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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. GRB 250821A: Swift-BAT refined analysis
  2. LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts
  3. LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz
  4. EP250821a: Swift/UVOT further observations.
  5. EP250821a: VLT X-shooter spectroscopic redshift z = 0.577
  6. LIGO/Virgo/KAGRA S250818k: GRAWITA TNG NIR observations of AT2025ulz
  7. GRB 250821A: VLT/FORS2 Optical Observation
  8. GRB 250821A: GECAM-B detection
  9. GRB 250821B: SVOM/GRM observation
  10. EP250821a: SVOM/VT optical observation
  11. Fermi GRB 250821B: Global MASTER-Net observations report
  12. GRB 250821A: NOT z-band upper limit
  13. GRB 250821C: Fermi GBM Final Real-time Localization
  14. GRB 250821A: KAIT optical upper limit
  15. LIGO/Virgo/KAGRA S250818k: Continued fading and reddening of AT2025ulz from Keck/LRIS imaging observations
  16. GRB 250821A: Swift-XRT refined Analysis
  17. Swift GRB 250821A: Global MASTER-Net observations report
  18. GRB 250821B: Fermi GBM Detection
  19. LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz
  20. GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672)
  21. LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits
  22. GRB 250821B: Fermi GBM Final Localization of a Bright Burst
  23. GRB 250821A: Enhanced Swift-XRT position
  24. GRB 250821A: Swift/UVOT Upper Limits
  25. EP250821a: Rapidly fading optical source detected in Las Cumbres followup
  26. EP250821a: Swift XRT and UVOT counterpart detection
  27. LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits
  28. EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations
  29. GRB 250821A: LCOGT Optical Upper Limits
  30. GRB 250814A: CMO SAI and SAO RAS izJ observations.
  31. LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz
  32. GRB 250821A: Prompt enhanced Swift-XRT position
  33. GRB 250821A: Swift detection of a burst
  34. LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz
  35. LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz
  36. EP250821a: Einstein Probe detection of an X-ray transient
  37. GRB 250819A: SVOM/GRM observation of a possible “long” duration type I burst
  38. GRB 250819A: Insight-HXMT detection
  39. LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz
  40. GRB 250818B: 10 GHz VLA detection
  41. LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz
  42. LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz
  43. LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections
  44. LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz
  45. GRB 250814A: Swift/UVOT Upper Limits
  46. GRB 250813B: Swift/UVOT Upper Limit
  47. IPN triangulation of GRB 250816A (short)
  48. IPN triangulation of GRB 250814C (short)
  49. IPN triangulation of GRB 250804B (short)
  50. GRB 250818B: FTW optical and NIR observations show reddening of the counterpart
  51. GRB 250818B: Kilonova-Catcher optical afterglow detection
  52. LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits
  53. GRB 250818B: Liverpool Telescope optical detections
  54. LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
  55. LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
  56. LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
  57. GRB 250818A: AstroSat CZTI detection
  58. LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
  59. LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
  60. GRB 250818B: Swift/UVOT Detection
  61. Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB
  62. LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
  63. GRB 250820A: Fermi GBM Final Real-time Localization
  64. GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq)
  65. GRB 250818B: TRT Optical Observation
  66. Fermi GRB 250816A: Global MASTER-Net observations report
  67. GRB 250818B: SAO RAS optical observations
  68. Swift GRB 250818A: Global MASTER-Net observations report
  69. GRB 250818B: NOT optical observations
  70. GRB 250818A: Swift-XRT refined Analysis
  71. GRB 250818B: EP-FXT counterpart detection
  72. GRB 250818A: Swift-BAT refined analysis
  73. Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB
  74. LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
  75. GRB 250818C: Fermi GBM Final Real-time Localization
  76. GRB 250818B: Keck redshift of the optical afterglow
  77. Fermi trigger No 776849714: Global MASTER-Net observations report
  78. GRB 250818B: KAIT optical observations
  79. GRB 250818A: Swift/UVOT Upper Limits
  80. LIGO/Virgo/KAGRA S250818t: Updated Sky localization
  81. LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility
  82. GRB 250818A: Enhanced Swift-XRT position
  83. GRB 250813B: SVOM/ECLAIRs refined analysis
  84. GRB 250818A: Fermi GBM Observation
  85. LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations
  86. GRB 250818B: SVOM/VT optical observation
  87. GRB 250814A: Further GRAWITA TNG NIR observations
  88. GRB 250818B: Swift-XRT afterglow detection
  89. GRB 250818B: GOTO optical counterpart candidate
  90. GRB 250818B: SVOM detection of a short burst
  91. LIGO/Virgo/KAGRA S250818t: Identification of a GW compact binary merger candidate
  92. GRB 250818A: Swift detection of a burst
  93. GRB 250818A: Fermi GBM Final Real-time Localization
  94. sb25081602: Swift-XRT observations
  95. GRB 250816A: SVOM/GRM observation of a short burst
  96. SVOM/sb25081602: detection of a likely X-ray transient
  97. Fermi GRB 250814B: Global MASTER-Net observations report
  98. GRB 250816A: Fermi GBM Final Real-time Localization
  99. GRB 250814A: Swift-BAT refined analysis
  100. GRB 250815A: Fermi GBM Final Localization

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