On 9 November 2021, the French Science Center (FSC) of the SVOM mission generated and sent its first alert to a ground-based robotic telescope for automatic photometric monitoring. This first test was carried out with the 50 cm robotic telescope IRiS (Initiation à la Recherche en Astronomie pour les Scolaires) located at the Observatoire de Haute Provence during a SVOM scientific meeting held there. The objective of this test was multiple:
- test the FSC’s alert generation and delivery system;
- test the real-time communication between the FSC and observers from outside the SVOM collaboration;
- ensure the proper transmission of SVOM alert information and optical tracking by ground-based robotic telescopes;
- analysing images taken in order to identify the transient source at the origin of the SVOM alert.
For this test, the FSC teams used the coordinates of a still relatively bright Type Ia supernova, SN2021achd/ZTF21aciwkzc (detected by the US ZTF optical survey on 7 November 2021), to simulate the position of a gamma-ray burst that would have been detected by the ECLAIRs instrument on board the SVOM satellite. On 27 October 2021 at 21:15 (local time), the FSC transmitted the alert to the IRiS telescope via a dedicated communication channel. Between the generation of the SVOM alert and its transmission to the IRiS telescope, only 1 to 2 seconds elapsed. It took a few more tens of seconds for the IRiS observation scheduling system to schedule the observation and point the telescope at the target to acquire the first follow-up image. In total, about 1 minute will have elapsed between the reception of the alert and the start of IRiS acquisitions. In total, a sequence of 79 images was taken during the rest of the night (60x5sec exposure with an sdss-r filter, then 9x60s in sdss-r and 10x60s in sdss-i). Each image taken by IRiS was stored in real time in a shared directory with the FSC, which was then able to analyse the images.
The precise analysis of the images is still underway, but supernova 2021achd was clearly detected in both the individual and cumulative images (see Figure 1).
In the context of the monitoring of a SVOM gamma-ray burst, the photometric analysis will make it possible to study the temporal behaviour of the optical transient source. This analysis will make it possible to identify whether the flux evolution of the transient source is in agreement with the expected light curve for a gamma-ray burst. The automation of this type of analysis is underway in order to be as reactive as possible as soon as IRiS images are taken.
This test was therefore conclusive in every way. It demonstrated the ability of the SVOM alert system to quickly deliver crucial information to the ground-based telescope teams in order to characterise the transient sources of interest as quickly as possible.
Further tests will take place in the coming months to collect more statistics on the stability of the SVOM alert system and to fine-tune the observing strategies. In parallel, real-time observations of gamma-ray burst alerts detected by the US Swift mission and redistributed by the FSC will also be scheduled in the coming months with SVOM partner telescopes.
Optical follow-up of gravitionnal waves
The first results of the space mission SVOM (for Space-based multi-band astronomical Variable Objects Monitor) have just been released before the launch scheduled for the end of 2021. How is this possible? Quite simply because this ambitious Franco-Chinese mission, which aims at studying gamma-ray bursts of the Universe, is also developing a network of ground-based cameras able to detect the emission of visible light that follows the outbreak of these bursts, the most violent known explosions. This network, dubbed Ground-based Wide Angle Camera (GWAC), is already in operation at the Xinglong Observatory in Northeast Beijing (China). Its test version, named Mini-GWAC, successfully concluded a first campaign of monitoring and real-time follow-up of gravitational wave sources discovered by the LIGO (USA) and Virgo (Italy) facilities. These results are being published in the journal Research in Astronomy and Astrophysics.
See the luminous echo of a gravitational wave
The SVOM team has just published the results obtained by the Mini-GWAC instrument of the follow-up of the second sequence of gravitational wave detection (run O2) by LIGO and Virgo that took place from November 2016 to August 2017. Not less than 14 potential events were published by LIGO in this interval, eight of which were monitored by Mini-GWAC. Other six events were subsequently retracted by LIGO.
For two of them, the most spectacular, GW170104 and GW170608 which have been confirmed as resulting from the fusion of two black holes, optical tracking has worked remarkably well. In the case of GW170104 (for Gravitational Wave of January 04, 2017), it is the result of the merging of two black holes of mass approximately 20 and 30 times that of the Sun.
Mini-GWAC was able to observe GW170104 a little more than 2 hours after the triggering of the alert and provide data for 10 hours, covering 62% of the error box. For GW170608 (June 8, 2018), nearly 20% of the region was covered. In both cases, no visible emission was detected, up to a magnitude of about mv = 12.
