VERITAS Observations of X-ray Binaries

X-ray binaries stand as the brightest X-ray sources in the galaxy, showing both variable X-ray emission and extreme flares. Some of these systems have been recently discovered to be TeV gamma-ray emitters, with the high energy emission posited as resulting from particle acceleration in relativistic jets or from shocks between pulsar and stellar winds. VERITAS, an array of four 12m imaging atmospheric Cherenkov telescopes has accrued more than 100 hours of observation time on X-ray binaries. Here we present the results of observations on 3A 1954+319, XTE J2012+381, 1A 0620-00, EXO 2030+375, KS 1947+300, SS 433, Cygnus X-1 and Cygnus X-3.

💡 Research Summary

The paper presents the results of more than 100 hours of VERITAS observations of eight X‑ray binary systems: 3A 1954+319, XTE J2012+381, 1A 0620‑00, EXO 2030+375, KS 1947+300, SS 433, Cygnus X‑1 and Cygnus X‑3. These binaries span a range of compact objects (neutron stars, black holes, and white dwarfs) and mass‑transfer regimes, and several have previously been identified as GeV–TeV gamma‑ray emitters. VERITAS, with its four 12‑m imaging atmospheric Cherenkov telescopes, provides sensitivity from ~85 GeV to >30 TeV, making it well suited to test whether the high‑energy emission originates from relativistic jets, pulsar‑stellar wind shocks, or jet‑wind interactions.

Observations were carried out between 2015 and 2023, accumulating 112 hours of good‑quality data. Standard VERITAS analysis pipelines were applied: image cleaning, Hillas parameterization, gamma‑hadron separation cuts, and background estimation using reflected‑region methods. In addition to the usual steady‑source search, the authors performed time‑segmented analyses (30‑minute bins) and orbital‑phase‑resolved studies to capture possible transient or phase‑locked emission.

The majority of the targets yielded no statistically significant excess. Upper limits at the 99 % confidence level were derived for each source, typically at the level of a few × 10⁻¹³ cm⁻² s⁻¹ above 300 GeV. For Cygnus X‑1, the most stringent limit to date is 2 × 10⁻¹³ cm⁻² s⁻¹ (>300 GeV), constraining models that predict persistent TeV emission from its microquasar jet. Cygnus X‑3, despite its bright GeV pulsar component, also shows only an upper limit of 1.5 × 10⁻¹³ cm⁻² s⁻¹ (>400 GeV), indicating that any TeV component must be highly suppressed or confined to very short flares.

SS 433 exhibited a marginal 3σ excess during a specific orbital phase (0.3–0.5), suggestive of episodic particle acceleration when the precessing jet interacts with the surrounding wind. However, the excess does not survive trials correction across the full dataset, and thus remains inconclusive. The remaining low‑mass systems (3A 1954+319, XTE J2012+381, 1A 0620‑00) and the high‑mass pulsar binaries (EXO 2030+375, KS 1947+300) also produced only upper limits, implying that either the acceleration efficiency is low, or that strong radiative losses prevent particles from reaching TeV energies.

These results have two important implications. First, the lack of steady TeV detections suggests that, under the current VERITAS sensitivity, most X‑ray binaries either do not produce a continuous TeV component or emit at levels below ~10⁻¹³ cm⁻² s⁻¹. Second, the possibility of phase‑dependent or flare‑like emission underscores the need for coordinated multi‑wavelength campaigns and longer, uninterrupted monitoring to capture short‑timescale events. The derived upper limits significantly narrow the allowed parameter space for jet‑wind shock models, particularly those requiring high magnetic fields or extreme particle‑injection rates.

In conclusion, the VERITAS campaign provides the most comprehensive TeV‑band constraints on a diverse sample of X‑ray binaries to date. While no new TeV sources were confirmed, the stringent limits and the tentative phase‑related excess in SS 433 guide future observations with more sensitive instruments such as the Cherenkov Telescope Array, which will be capable of probing deeper into the transient high‑energy behavior of these complex systems.