5. Determining GRB Follow Up

5. Determining GRB Follow Up#

GRB follow-up in GRANDMA is guided by scientific interest, localization quality, and operational feasibility. GRB events are reported in the GCN Circulars.

5.1 What GRBs Should Be Observed?#

The following classes of GRBs are prioritized:

  1. Long GRBs with X-ray Counterparts

    • These may indicate X-ray-rich, optically faint bursts in high-density environments (possibly merger-driven).

    • Prioritize GRBs detected in X-rays by Swift/XRT or similar instruments.

  2. Very Energetic GRBs

    • GRBs with >10–100 GeV emission are rare and scientifically valuable.

    • Detected by high-energy instruments like Fermi-LAT.

  3. Extremely Bright GRBs (BOAT-like)

    • Defined by exceptionally high peak flux and fluence.

    • Thresholds for triggering are being developed on a case-by-case basis.

  4. GRBs with Known or Estimated Redshift

    • Nearby GRBs (z < 1): More easily detectable, may be associated with supernovae.

    • High-z GRBs (z > 4): Important for early-Universe studies.

  5. Short GRBs (T90 < 2s)

    • High-priority due to potential kilonova association.

    • Often poorly localized (e.g., Fermi/GBM) — require tiling strategies for follow-up.

  6. Well-localized Long GRBs with Optical Counterparts

    • Long GRBs (T90 > 2s) with < 5 arcmin localization uncertainty and an optical counterpart reported by another team/instrument.

Other scenarios, such as off-axis jets, may be considered individually by GRB chairs and the Core Team.

5.1.1 Operational Notes on GRB Alerts#

  • GRB alerts are not automatically ingested into Slack or SkyPortal like GW alerts are.

  • GRBs are announced through GCN Circulars, which must be monitored via email.

  • Prioritize Swift-detected GRBs:

    • Swift GRBs are automatically ingested into SkyPortal.

    • Fermi GRBs are not ingested and ZTF follow-up is not automated, making them lower priority unless exceptional.

  • If no counterpart is reported, observing is often unproductive. Follow-up should be reserved for events with:

    • X-ray or optical detections

    • Accurate localization

    • External team reports confirming visibility

5.2 Procedure for Observing GRBs#

  1. Trigger the GRANDMA Network

    • Share alert and relevant context via email and Slack.

  2. Trigger TAROT via SkyPortal

    • If the GRB is observable from TAROT and counterpart info is promising, trigger directly.

    • Analyze images within ~5 hours to evaluate detection.

  3. Stop Conditions

    • If nothing is visible in the early images, STOP GRANDMA.

    • Regardless of detection, observations should end after 2–3 days.

  4. If Detection Occurs

    • Update the RA/Dec on SkyPortal with refined coordinates.

    • Continue observing until the afterglow becomes too faint for the network (e.g., mag > 21).

  5. If External Request Is Received

    • If another collaboration requests follow-up with a reasonable science case,
      the GRB Chairs and Core Team can approve it.

    • In such cases, follow the standard strategy above.

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