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The alert network based on a French design is one of the key features of the SVOM mission enabling the near real time dissemination of information between the satellite and the ground. It plays a crucial role in the optimization of the synergy for the GRB multiwavelength follow-up. This effort is needed to ensure that the distance to the GRB can be measured.

Given the rather short duration of GRBs, the alert network is designed to dispatch, as soon as possible, all science data needed for ground telescopes to rapidly follow-up of the GRBs detected by ECLAIRs. Once a GRB is detected onboard, the alert message is transmitted to the ground in about 10 seconds at very high frequency (VHF) in a frequency band between 137 and 138 MHz by an onboard antenna. The message is then downlinked through a network of radio stations spread around the Earth’s equator.

Map of the SVOM mission alert network on May 1, 2021, in green the deployed stations, in gray the stations to come, credits: CNES.
Map of the SVOM mission alert network on May 1, 2021, in green the deployed stations, in gray the stations to come, credits: CNES.

At least 43 VHF antennae will be homogeneously deployed in the inter-tropical zone around the Earth, between latitudes -30° and +30°. They will relay every alert message from the satellite to the French Science Center (FSC) located at Saclay in France.

The VHF messages will summarize the main GRB properties that are needed for the follow-up. After a first analysis at the FSC, the messages will be distributed to the whole scientific community, in particular to the GFTs robotic telescopes that will refine the GRB position and give an initial indication of distance. A summary of the GFT results will then be sent back to the FSC in order to share their findings with large observing facilities.
The performance objective of this network is to enable the alert message to be transmitted to the robotic telescopes in less than 30 seconds after detection on board the satellite. The results of the observations from the GFT telescopes are sent back to the FSC and finally disseminated to the large telescopes. These large telescopes, with a smaller field of view, will allow the acquisition of the spectrum of the burst and thus the estimation, by measuring the redshift, of its distance. If everything goes smoothly, it should take less than 4 min to start acquiring GRB optical spectra with large telescopes.

As of December 1rst, 2021 the deployed network is composed of 27 stations.

Chronology of station installation


The alert network

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2021-12-08 23:57:15
2021-12-08 21:57:15

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2016-01-28 20:07:01
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The VT telescope (Visible Telescope) is a dedicated optical tracking telescope, onboard the SVOM satellite. Its main purpose is to detect and observe the emission in visible light, produced immediately after a gamma burst. In less than 10 minutes after the alert given by the ECLAIRS camera and using the refined position provided by the MXT instrument, this instrument is able to reconstruct the position of the gamma burst with an accuracy of a few seconds of arc. The VT is expected to detect and locate about 60 gamma bursts per year.

VT
The VT is composed by a mirror 40 cm in diameter. The focal plane includes two CCD cameras, one red channel and one blue channel. Credit: NAOC

The VT telescope is a Ritchey-Christian telescope with a 40 cm primary mirror and a field of view of 26 arc minutes x 26 arc minutes. Its focal plane is equipped with two 2048 × 2048 CCD cameras covering two wavelength ranges: the blue channel from 450 to 650 nm and the red channel from 650 to 1000 nm. The blue channel CCD is a thinned, backlit detector, while the red channel CCD is specially processed to achieve high sensitivity at long wavelengths. The quantum efficiency of the red channel CCD is greater than 50% at 0.9 ?m, allowing the VT telescope to reach the visual magnitude of 22.5 in 300 seconds. The VT is expected to detect very far-distant gamma-ray bursts with a redshift greater than 6.5, corresponding to distances more than 12 billion years away.

Optical paths (left) and spectral responses (right) of the two red and blue channels of the VT telescope. @NAOC
Optical paths (left) and spectral responses (right) of the two red and blue channels of the VT telescope. @NAOC

In order to quickly provide the position of gamma ray bursts with a precision less than the arc second, the VT telescope carries out on-board data processing. After the localization of a gamma burst by the MXT instrument co-aligned with the VT telescope, lists of possible sources are extracted from the successive images obtained by the VT telescope, centered on the gamma burst position provided by the MXT.
These lists are transmitted in near real time via the VHF high-frequency network, to allow the ground-based software to produce finding charts  and to search for the optical equivalent of the gamma-ray burst, by comparing to existing catalogs. If a counterpart is identified, an alert is broadcast to the world astronomical community to trigger observations using large ground-based telescopes, in particular to measure the redshift of the gamma-ray burst.

VT telescope test model and test facility at Xinglong Observatory (Beijing, China). @NAOC
VT telescope test model and test facility at Xinglong Observatory (Beijing, China). @NAOC

According to the results of the Swift mission, confirmed high red-shift gamma-ray bursts are rare, contrary to theoretical calculations that predict a fraction greater than 5 to 7%. This is probably because, for most of the gamma-ray bursts detected by Swift, optical tracking images are not deep enough to allow for rapid identification, preventing large ground-based telescopes from performing spectroscopic observations.
The SVOM mission will greatly improve this situation thanks to the high sensitivity of the VT telescope, especially for long wavelengths, and the generation of quick alerts on the visible counterpart of the burst. In addition, the SVOM pointing strategy, in the opposite direction to the sun, allows gamma ray bursts to be observed very early using large ground-based spectroscopic telescopes. As a result, it is expected that more high redshift gamma ray bursts will be identified by SVOM.

Institutes: NAOC Beijing, XIOPM Xian (China)

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