The final version of the GWAC installation that was commissioned in late 2017 at the Xinglong Observatory (China) is now able to track gravitational wave alerts down to a visible magnitude mv = 16, more than 40 times fainter than the test version. In some cases, the emission of gravitational waves is also accompanied by gamma-ray bursts that will also be detected by the other SVOM instruments [1]. The entire GWAC device is now ready and able to detect the visible counterparts of these events, even before the launch of the SVOM mission.
[1] SVOM is a Franco-Chinese mission for the observation of the gamma-ray bursts of the Universe. Its launch is currently scheduled for the end of 2021.
[2] GWAC is one of the components of the SVOM mission which also includes 4 instruments (ECLAIRs, MXT, GRM and VT) * which will be onboard the SVOM satellite. GWAC aims to complete observations from the ground to study and identify gamma-ray bursts detected from space by the SVOM satellite.
GWAC consists of 10 mounts each carrying 4 cameras of 18 cm in diameter and thus covering a total field of view of about 5000 square degrees. Each of the 40 cameras is equipped with a 4096 × 4096 E2V CCD operating in the 0.5 to 0.85 ?m wavelength band with a field of view (FoV) of 150 deg². GWAC provides source locations up to a visible magnitude mv = 16 with an accuracy of 11 arcsec (for a 10 s exposure).
The preliminary Mini-GWAC test version consisted of a system of six mounts, each equipped with two cameras (Canon 85 / f1.2).
Publication :
“The mini-GWAC optical follow-up of the gravitational wave alerts.
Results from the O2 campaign and prospects for the upcoming O3 run.”
D. Turpin et al., Research in Astron. Astrophys. 2019 (in press), see 
the publication in (PDF)
Meeting with Stéphane Basa, who presents the French robotic telescope F-GFT.
Click on CC to activate the English subtitle
The third gravitational wave event (GW170104) discovered by the LIGO-Virgo collaboration confirms the existence of massive stellar black holes with a mass larger than 20 solar masses. These two in-orbit stellar masses stars finally merged to form a compact object of nearly 50 solar masses. During this process, a huge amount of gravitational wave, predicted by the general relativity theory, is emitted. This is precisely the signal of this fusion that the two LIGO interferometers detected on earth for the third time.
Searching for an electromagnetic counterpart to such event is today a challenge, both on a observational side or on a theoretical aspect. Many multi-wavelength networks, on ground and in space, share this goal. Among them, the SVOM mission has started to survey the GW candidates with a dedicated instrument, the SVOM/mini-GWAC equipment, pathfinder towards a more complete SVOM instrument. Starting just a few hours after the communication to the scientific community of the GW position, the following text describes the different steps of this search.
GW170104
The gravitational wave event GW170104 has been detected on 4 January 2017 at 10:11:58 UTC by the LIGO-Virgo consortium with the twin advanced interferometers Hanford and Livingston located in the United States.
This event is the result of the coalescence of a pair of stellar-mass black holes with respectively 31.2(+8.4;?6.0) and 19.4 (+5.3;?5.9) solar masses. The source luminosity distance is 880 (+450;?390)??Mpc corresponding to a redshift of z=0.18 (+0.08;?0.07). The signal was measured with a signal-to-noise ratio of 13 and a false alarm rate less than 1 in 70?000 years.
Timeline and Localization error box
The GW alert has been delivered to the astronomical community with a delay of 6.3 h.
The initial 90% localization error box covers an area on the sky of 2065 deg2.
An update of the localization error box has been delivered 4 months later with a 22% reduction of the localization error box.
SVOM activity
As soon as the GW alert was delivered to the astronomical community (6h after the trigger time e.g 4 hours after the beginning of the Chinese night), the probability skymap was quickly digested in order to produce the follow-up observation plan.
The mini-GWAC telescopes covered 80% of the error box in 14exposures of variable duration between 1 and 3 hours.
The SVOM group reported their follow-up observations in a circular published at the end of the night.
SVOM/Mini-GWAC follow-up observations were unique in the optical band: SVOM/mini-GWAC performed the largest probability coverage of GW170401 localization in shortest latency for optical band.
At this time, we didn’t detect a relevant electromagnetic counterpart but for the future, we look forward the whole GWAC system (limit mag 16), in operation at fall 2017.
SVOM science is on the way !
Meeting with Alain Klotz, who presents us the TAROT telescope.
Côte d’Azur observatory located in the Plateau de Calern, August 2015.
Click on CC to activate the English subtitle