D. J. van der Walt

Very high energy gamma rays from the direction of Sagittarius A

H.E.S.S. – the High Energy Stereoscopic System– is a new system of large atmospheric Cherenkov telescopes for GeV/TeV astronomy. Each of the four telescopes of 107 m mirror area is equipped with a 960-pixel photomulitiplier-tube camera. This paper describes the methods used to convert the photomultiplier signals into the quantities needed for Cherenkov image analysis. Two independent calibration techniques have been applied in parallel to provide an estimation of uncertainties. Results on the long-term stability of the H.E.S.S. cameras are also presented.

A New Population of Very High Energy Gamma-Ray Sources in the Milky Way

Very high energy γ-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy γ-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of “dark” nucleonic cosmic ray sources.

H.E.S.S. observations of PKS 2155-304

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Calibration of cameras of the H.E.S.S. detector

We report the detection of a point-like source of very high en ergy (VHE) γ-rays coincident within 1 ′ of Sgr A, obtained with the H.E.S.S. array of Cherenkov telescopes. The γ-rays exhibit a power-law energy spectrum with a spectral in dex of −2.2±0.09±0.15 and a flux above the 165 GeV threshold of (1 .82±0.22)·10−7m−2s−1. The measured flux and spectrum di ffer substantially from recent results reported in particular b y the CANGAROO collaboration.

Serendipitous discovery of the unidentified extended TeV gamma-ray source HESS J1303-631

The serendipitous discovery of an unidentified extended TeV γ-ray source close to the galactic plane named HESS J1303-631 at a significance of 21 standard deviations is reported. The observations were performed between February and June 2004 with the HESS stereoscopic system of Cherenkov telescopes in Namibia. HESS J1303-631 was discovered roughly 0.6 ◦ north of the binary system PSR B1259-63/SS 2883, the target object of the initial observation campaign which was also detected at TeV energies in the same field of view. HESS J1303-631 is extended with a width of an assumed intrinsic Gaussian emission profile of σ = (0.16 ± 0.02) ◦ and the integral flux above 380 GeV is compatible with constant emission over the entire observational period of (17 ± 3)% of the Crab Nebula flux. The measured energy spectrum can be described by a power-law dN/dE ∼ E −Γ with a photon index of Γ= 2.44 ± 0.05stat ± 0.2syst. Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.

Periodic class II methanol masers in G9.62+0.20E

We present the light curves of the 6.7 and 12.2 GHz methanol masers in the star forming region G9.62+0.20E for a time span of more than 2600 days. The earlier reported period of 244 days is confirmed. The results of monitoring the 107 GHz methanol maser for two flares are also presented. The results show that flaring occurs in all three masing transitions. It is shown that the average flare profiles of the three masing transitions are similar. The 12.2 GHz masers are the most variable of the three masers with the largest relative amplitude having a value of 2.4. The flux densities for the different masing transitions are found to return to the same level during the low phase of the masers, suggesting that the source of the periodic flaring is situated outside the masing region, and that the physical conditions in the masing region are relatively stable. On the basis of the shape of the light curve we excluded stellar pulsations as the underlying mechanism for the periodicity. It is argued that a colliding wind binary can account for the observed periodicity and provide a mechanism to qualitatively explain periodicity in the seed photon flux and/or the pumping radiation field. It is also argued that the dust cooling time is too short to explain the decay time of about 100 days of the maser flare. A further analysis has shown that for the intervals from days 48 to 66 and from days 67 to 135 the decay of the maser light curve can be interpreted as due to the recombination of a thermal hydrogen plasma with densities of approximately 1.6× 10 6 cm 3 and 6.0× 10 5 cm 3 respectively.

Short-period variability in the Class II methanol maser source G12.89+0.49 (IRAS 18089-1732)

Time series are presented for the class II methanol maser source G12.89+0.49, which has been monitored for nine years at the Hartebeesthoek Radio Astronomy Observatory. The 12.2 and 6.7 GHz methanol masers were seen to exhibit rapid, correlated variations on timescales of less than a month. Daily monitoring has revealed that the variations have a periodic component with a period of 29.5 days. The period seems to be stable over the 110 cycles spanned by the time series. There are variations from cycle to cycle, with t he peak of the flare occurring anywhere within an eleven day window but the minima occur at the same phase of the cycle. Time delays of up to 5.7 days are seen between spectral features at 6.7 GHz and a delay of 1.1 day is seen between the dominant 12.2 GHz spectral feature and its 6.7 GHz counterpart.

ON THE METHANOL MASERS IN G9.62+0.20E AND G188.95+0.89

A comparison between the observed light curves of periodic masers in G9.62+0.20E and G188.95+0.89 and the results of a simple colliding-wind binary model is made to establish whether the flaring and other time-dependent behavior of the masers in these two star-forming regions can be ascribed to changes in the environment of the masers or in the continuum emission from parts of the background H II region. It is found that the light curves of widely different shape and amplitude in these two objects can be explained within the framework of a periodic pulse of ionizing radiation that raises the electron density in a volume of partially ionized gas against which the masers are projected. It is also shown that the decay of the 11.405 km s-1 maser in G188.95+0.89 can be explained very well in terms of the recombination of the ionized gas against which the maser is projected, while it would require very special conditions to explain it in terms of changes in the environment of the maser. We conclude that for G9.62+0.20E and G188.95+0.89 the observed changes in the masers are most likely due to changes in the background free-free emission which is amplified by the masers.

Upper limits to the SN1006 multi-TeV gamma-ray flux from HESS observations

Observations of the shell-type supernova remnant SN1006 have been carried out with the HESS system of Cherenkov telescopes during 2003 (18.2 h with two operating telescopes) and 2004 (6.3 h with all four telescopes). No evidence for TeV

Very Long Baseline Array imaging of a periodic 12.2-GHz methanol maser flare in G9.62+0.20E

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