early_warning.rst

Early-Warning Alerts

:term:`BNS` mergers spend several minutes in band of the Advanced ground-based gravitational-wave detectors. For some loud and nearby :term:`BNS` mergers, it is possible to accumulate enough :term:`SNR` and detect them several tens of seconds before merger. During O3, automated public alerts for :term:`CBC` events have been sent within as little as 2 minutes after merger. In O4, we have commissioned search pipelines that can detect :term:`BNS` events before merger if the signal is sufficiently strong.

Since it is generally assumed that detectable electromagnetic (or neutrino) emission starts shortly after merger, a pre-merger gravitational-wave detection would provide early warning of an impending electromagnetic transient and might make it possible for automated follow-up facilities to capture any prompt emission from the merger environment, the jet, and other unknown activity.

We had previously conducted a trial early warning public alert infrastructure in June 2020 replaying an 8-day period of archival LIGO data from O3. Results from this study were published in [1]. This study demonstrated that in principle it is possible to send out :term:`GCN` Notices in advance of a :term:`BNS` merger.

Detection Method

All of our :term:`CBC` search pipelines (GstLAL [2], MBTA, PyCBC [3], and SPIIR [4]) are participating in early-warning alerts. See the :doc:`content` section for details about alert types and contents. See the :doc:`analysis/searches` section for details on these analyses. Localizations will be produced with BAYESTAR; see the :doc:`analysis/parameter_estimation` section for details.

:term:`BNS` signals sweep up smoothly in frequency for a few minutes across the Advanced LIGO band. In that time, they may accumulate enough :term:`SNR` to be detected before merger. A GW170817-like system with a total network :term:`SNR` of 32 will already accumulate an :term:`SNR` of 11 by the time the signal sweeps up to 30 Hz, about a minute before merger.

The time evolution of the gravitational-wave frequency and the cumulative :term:`SNR` for a GW170817-like :term:`BNS` system.

The early warning search is a matched-filter search that uses templates that have been truncated at a selection of end frequencies---or equivalently, cut off at a selection of times before merger. The following tables give, for each search pipeline, the ending frequencies and corresponding times before merger. The relationship between frequency and time is approximate and assumes a :term:`BNS` with masses 1.4 \, M_\odot and 1.4 \, M_\odot.

GstLAL:

Frequency (Hz)	Time before merger (s)
29	60
32	46
38	29
49	15
56	10

MBTA:

Frequency (Hz)	Time before merger (s)
34	39
42	22
50	14
58	9

PyCBC Live:

Frequency (Hz)	Time before merger (s)
29	60
32	46
38	29
44	20
49	15
56	10

SPIIR:

Frequency (Hz)	Time before merger (s)
29	60
31	50
34	40
38	30
44	20
56	10

Source Classification

The automated :doc:`source classification and properties <analysis/inference>` have not been trained or tested extensively for early warning alerts. However, the early warning analysis is only optimized for :term:`BNS`-mass mergers. As a result, the assumed source class in early warning :term:`GCN` Notices will always be :term:`BNS` or :term:`Terrestrial`, with an assumption of a 0% chance of :term:`NSBH` or :term:`BBH`. The HasNS and HasRemnant fields will likewise always show 100%.

Localization

Sky localizations for early warning alerts are typically very coarse because the early warning analysis inherently does not make use of the full duration and bandwidth of the gravitational-wave signal. The localization improves slowly up until the last second before merger, and then converges rapidly in the last second.

The animations below taken from the data release [5] show the evolution of early-warning sky maps for three representative events with different :term:`SNR` values. Note that this study assumed the detectors to be operating at their final design sensitivity.

Final SNR	11	18	25
Distance	250 Mpc	210 Mpc	160 Mpc
Sky map (animated GIF)
Frequency	Localization accuracy (90% credible area)
29 Hz	Not detected	Not detected	12000 deg2
32 Hz			10000 deg2
38 Hz		9200 deg2	8200 deg2
49 Hz	2300 deg2	1000 deg2	730 deg2
56 Hz	1000 deg2	700 deg2	250 deg2
1024 Hz	10 deg2	31 deg2	5.4 deg2

[1]	Magee, R., Chatterjee, D., Singer, L. P., Sachdev, S., et al. 2022. :doi:`10.3847/2041-8213/abed54`

[2]	Sachdev, S., Magee, R., Hanna, C., et al. 2020. :doi:`10.3847/2041-8213/abc753`

[3]	Nitz A. H., Schäfer M., Dal Canton T. 2020. :doi:`10.3847/2041-8213/abbc10`

[4]	Kovalam, M., Kaium Patwary, Md A., Sreekumar, A. K., et. al. 2022. :doi:`10.3847/2041-8213/ac5687`

[5]	https://gstlal.docs.ligo.org/ewgw-data-release/