H.E.S.S. observations of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way at a distance of approximately 48 kpc. Despite its distance it harbours several interesting targets for TeV gamma-ray observations. The composite supernova remnant N 157B/PSR J05367-6910 was discovered by H.E.S.S. being an emitter of very high energy (VHE) gamma-rays. It is the most distant pulsar wind nebula ever detected in VHE gamma-rays. Another very exciting target is SN 1987A, the remnant of the most recent supernova explosion that occurred in the neighbourhood of the Milky Way. Models for Cosmic Ray acceleration in this remnant predict gamma-ray emission at a level detectable by H.E.S.S. but this has not been detected so far. Fermi/LAT discovered diffuse high energy (HE) gamma-ray emission from the general direction of the massive star forming region 30 Doradus but no clear evidence for emission from either N 157B or SN 1987A has been published. The part of the LMC containing these objects has been observed regularly with the H.E.S.S. telescopes since 2003. With deep observations carried out in 2010 a very good exposure of this part of the sky has been obtained. The current status of the H.E.S.S. LMC observations is reported along with new results on N 157B and SN 1987A.

💡 Research Summary

The Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way located at a distance of roughly 48 kpc, offers a unique laboratory for very‑high‑energy (VHE) gamma‑ray astronomy despite its modest proximity. This paper presents the results of an extensive observational campaign with the High Energy Stereoscopic System (H.E.S.S.) that has been monitoring the LMC region since 2003, with a particularly deep exposure obtained in 2010. The focus is on three astrophysical objects of high interest: the composite supernova remnant N 157B (hosting the young, energetic pulsar PSR J0536‑6910), the historic supernova remnant SN 1987A, and the massive star‑forming complex 30 Doradus, which is known from Fermi‑LAT observations to emit diffuse high‑energy (HE) gamma rays.

N 157B / PSR J0536‑6910 – The H.E.S.S. data reveal a clear VHE gamma‑ray signal coincident with N 157B. The source is detected with a significance exceeding 7σ after more than 30 h of good quality exposure, and its spectrum above 1 TeV follows a power‑law with photon index Γ≈2.3 and an integral flux of ≈1.5 × 10⁻¹³ ph cm⁻² s⁻¹. At a distance of 48 kpc this corresponds to a gamma‑ray luminosity of ≈10³⁴ erg s⁻¹, i.e. roughly 0.1 % of the pulsar’s spin‑down power (≈5 × 10³⁸ erg s⁻¹). This efficiency is comparable to that of Galactic pulsar wind nebulae (PWNe), confirming that the same particle‑acceleration mechanisms operate even in a much more distant environment. The morphology is point‑like within the H.E.S.S. point‑spread function, indicating that the VHE emission originates from the compact nebula rather than from the surrounding supernova shell.

SN 1987A – Theoretical models of cosmic‑ray acceleration in young supernova remnants predict that SN 1987A should become a detectable VHE source within a few decades, as the expanding shock interacts with the dense circumstellar ring. H.E.S.S. accumulated ≈120 h of exposure on the SN 1987A position (including the deep 2010 data) but found no statistically significant excess. The derived 95 % confidence upper limit above 1 TeV is F(>1 TeV) < 2 × 10⁻¹³ ph cm⁻² s⁻¹, which is already below several optimistic model predictions. This non‑detection suggests either that the efficiency of particle acceleration is lower than assumed, that the magnetic field in the shock region is weaker, or that the density of target material for hadronic interactions is smaller than inferred from optical and X‑ray studies. The result therefore provides a stringent constraint on the early evolution of cosmic‑ray production in a supernova remnant that is uniquely well‑characterised at other wavelengths.

30 Doradus and Diffuse Emission – The Fermi‑LAT instrument has reported a spatially extended HE gamma‑ray component associated with the massive star‑forming region 30 Doradus. H.E.S.S. examined the same area for VHE counterparts. While no distinct point sources were identified within the LAT error region, the H.E.S.S. sky map shows a modest, diffuse excess that aligns with the large‑scale interstellar medium illuminated by massive stellar winds and supernova activity. This suggests that collective processes (e.g., superbubble acceleration) may generate VHE photons, albeit at a level below the current detection threshold.

Methodology – The analysis employed the full four‑telescope array (later five telescopes) in stereoscopic mode, using the Model++ reconstruction algorithm to achieve optimal angular resolution (≈0.07°) and energy resolution (≈15 %). Background estimation relied on the reflected‑region method, and systematic uncertainties were kept below 20 % through extensive Monte‑Carlo simulations and cross‑checks with independent analysis chains. The combined dataset spans observations from 2003 to 2009 (regular monitoring) and the dedicated deep campaign of 2010, yielding an effective exposure of roughly 120 h on the LMC field.

Implications and Outlook – The detection of N 157B establishes the most distant VHE pulsar wind nebula known to date, extending the reach of ground‑based gamma‑ray astronomy beyond the Milky Way. The stringent upper limits for SN 1987A challenge optimistic acceleration scenarios and highlight the need for deeper, more sensitive observations. The forthcoming Cherenkov Telescope Array (CTA), with an order‑of‑magnitude improvement in sensitivity and a broader energy coverage, will be capable of probing the faint VHE emission expected from SN 1987A and the diffuse component of 30 Doradus. Moreover, the LMC’s distinct metallicity, star‑formation rate, and interstellar environment make it an invaluable testbed for cosmic‑ray physics under conditions that differ from those in our own Galaxy.

In summary, the H.E.S.S. LMC program has delivered the first VHE detection of an extragalactic PWN, placed robust constraints on the gamma‑ray output of the iconic SN 1987A, and provided valuable context for the diffuse high‑energy emission observed by Fermi‑LAT. These results enrich our understanding of particle acceleration in diverse astrophysical settings and set the stage for the next generation of gamma‑ray observatories to explore the LMC in unprecedented detail.