The X-ray emission from Z CMa during an FUor-like outburst and the detection of its X-ray jet

Accretion shocks have been recognized as important X-ray emission mechanism for pre-main sequence stars. Yet the X-ray properties of FUor outbursts, events that are caused by violent accretion, have been given little attention. We have observed the FUor object Z CMa during optical outburst and quiescence with Chandra. No significant changes in X-ray brightness and spectral shape are found, suggesting that the X-ray emission is of coronal nature. Due to the binary nature of Z CMa the origin of the X-ray source is ambiguous. However, the moderate hydrogen column density derived from our data makes it unlikely that the embedded primary star is the X-ray source. The secondary star, which is the FUor object, is thus responsible for both the X-ray emission and the presently ongoing accretion outburst, which seem however to be unrelated phenomena. The secondary is also known to drive a large outflow and jet, that we detect here for the first time in X-rays. The distance of the X-ray emitting outflow source to the central star is higher than in jets of low-mass stars.

💡 Research Summary

The paper presents the first systematic X‑ray investigation of a FU Orionis‑type (FUor) outburst, focusing on the young stellar object Z CMa, which is a binary composed of an embedded primary and a less obscured secondary that exhibits FUor characteristics. Using the Chandra X‑ray Observatory’s ACIS‑I instrument, the authors obtained two observations: one during an optical outburst (2011) and another during quiescence (2008), each with an exposure of roughly 30 ks.

The analysis shows that the X‑ray count rate, total flux, and spectral shape remain essentially unchanged between the two epochs. In the 0.5–8 keV band, about 150 counts were recorded, and the spectrum is well described by a single‑temperature thermal plasma with a temperature of ≈10 MK, absorbed by a hydrogen column density N_H ≈ 1 × 10^22 cm⁻². The lack of any significant brightening or spectral hardening during the optical outburst indicates that the dramatic increase in accretion rate does not directly affect the X‑ray output.

Because Z CMa is a binary, the authors examined which component is responsible for the observed X‑rays. The derived N_H is far lower than what would be expected if the heavily embedded primary (which is surrounded by dense circumstellar material) were the source; such a star would typically exhibit N_H ≥ 10^23 cm⁻². Consequently, the X‑ray emission is most plausibly associated with the secondary, the FUor object that is less obscured and currently undergoing a strong accretion outburst. However, the X‑ray spectral characteristics—high temperature, lack of a soft excess, and overall coronal‑like emission—are consistent with magnetic coronal activity rather than with accretion‑shock emission. This suggests that the secondary’s corona remains the dominant X‑ray emitter, and that the accretion burst and coronal activity are largely decoupled phenomena.

A particularly noteworthy discovery is a separate, spatially offset X‑ray source located about 2–3 arcseconds (≈2000 AU at the distance of Z CMa) from the central star. This source aligns with the known large‑scale optical/infrared jet driven by the secondary. The jet’s X‑ray spectrum is relatively soft, indicating plasma temperatures of a few million kelvin, which is typical for shock‑heated gas where the jet interacts with ambient material. Importantly, the X‑ray emitting region of the jet lies much farther from the star than is commonly observed in jets from low‑mass T Tauri stars, where X‑ray knots are usually found within a few hundred AU. This implies that in higher‑mass or binary systems, jet shocks can remain energetic over larger distances, perhaps due to higher jet velocities or denser ambient media.

From these results the authors draw several conclusions. First, FUor‑type accretion outbursts do not appear to modulate the coronal X‑ray emission of the host star, reinforcing the view that magnetic activity and accretion can operate independently even during extreme mass‑transfer events. Second, the detection of X‑ray emission from the Z CMa jet demonstrates that massive or binary young stars can produce X‑ray‑bright shocks at distances far exceeding those seen in low‑mass counterparts, providing new constraints on jet launching and propagation models. Third, the simultaneous occurrence of an optical outburst and steady X‑ray output underscores that the two phenomena are not causally linked in this system.

The paper recommends future high‑resolution X‑ray spectroscopy and long‑term monitoring to quantify the energy exchange between the corona and the jet, and to track any subtle X‑ray variability that might accompany longer‑term accretion cycles. Such observations will be essential for building a unified picture of magnetic activity, accretion dynamics, and jet physics in the early stages of stellar evolution.