ENGRAVEr's mind under pressure - Dry run before Christmas

The ENGRAVE collaboration finished their first dry run in preparation for the LIGO/VIRGO engineering run ER13 starting on 14 December. With new gravitational wave detections on the horizon four ENGRAVE operations teams tested for the first time telescope triggers, data reduction step-by-step procedure and their observation strategies and capabilities. We would like to thank all participants for their efforts!

The event was organised by the ENGRAVE Executive Committee (EC), who set up a mock Astronomical Observatory identical to ESO Paranal and provided the teams (A, B, C and D) with observational data, if requested. To trigger Target of Opportunity (ToO) observations, a web page similar to the ESO trigger web page was set up.

To test their reactivity the teams were only told that the dry run would happen in the week of 3-7 December 2018. Finally, on Wednesday December 5 at 12:26 CET, the participants received the mock gravitational wave trigger together with the following announcement:

DRY-RUN ENGRAVE CIRCULAR 001 - Gravitational wave detection of a likely black hole - neutron star merger

THIS MESSAGE REFERS TO THE ENGRAVE DRY-RUN. IT DOES NOT REFERENCE REAL OBSERVATIONS AND SHOULD BE USED FOR DRY-RUN PRACTICE ONLY.

“At Dec 05 09:16 UT the network of gravitational wave detectors identified a likely chirp signal. The signal is consistent with the merger of a neutron star with a black hole. The current significance estimate is ~1/300 years. At the present time the localisation is centred at

RA: 03:58:00

DEC: -06:35:00

With an error radius of 8 degrees (90% containment). This error may be improved with subsequent analysis. The luminosity distance inferred from the gravitational wave signal is d_L = 84.7 +/- 24 Mpc. ”

The teams immediately began their search for hosts of the neutron star - black hole merger. But shortly after new data became available:

DRY-RUN ENGRAVE CIRCULAR 002 - Gamma-ray analysis around GW trigger

THIS MESSAGE REFERS TO THE ENGRAVE DRY-RUN. IT DOES NOT REFERENCE REAL OBSERVATIONS AND SHOULD BE USED FOR DRY-RUN PRACTICE ONLY.

“We have examined gamma-ray observations in the 15-150 keV range at the source location and time of GW181205. We note that there are no coincident gamma-ray signals readily visible within 10s of the GW merger time.

However, we do identify a weak, long duration GRB at a time of 09:48 UT. The source fluence of 5e-6 erg/cm^2 means that the source localisation is poor, but the best available position is

RA 05:23:41 DEC 10:27:30

With an error radius of 30 degrees (90% containment). “

Gamma-ray observations provided a much higher confidence of the reality of the GW signal in the case of the first neutron star merger GW 170817. This time however, only a weak, long duration gamma-ray burst (GRB) was observed, which did not significantly improve the sky location. Moreover, the GRB took place 30 minutes after the gravitational wave event. The ENGRAVE operations teams correctly deduced that such a GRB is unrelated to the neutron star - black hole merger.

DRY-RUN ENGRAVE CIRCULAR 003 - Optical transients in the error box of GW181205

THIS MESSAGE REFERS TO THE ENGRAVE DRY-RUN. IT DOES NOT REFERENCE REAL OBSERVATIONS AND SHOULD BE USED FOR DRY-RUN PRACTICE ONLY. –>

“We report observations of GW181205 with a 0.4m robotic telescope sited in South Africa in the time range 2018 Dec 05 11:40 UT to 2018 Dec 05 12:40 UT. The instantaneous field of view was 12 square degrees and multiple epochs of observations were obtained. Transients have been identified relative to a template image obtained on 2018-01-20. We list them with their approximate R-band AB discovery magnitudes below.

GWT1: 03:55:50.1, -04:06:20, 14.7

GWT2: 04:11:34.02, -09:07:21.2, 17.5

GWT3: 03:51:59.81, -08:51:18.5, 18.3

GWT4: 03:49:55.34, -14:01:55.8, 19.1

GWT5: 04:22:17.87, -05:28:05.41, 17.8

Further observations are planned.”

The teams had to decide which, if any, of the five possible targets they want to observe. Only one of them could be responsible for the gravitational wave signal. But then another circular came in…

DRY-RUN ENGRAVE CIRCULAR 004 - Discovery of possible counterpart to GW181204

THIS MESSAGE REFERS TO THE ENGRAVE DRY-RUN. IT DOES NOT REFERENCE REAL OBSERVATIONS AND SHOULD BE USED FOR DRY-RUN PRACTICE ONLY.

“We have undertaken a search of galaxies in and around the error region of GW181205 over the time span 5 Dec 12:00-13:00 UT. In comparison with archival PanSTARRS images we identify a new source in the galaxy UGC 02963 (z=0.0177, d_L ~ 77 Mpc). The source is offset approximately 4 kpc in projection from the centre of the galaxy and has a location of

RA 04:07:41.52

DEC 03:58:11.24

The magnitude of the source is R=18.2 (M_R ~ -16.2) is comparable to the early absolute magnitude of AT2017gfo.

Further observations are strongly encouraged.”

Six transients have been identified in the sky region corresponding to the gravitational wave signal. By now, the teams have triggered observations of some of the sources, and began reducing the data.

Operations Team A was first to claim the detection of the possible kilonova by sending a notification to the other teams:

DRY-RUN ENGRAVE CIRCULAR 005 - GW 181205A: VLT/X-shooter observations

THIS MESSAGE REFERS TO THE ENGRAVE DRY-RUN. IT DOES NOT REFERENCE REAL OBSERVATIONS AND SHOULD BE USED FOR DRY-RUN PRACTICE ONLY.

Chen, Dall’Ora, D’Elia , Fraser, Gillanders, Gromadzki, Lena, Lyman, McBrien, Patat, Schady, Smartt and Talia, i.e., the Dryrun-ops a report on behalf of the ENGRAVE collaboration:

We observed the candidate optical counterpart of GW181205A (DRY-RUN ENGRAVE CIRCULAR 001 and 004) with the ESO-VLT UT2 equipped with the X-shooter spectrograph. We acquired two 1200 second spectra on 2018-12-05, starting from 01:56:51.628 UT , approximately 5 hours after the fake GW alert.

Continuum is detected in the reduced spectrum across the entire spectral range of the instrument (blue limit of 3000 Angstroms). After a preliminary reduction the continuum does not resemble that of a SN. In fact, no Ca II, HI or He I lines are detected. On the other hand, the spectrum appears to be similar to that of a kilonova, and strongly resembles that of GW170817A.

We acknowledge the ESO observing staff at the fake Paranal, including Daniele Bjørn Malesani and the EC.

This circular caused confusion among the other teams - when they reduced the data of this source they couldn’t find the signal of a kilonova! It took the teams a while to figure out that the problem originated from the quick reduction pipeline, which defaulted to reducing the spectrum of last year’s kilonova.

In the end, all four ENGRAVE teams successfully triggered follow-up observations with various instruments. The following observations were requested: