TITLE:   GCN CIRCULAR
NUMBER:  20854
SUBJECT: LIGO/Virgo G275697: GRAWITA NOT optical observations of Swift/XRT Source 23
DATE:    17/03/12 15:27:55 GMT
FROM:    Daniele Malesani at DARK/NBI  <malesani@dark-cosmology.dk>

Daniele Malesani (DARK/NBI), Elena Pian (SNS), L. Amati (INAF-IASF Bo), 
L. A. Antonelli, S. Ascenzi (INAF-OAR), M.T. Botticella (INAF-OAC), M. 
Branchesi (Urbino University/INFN Firenze), S. Campana (INAF-OABr), E. 
Cappellaro (INAF OAPd), S. Covino, P. D'Avanzo (INAF-OABr), V. D'Elia 
(INAF-ASDC), F. Getman, A. Grado (INAF-OAC), G. Greco (Urbino 
University, INFN Firenze), L. Limatola (INAF-OAC), M. Lisi (INAF-OAR), 
A. Melandri (INAF-OABr), L. Nicastro, E. Palazzi (INAF-IASF Bo), S. 
Piranomonte, L. Pulone (INAF-OAR), A. Rossi (INAF-IASF Bo), G. Stratta 
(Urbino University/INFN Firenze), G. Tagliaferri (INAF-OABr), V. Testa 
(INAF-OAR), L. Tomasella, S. Yang (INAF-OAPd), E. Brocato (INAF-OAR), on 
behalf of the GRAvitational Wave Inaf TeAm (GRAWITA), report:

We observed the location of "Source 23" (Evans et al., LVC Circ. GCN 
20841) within the error uncertainty of the LIGO/Virgo event G275697 (LVC 
Circ. 20763), using the Nordic Optical Telescope (NOT) equipped with the 
ALFOSC imaging camera. Observations were carried out at high air mass 
(average 2.1) around twilight using the SDSS i filter. The mean epoch of 
the observation is March 10.26458 UT (10.47 days after the GW event). 
Two images with 2 min exposure and and eight with 1 min were secured (12 
min in total) in the SDSS i filter.

The location of XRT Source 23 lies close to an unrelated bright star (R 
~ 11), which complicates the analysis inside the XRT error circle. Two 
objects, both visible in the Pan-STARRS archival images, are detected 
consistent with the XRT error circle. Their properties are summarized below:

O1:
RA(J2000) = 21:33:15.28
Dec(J2000) = +39:41:11.6
i = 19.76 +- 0.08 AB (NOT data)
i = 19.90 +- 0.15 AB (Pan-STARRS data)

O2:
RA(J2000) = 21:33:14.74
Dec(J2000) = +39:41:07.6
i = 21.94 +- 0.16 AB (NOT data)
i = 21.90 +- 0.15 AB (Pan-STARRS data)

Photometry was computed against nearby stars from the Pan-STARRS 
catalog. For the Pan-STARRS images, we re-measured the photometry to 
properly account for the glare of the bright star, so the quoted values 
differ from those reported in the Pan-STARRS catalog. These values are 
very sensitive to the choice of the background area, hence the large 
uncertainties.

Neither object shows significant variability.

No other objects are detected within the XRT error area, down to a 
3-sigma limiting magnitude i = 21.5 (AB).