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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: GTC/OSIRIS Optical Detection of AT2025ulz
  2. LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz
  3. LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz
  4. GRB 250806A: Gemini observations of host galaxy candidate
  5. IPN triangulation of GRB 250821B (long/bright)
  6. GRB 250821A: Kilonova-Catcher optical upper limits
  7. EP250821a: Swift/XRT counterpart fading
  8. GRB 250812A: SVOM/ECLAIRs refined analysis
  9. GRB 250821A: Swift-BAT refined analysis
  10. LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts
  11. LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz
  12. EP250821a: Swift/UVOT further observations.
  13. EP250821a: VLT X-shooter spectroscopic redshift z = 0.577
  14. LIGO/Virgo/KAGRA S250818k: GRAWITA TNG NIR observations of AT2025ulz
  15. GRB 250821A: VLT/FORS2 Optical Observation
  16. GRB 250821A: GECAM-B detection
  17. GRB 250821B: SVOM/GRM observation
  18. EP250821a: SVOM/VT optical observation
  19. Fermi GRB 250821B: Global MASTER-Net observations report
  20. GRB 250821A: NOT z-band upper limit
  21. GRB 250821C: Fermi GBM Final Real-time Localization
  22. GRB 250821A: KAIT optical upper limit
  23. LIGO/Virgo/KAGRA S250818k: Continued fading and reddening of AT2025ulz from Keck/LRIS imaging observations
  24. GRB 250821A: Swift-XRT refined Analysis
  25. Swift GRB 250821A: Global MASTER-Net observations report
  26. GRB 250821B: Fermi GBM Detection
  27. LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz
  28. GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672)
  29. LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits
  30. GRB 250821B: Fermi GBM Final Localization of a Bright Burst
  31. GRB 250821A: Enhanced Swift-XRT position
  32. GRB 250821A: Swift/UVOT Upper Limits
  33. EP250821a: Rapidly fading optical source detected in Las Cumbres followup
  34. EP250821a: Swift XRT and UVOT counterpart detection
  35. LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits
  36. EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations
  37. GRB 250821A: LCOGT Optical Upper Limits
  38. GRB 250814A: CMO SAI and SAO RAS izJ observations.
  39. LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz
  40. GRB 250821A: Prompt enhanced Swift-XRT position
  41. GRB 250821A: Swift detection of a burst
  42. LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz
  43. LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz
  44. EP250821a: Einstein Probe detection of an X-ray transient
  45. GRB 250819A: SVOM/GRM observation of a possible “long” duration type I burst
  46. GRB 250819A: Insight-HXMT detection
  47. LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz
  48. GRB 250818B: 10 GHz VLA detection
  49. LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz
  50. LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz
  51. LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections
  52. LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz
  53. GRB 250814A: Swift/UVOT Upper Limits
  54. GRB 250813B: Swift/UVOT Upper Limit
  55. IPN triangulation of GRB 250816A (short)
  56. IPN triangulation of GRB 250814C (short)
  57. IPN triangulation of GRB 250804B (short)
  58. GRB 250818B: FTW optical and NIR observations show reddening of the counterpart
  59. GRB 250818B: Kilonova-Catcher optical afterglow detection
  60. LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits
  61. GRB 250818B: Liverpool Telescope optical detections
  62. LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
  63. LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
  64. LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
  65. GRB 250818A: AstroSat CZTI detection
  66. LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
  67. LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
  68. GRB 250818B: Swift/UVOT Detection
  69. Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB
  70. LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
  71. GRB 250820A: Fermi GBM Final Real-time Localization
  72. GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq)
  73. GRB 250818B: TRT Optical Observation
  74. Fermi GRB 250816A: Global MASTER-Net observations report
  75. GRB 250818B: SAO RAS optical observations
  76. Swift GRB 250818A: Global MASTER-Net observations report
  77. GRB 250818B: NOT optical observations
  78. GRB 250818A: Swift-XRT refined Analysis
  79. GRB 250818B: EP-FXT counterpart detection
  80. GRB 250818A: Swift-BAT refined analysis
  81. Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB
  82. LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
  83. GRB 250818C: Fermi GBM Final Real-time Localization
  84. GRB 250818B: Keck redshift of the optical afterglow
  85. Fermi trigger No 776849714: Global MASTER-Net observations report
  86. GRB 250818B: KAIT optical observations
  87. GRB 250818A: Swift/UVOT Upper Limits
  88. LIGO/Virgo/KAGRA S250818t: Updated Sky localization
  89. LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility
  90. GRB 250818A: Enhanced Swift-XRT position
  91. GRB 250813B: SVOM/ECLAIRs refined analysis
  92. GRB 250818A: Fermi GBM Observation
  93. LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations
  94. GRB 250818B: SVOM/VT optical observation
  95. GRB 250814A: Further GRAWITA TNG NIR observations
  96. GRB 250818B: Swift-XRT afterglow detection
  97. GRB 250818B: GOTO optical counterpart candidate
  98. GRB 250818B: SVOM detection of a short burst
  99. LIGO/Virgo/KAGRA S250818t: Identification of a GW compact binary merger candidate
  100. GRB 250818A: Swift detection of a burst

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