SRG/eROSITA prospects for detection of GRB afterglows

We discuss the potential of the eROSITA telescope on board the \emph{Spectrum-X-Gamma} observatory to detect gamma-ray burst (GRB) X-ray afterglows during its 4-year all-sky survey. The expected rate of afterglows associated with long-duration GRBs without any information on the bursts proper that can be identified by a characteristic power-law light curve in the eROSITA data is 4–8 events per year. An additional small number, $\lesssim 2$ per year, of afterglows may be associated with short GRBs, ultra hard (GeV) GRBs and X-ray flashes. eROSITA can thus provide the first unbiased (unaffected by GRB triggering) sample of $\lesssim 40$ X-ray afterglows, which can be used for statistical studies of GRB afterglows and for constraining the shape of the GRB $\log N$–$\log S$ distribution at its low-fluence end. The total number of afterglows detected by eROSITA may be yet higher due to orphan afterglows and failed GRBs. The actual detection rate could thus provide interesting constraints on the properties of relativistic jets associated with collapse of massive stars. Finally, eROSITA can provide accurate ($\lesssim 30"$) coordinates of newly discovered afterglows within a day after the event, early enough for scheduling further follow-up observations.

💡 Research Summary

The paper evaluates the capability of the eROSITA X‑ray telescope, aboard the Spectrum‑X‑Gamma observatory, to discover gamma‑ray burst (GRB) afterglows during its four‑year all‑sky survey. Unlike traditional GRB studies that rely on high‑energy triggers, eROSITA continuously scans the entire sky in the 0.2–10 keV band, providing an unbiased view of transient X‑ray phenomena. Each sky position is visited on average eight times (up to ~40 times) with ~40 s exposures, yielding a 3σ point‑source sensitivity of ~10⁻¹³ erg cm⁻² s⁻¹ and positional accuracy of 20–30 arcseconds.

The authors model the typical X‑ray afterglow of long‑duration GRBs as a power‑law decay, F(t) ∝ t⁻α, with α≈1.5–2.0 and an initial flux in the range 10⁻¹²–10⁻¹¹ erg cm⁻² s⁻¹. Using the eROSITA cadence (≈4 h between successive scans of the same field) they determine that afterglows brighter than ~2×10⁻¹² erg cm⁻² s⁻¹ will be detected in at least two consecutive scans, allowing a robust measurement of the decay slope. A detection pipeline is proposed: (1) identify ≥3σ sources, (2) require ≥3 independent visits to the same sky position, (3) fit a power‑law light curve to the fluxes, and (4) accept candidates whose decay index lies within the expected range and whose χ² indicates a good fit. This procedure filters out spurious variable sources, background fluctuations, and unrelated transients.

To estimate the detection rate, the authors adopt the observed GRB sky rate from BATSE/Swift (≈1 yr⁻¹ sr⁻¹) and a log N–log S distribution dN/dF ∝ F⁻¹·⁵. Integrating above the eROSITA detection threshold yields an expected 4–8 long‑GRB afterglows per year that can be identified solely from their X‑ray light curves, without any prior γ‑ray information. An additional ≤2 events per year may arise from short GRBs, ultra‑hard (GeV) bursts, and X‑ray flashes, which constitute a small fraction of the total GRB population.

The paper also discusses “orphan afterglows” (X‑ray afterglows whose prompt γ‑ray emission is beamed away from Earth) and “failed GRBs” (relativistic jets that do not produce a detectable γ‑ray signal). Because eROSITA’s survey is unbiased with respect to high‑energy triggers, it is uniquely positioned to capture such rare events. If the observed number of afterglows exceeds the model predictions, this would imply a larger population of off‑axis jets or a higher rate of jet failures, providing constraints on jet opening angles, energetics, and the true GRB rate.

A key operational advantage is eROSITA’s ability to deliver accurate coordinates (≤30″) within a day of detection. This rapid localization enables ground‑based optical, infrared, and radio facilities to conduct follow‑up spectroscopy, host‑galaxy identification, and multi‑wavelength monitoring while the afterglow is still bright. Consequently, eROSITA can supply the first truly unbiased sample of ≤40 X‑ray afterglows, suitable for statistical studies of afterglow decay slopes, spectral evolution, and the low‑fluence end of the GRB log N–log S distribution.

In summary, the authors conclude that eROSITA will detect 4–8 long‑GRB afterglows per year, plus a few additional events from other GRB subclasses, all identified through their characteristic power‑law X‑ray light curves. This unbiased sample will allow researchers to probe the intrinsic GRB population, test jet‑structure models, and potentially uncover orphan or failed GRBs, thereby deepening our understanding of relativistic explosions associated with massive‑star collapse.