[Paper Review] Status and recent results from H.E.S.S

Berrie Giebels, Collaboration, H. E. S. S.|arXiv (Cornell University)|Mar 12, 2013

Astrophysics and Cosmic Phenomena5 references3 citations

TL;DR

This paper presents the status and recent scientific results from the H.E.S.S. Cherenkov telescope array, highlighting the enhanced sensitivity from the addition of the 28-meter H.E.S.S. II telescope and upgrades to H.E.S.S. I. Key results include the detection of VHE gamma-ray emission from Galactic sources like W49B and binary systems, constraints on dark matter annihilation at the Galactic Center, and improved limits on quantum gravity effects and the extragalactic background light using PKS 2155-304 observations.

ABSTRACT

The H.E.S.S. instrument consists of four 13 m (H.E.S.S. I) and one 28 m diameter (H.E.S.S. II) Atmospheric Cherenkov Telescopes (ACTs) located in the Khomas Highland in Namibia, 1800 m above sea level. The H.E.S.S. I array began operations in 2003, and has achieved recent scientific results allowed by the synergy with the Fermi-LAT, some of which are outlined here. The H.E.S.S. II telescope started operations in July 2012, and is expected to provide its first scientific results by the end of the year. Since the inauguration of the first telescope in September 2002, H.E.S.S. has taken 9415 hours of data, with 4234 hours in the band of the Galaxy and 5181 hours in extragalactic space, discovered over 80 new very high energy (VHE; E > 100 GeV) γ-ray sources (according to TeVCat listings), among them more than 60 galactic objects and 19 extragalactic sources.

Motivation & Objective

• To report on the operational status and scientific achievements of the H.E.S.S. Cherenkov telescope array following the integration of H.E.S.S. II and upgrades to H.E.S.S. I.
• To investigate the nature of very high energy (VHE; E > 100 GeV) gamma-ray sources in the Galactic plane, particularly pulsar wind nebulae and supernova remnants.
• To probe exotic physics, including dark matter annihilation at the Galactic Center and Lorentz invariance violation via photon energy-dependent time delays.
• To measure the extragalactic background light (EBL) density using VHE gamma-ray absorption in distant AGNs, and to test quantum gravity models.
• To improve sensitivity and energy threshold for VHE gamma-ray detection through advanced reconstruction techniques and hybrid operation of all five telescopes.

Proposed method

• Utilization of four 13 m (H.E.S.S. I) and one 28 m (H.E.S.S. II) atmospheric Cherenkov telescopes located in Namibia to detect VHE gamma rays via Cherenkov light from extensive air showers.
• Employment of advanced shower reconstruction techniques, including cascade fits using all pixels, 3D image characterization, and multivariate methods like boosted decision trees, improving flux sensitivity by a factor of ~2.
• Implementation of hybrid mode operation with all five telescopes to enhance angular resolution and reduce background, enabling better source localization and sensitivity.
• Application of stereoscopic imaging and improved optical efficiency from mirror recoating (completed over two years) to increase effective collection area and signal-to-noise.
• Use of simultaneous Fermi-LAT and H.E.S.S. observations to cross-calibrate and extend spectral coverage from GeV to TeV energies.
• Employment of event-by-event likelihood fitting on lightcurves (e.g., PKS 2155-304) to constrain photon energy-dependent time delays for testing quantum gravity models.

Experimental results

Research questions

• RQ1 What is the nature of the VHE gamma-ray emission from the W49B supernova remnant, and does it support a hadronic interaction origin?
• RQ2 How do the VHE and GeV gamma-ray emissions from PSR B1259-63 and 1FGLJ1018-5856 compare, and what do they reveal about particle acceleration in binary systems?
• RQ3 What constraints can be placed on dark matter annihilation at the Galactic Center using H.E.S.S. observations of the central 1.5° region?
• RQ4 Can time delays in VHE photon arrival times from flaring AGNs like PKS 2155-304 be used to set limits on quantum gravity energy scales?
• RQ5 To what extent can VHE gamma-ray absorption by the extragalactic background light (EBL) be used to directly measure EBL density at 0.1–10 μm wavelengths?

Key findings

• H.E.S.S. detected VHE gamma-ray emission from W49B, with a smooth connection between the GeV and TeV spectra, favoring a hadronic origin due to high GeV luminosity and interaction with dense molecular material.
• The H.E.S.S. Galactic Plane Survey revealed over 80 new VHE sources, with pulsar wind nebulae being the most abundant type, and identified HESSJ1018-586 as a candidate VHE counterpart to the Fermi-LAT binary 1FGLJ1018-5856 with a photon index Γ = 2.7 ± 0.5.
• No significant excess was found in the Galactic Center region for WIMP annihilation, leading to the best current constraints on the dark matter annihilation cross-section for masses between ~300 GeV and ~10 TeV, for both NFW and Einasto profiles.
• From the extreme flare of PKS 2155-304 (8000+ events in 85 min), a 95% CL lower limit of 2.1 × 10^18 GeV was set on the quantum gravity energy scale for the linear term in the photon dispersion relation.
• H.E.S.S. provided the first direct detection of the extragalactic background light (EBL) using VHE gamma-ray absorption, with results in excellent agreement with Fermi-LAT measurements in overlapping energy ranges.
• The addition of H.E.S.S. II and upgraded H.E.S.S. I telescopes improved sensitivity by a factor of ~2 and lowered the energy threshold, enabling better detection of pulsed emission and transient phenomena like gamma-ray bursts.