¹⁰ cm⁻² s⁻¹ integrated in the 1–100 TeV energy range. This fraction reduces to ∼20% for energy cutoff reaching the cosmic-ray proton knee around 5 PeV.