Microquasar observations with the MAGIC telescope

Microquasars, X-ray binaries displaying relativistic jets driven by accretion onto a compact object, are some of the most efficient accelerators in the Galaxy. Theoretical models predict Very High Energy (VHE) emission at the base of the jet where particles are accelerated to multi-TeV energies. This emission could be detected by present IACTs. %Moreover, gamma-ray fluxes should increase during flaring events when accretion rates are enhanced. The MAGIC telescope observed the microquasars GRS 1915+105, Cyg X-3, Cyg X-1 and SS433 for ~ 150 hours in total from 2005 to 2008. We triggered our observations by using multi wavelength information through radio flaring alerts with the RATAN telescope as well as by ensuring the low/hard state of the source through RXTE/ASM and Swift/BAT monitoring data. We report on the upper limits on steady and variable emission from these sources over this long period.

💡 Research Summary

The paper presents a comprehensive search for very‑high‑energy (VHE; >100 GeV) gamma‑ray emission from four well‑studied Galactic microquasars—GRS 1915+105, Cygnus X‑3, Cygnus X‑1, and SS 433—using the MAGIC imaging atmospheric Cherenkov telescope. Microquasars are X‑ray binaries that launch relativistic jets powered by accretion onto a compact object (black hole or neutron star). Theoretical models predict that particle acceleration at the jet base or at internal shocks can produce multi‑TeV electrons and protons, which in turn should generate detectable VHE photons via inverse‑Compton scattering, neutral‑pion decay, or synchrotron processes. Detecting such emission would provide a direct probe of jet physics, particle acceleration efficiency, and the coupling between accretion states and outflows.

Observations were carried out between 2005 and 2008 for a total exposure of roughly 150 hours. The authors adopted a two‑pronged trigger strategy. First, they used real‑time radio flare alerts from the RATAN‑600 telescope to identify epochs when the jet activity was heightened. Second, they monitored the X‑ray hardness ratio with RXTE/ASM and Swift/BAT to ensure that each source was in the low/hard state, a regime historically associated with steady compact jets. By combining these criteria, the team aimed to maximise the probability of catching VHE emission either as a persistent component or as a short‑lived flare coincident with radio outbursts.

Data reduction followed the standard MAGIC pipeline: image cleaning, Hillas parameterisation, and background suppression using a Random Forest classifier trained on Monte‑Carlo gamma‑ray simulations and real background data. Energy reconstruction covered the 80 GeV–10 TeV range, with an analysis threshold of ~200 GeV after quality cuts. The significance of any excess was evaluated using the Li & Ma method, and upper limits were derived at the 95 % confidence level using the Rolke approach.

No statistically significant VHE signal was found from any of the four microquasars, either in the time‑integrated data set or in time‑resolved searches around radio flares. The resulting integral flux upper limits (E > 200 GeV) are of order 2 × 10⁻¹² cm⁻² s⁻¹ for each source, corresponding to a few percent of the Crab Nebula flux. These limits are generally below the predictions of several hadronic and leptonic jet models that assume efficient particle acceleration during flaring episodes. In particular, the lack of detection during the radio‑flare windows suggests that either the VHE production efficiency is lower than expected, the emission region is heavily absorbed (e.g., by the intense stellar photon field in Cygnus X‑3), or the flares are intrinsically rare and short‑lived beyond the temporal resolution of the observations.

The authors compare their results with previous claims of transient VHE emission, notably the 2006 MAGIC detection of a brief flare from Cygnus X‑1. Their long‑term monitoring did not reproduce such an event, indicating that the earlier detection may represent an exceptional outlier rather than a common characteristic of microquasars. The paper also discusses systematic uncertainties, including atmospheric transmission, telescope calibration, and the assumed spectral shape (a power law with photon index –2.5) used for the upper‑limit calculation.

In the concluding section, the authors argue that the current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) is approaching the sensitivity required to test realistic microquasar emission models, but that further improvements are needed. They highlight the upcoming Cherenkov Telescope Array (CTA) as a crucial instrument: its order‑of‑magnitude better sensitivity, lower energy threshold, and faster slewing capabilities will enable both deeper steady‑state searches and rapid response to multi‑wavelength alerts. Additionally, they advocate for tighter coordination with radio, X‑ray, and possibly neutrino observatories to build a truly simultaneous multi‑messenger campaign, which would help disentangle the contributions of leptonic versus hadronic processes and clarify the role of accretion state transitions in governing VHE output.

Overall, the study provides the most stringent constraints to date on VHE gamma‑ray emission from the four canonical microquasars, demonstrates the effectiveness of a multi‑wavelength triggered observing strategy, and outlines a clear roadmap for future observations that could finally reveal the high‑energy secrets of relativistic jets in our Galaxy.