ATOM: A trailer, a trigger and a tug for H.E.S.S.

May 2020

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The H.E.S.S. array is a very advanced and efficient observatory for ground-based gamma-ray astronomy, sensitive enough to trace rapid variations in flaring sources on time-scales of minutes. Nonetheless, most gamma-ray sources require many tens of hours of observations to be detected and the limited-though-large (5 degree diameter) field-of-view requires dedicated pointings. In order to allow for a rapid response and gamma-ray observations of variable objects, atmospheric Cerenkov telescopes benefit greatly from triggers provided by other instruments that survey large fractions of the sky or a large number of sources in a short time, identify flares promptly and provide instantaneous information for the gamma-ray telescopes. The H.E.S.S. array not only uses publicly available information from satellite-based instruments such as e.g. BAT and XRT onboard SWIFT or LAT and GBM onboard Fermi in the X-ray and MeV-Gamma-ray bands, respectively - it also is tugged, triggered and trailed by its own dedicated optical telescope ATOM (Automatic Telescope for Optical Monitoring). This fully robotic facility surveys a large number of known or potential gamma-ray sources at optical wavelengths, searching for variations that correspond to pre-defined patterns or exceed source-specific thresholds. The information can be used in order to adjust the observing schedule of the H.E.S.S. telescopes (just as a small tugboat steers a mighty vessel), it can trigger Target of Opportunity observations of unanticipated variations and flares, and it can trail whichever event or routine observation is conducted by the Cerenkov telescopes. In addition to providing trigger signals, the simultaneously conducted observations in the gamma-ray and optical bands facilitate the scientific interpretation as they ensure that time-dependent studies are actually followed simultaneously throughout the electromagnetic spectrum. For very many gamma-ray sources, the optical band, monitored by ATOM covers the high-energy, optically-thin end of the synchrotron emission, while the gamma-ray emission observed with H.E.S.S. records the corresponding high-energy end of Compton-scattered radiation.

ATOM is an 80 cm Ritchey-Chrétien telescope manufactured by ZEISS, Germany and retrofitted with state-of-the-art electronics for robotic operations at the H.E.S.S. site in Göllschau, Namibia during the early days of the H.E.S.S. experiment ([1]). Being placed less than one kilometer away from the H.E.S.S. array it is subject to the same atmospheric conditions. This enables ATOM measurements to be used for photometric monitoring and ensures that the skies are clear for ATOM whenever H.E.S.S. is taking data and vice-versa. ATOM went through stages of remote-control-, and automatic operations through to a full robotic mode since 2007. For more than a decade, data were obtained with a single CCD that allowed subsequent multiband photometry of stars and extragalactic sources within its 8 arcmin field-of-view. Subsequent upgrades kept its sensors and auxiliary instruments state-of-the art. Since 2019 ATOM operates with an upgraded instrument that extend its performance in three aspects: A dual-channel design allows for simultaneous observations in a blue and red arm. This increases the efficiency and ensures strictly simultaneous data-taking in case of rapid changes. The red arm is facilitated with a polarimeter that allows polarimetric measurements to be obtained as part of routine monitoring or in response to triggers. This also provides for routine monitoring of polarimetric properties in the Southern Sky - which is still a very rare resource. Lastly, one of the CCDs in an (electron-multiplying) EMCCD, which allows for very short integration times (down to milli-seconds). ATOM lives in its own separately fenced-off hut, overseeing the H.E.S.S. array and connected with the internal network connection in the H.E.S.S. control room, while its night-by-night operation are conducted robotically. Results are recorded real-time and can be communicated as triggers within a minute after read-out.

Fig. 1:The ATOM telescope during sunset over Khomas highland. The robotically-controlled roof has slided-off to the left (where one of the H.E.S.S. telescopes is seen in the back) - ready for another long night worth of observing. The little specs above the mountain at the horizon to the right are birds – not clouds.

The main objectives of ATOM are to (1) obtain simultaneous optical observations of any potentially variable target of H.E.S.S. gamma-ray observations to provide instantaneous multiband information, (2) monitor all potentially variable targets foreseen for H.E.S.S. observations in one observing season to assess the general state of a source or optimize the period when gamma-ray observations are to be conducted, (3) monitor a collection of about 300 sources searching for flares to trigger subsequent observations with H.E.S.S. The cadence of the monitoring is pre-defined according to a set of scientific and operational criteria, but is adapted by an automatic scheduler that takes into account the demands on observing time by the objectives 1 and 2 as well as current activities of the sources and changing weather conditions. Side benefits of the program are a continuous monitoring of photometric parameters through daily observations of photometric standard stars and the assembly of a long-term archive of gamma-ray sources for timing studies. ATOM data have been published along with H.E.S.S. measurements in nearly all papers on variable sources studied with the H.E.S.S. telescopes since 2006.

Fig. 1:The distribution of sources monitored with ATOM in an Aitoff projection (equatorial coordinates). While the regular monitoring is dominated by Active Galactic Nuclei (AGN), a small number of Galactic sources are observed routinely as well. They smoothly fill in what would otherwise be the Zone of Avoidance around the Galactic equator where extragalactic targets are apparently less numerous due to absorption and crowding in source identification.

One of the many quasars monitored with ATOM is 3C 279, one of the most luminous gamma-ray sources in the local universe. Figure 3 illustrates the data and the principle of the mission of ATOM. Given its gamma-ray history it is observed with a rather dense cadence (weather permitting) as illustrated in the beginning of the series. The exceptionally rainy February 2020 (in Namibia) caused rather extended periods with much reduced cadence as indicated by yellow-hatched bars in the light-curve. After the clouds had cleared, 3C 279 was found to have increased in flux by more than 100% (more than one magnitude) in the R-band (about 600–700 nm) to a level that is exceeded only about once or twice per decade. The increase triggered observations with the H.E.S.S. telescopes (the results of which will be reported in a future edition of the H.E.S.S. Source-of-the-Month) , which, in turn, resulted in an enhanced cadence of ATOM data (including searches for rapid variations on minute time-scales), polarimetric measurements, and an extension of the wavelength coverage of ATOM observations (as illustrated by the additional measurements in the V and I bands). In addition, the detection of the flare resulted in further observations obtained in other waveband ranges. The specific event is rather peculiar given the shape of the light-curve, the high peak flux and the unusual ratio of synchrotron-to-Compton emission.

Fig. 3:ATOM Observations of 3C 279 in early 2020 reveal a clear outburst in March 2020 despite multiple spells of poor weather. The bright flare displays an exponential fall-off (linear trend in (preliminary calibrated) magnitudes vs. time)

This particular event is but one example of the variations identified with the ATOM facility in 2020. Others include, e.g., a detailed monitoring of the flares of PKS 0903-57 that had been observed in the MeV/GeV gamma-ray bands ([2], [3]) and with H.E.S.S. in the TeV gamma-ray range ([4]), and the recent detection of an outburst in 1ES 1218+304 ([5]).


[1] Hauser, M. et al.: 2004, Astron.Nachr., 325, 659
[2] Lucarelli, F., et al.: ATEL 13602 (2020) AGILE detection of enhanced gamma-ray activity from the blazar PKS 0903-57
[3] Buson, S., for the Fermi LAT collaboration.: ATEL 13604 (2020) Fermi-LAT discovery of VHE emission from blazar PKS 0903-57
[4] Wagner, S.J., for the H.E.S.S. collaboration: ATEL 13638 (2020) Detection of Very-High-Energy Gamma-rays from PKS 0903-57 by H.E.S.S.
[5] Jankowsky, F. and Wagner, S.J.: ATEL 13xxx (2020) xxx