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GCN Circulars

GCN Circulars are rapid astronomical bulletins submitted by and distributed to community members worldwide. They are used to share discoveries, observations, quantitative near-term predictions, requests for follow-up observations, or future observing plans related to high-energy, multi-messenger, and variable or transient astrophysical events. See the documentation for help with subscribing to or submitting Circulars.

Search for Circulars by submitter, subject, or body text (e.g. 'Fermi GRB').
To navigate to a specific circular, enter the associated Circular ID (e.g. 'gcn123', 'Circular 123', or '123').
Advanced Search

To narrow the search results, use advanced search syntax. This allows for specifying which circular field to search (submitter, subject, and/or body). For additional information, refer to the advanced search documentation.

Advanced Search Examples (click to copy):

subject:"Swift"
body:"GRB"
submitter:"Judith Racusin"
  1. LIGO/Virgo/KAGRA S250818k: Continued fading and reddening of AT2025ulz from Keck/LRIS imaging observations
  2. GRB 250821A: Swift-XRT refined Analysis
  3. Swift GRB 250821A: Global MASTER-Net observations report
  4. GRB 250821B: Fermi GBM Detection
  5. LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz
  6. GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672)
  7. LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits
  8. GRB 250821B: Fermi GBM Final Localization of a Bright Burst
  9. GRB 250821A: Enhanced Swift-XRT position
  10. GRB 250821A: Swift/UVOT Upper Limits
  11. EP250821a: Rapidly fading optical source detected in Las Cumbres followup
  12. EP250821a: Swift XRT and UVOT counterpart detection
  13. LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits
  14. EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations
  15. GRB 250821A: LCOGT Optical Upper Limits
  16. GRB 250814A: CMO SAI and SAO RAS izJ observations.
  17. LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz
  18. GRB 250821A: Prompt enhanced Swift-XRT position
  19. GRB 250821A: Swift detection of a burst
  20. LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz
  21. LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz
  22. EP250821a: Einstein Probe detection of an X-ray transient
  23. GRB 250819A: SVOM/GRM observation of a possible “long” duration type I burst
  24. GRB 250819A: Insight-HXMT detection
  25. LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz
  26. GRB 250818B: 10 GHz VLA detection
  27. LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz
  28. LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz
  29. LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections
  30. LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz
  31. GRB 250814A: Swift/UVOT Upper Limits
  32. GRB 250813B: Swift/UVOT Upper Limit
  33. IPN triangulation of GRB 250816A (short)
  34. IPN triangulation of GRB 250814C (short)
  35. IPN triangulation of GRB 250804B (short)
  36. GRB 250818B: FTW optical and NIR observations show reddening of the counterpart
  37. GRB 250818B: Kilonova-Catcher optical afterglow detection
  38. LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits
  39. GRB 250818B: Liverpool Telescope optical detections
  40. LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
  41. LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
  42. LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
  43. GRB 250818A: AstroSat CZTI detection
  44. LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
  45. LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
  46. GRB 250818B: Swift/UVOT Detection
  47. Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB
  48. LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
  49. GRB 250820A: Fermi GBM Final Real-time Localization
  50. GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq)
  51. GRB 250818B: TRT Optical Observation
  52. Fermi GRB 250816A: Global MASTER-Net observations report
  53. GRB 250818B: SAO RAS optical observations
  54. Swift GRB 250818A: Global MASTER-Net observations report
  55. GRB 250818B: NOT optical observations
  56. GRB 250818A: Swift-XRT refined Analysis
  57. GRB 250818B: EP-FXT counterpart detection
  58. GRB 250818A: Swift-BAT refined analysis
  59. Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB
  60. LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
  61. GRB 250818C: Fermi GBM Final Real-time Localization
  62. GRB 250818B: Keck redshift of the optical afterglow
  63. Fermi trigger No 776849714: Global MASTER-Net observations report
  64. GRB 250818B: KAIT optical observations
  65. GRB 250818A: Swift/UVOT Upper Limits
  66. LIGO/Virgo/KAGRA S250818t: Updated Sky localization
  67. LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility
  68. GRB 250818A: Enhanced Swift-XRT position
  69. GRB 250813B: SVOM/ECLAIRs refined analysis
  70. GRB 250818A: Fermi GBM Observation
  71. LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations
  72. GRB 250818B: SVOM/VT optical observation
  73. GRB 250814A: Further GRAWITA TNG NIR observations
  74. GRB 250818B: Swift-XRT afterglow detection
  75. GRB 250818B: GOTO optical counterpart candidate
  76. GRB 250818B: SVOM detection of a short burst
  77. LIGO/Virgo/KAGRA S250818t: Identification of a GW compact binary merger candidate
  78. GRB 250818A: Swift detection of a burst
  79. GRB 250818A: Fermi GBM Final Real-time Localization
  80. sb25081602: Swift-XRT observations
  81. GRB 250816A: SVOM/GRM observation of a short burst
  82. SVOM/sb25081602: detection of a likely X-ray transient
  83. Fermi GRB 250814B: Global MASTER-Net observations report
  84. GRB 250816A: Fermi GBM Final Real-time Localization
  85. GRB 250814A: Swift-BAT refined analysis
  86. GRB 250815A: Fermi GBM Final Localization
  87. GRB 250813B: Liverpool Telescope optical upper limits
  88. GRB 250814A: Further COLIBRÍ optical upper limit
  89. GRB 250814A: detection of the afterglow candidate with Liverpool Telescope
  90. GRB 250814A: GRAWITA TNG NIR observations
  91. GRB 250812A: SVOM/VT optical continued observation
  92. GRB 250814A: Mondy optical observations of the candidate counterpart to LVK S250814bg
  93. GRB 250813B: Calapai Observatory, Massa S. Giorgio (Messina), upper limit
  94. GRB250814A: TRT optical upper limits
  95. GRB 250814B: SVOM/GRM observation
  96. GRB 250814C: SVOM/GRM observation of a short burst
  97. LIGO/Virgo/KAGRA S250813k: Swift XRT observations, 2 X-ray sources
  98. GRB 250813B: SVOM/GRM observation
  99. GRB 250814A: P200/WIRC observations of the afterglow candidate
  100. GRB 250814A: possible optical afterglow candidate at Spanish observatories

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