TITLE:   GCN CIRCULAR
NUMBER:  11862
SUBJECT: GRB110328A/Swift J164449.3+573451: Fermi LAT Observations
DATE:    11/04/02 06:06:13 GMT
FROM:    Nicola Omodei at Stanford U.  <nicola.omodei@slac.stanford.edu>

N. Omodei (Stanford), E. Troja (NASA/GSFC/ORAU), R. Corbet (CRESST/UMBC/GSFC), J. S. Perkins (CRESST/UMBC/GSFC), J. L. Racusin (NASA/GSFC),  J. E. McEnery (NASA/GSFC) on behalf of the Fermi Large Area Telescope collaboration.


At the time of the first Swift trigger (Cummings et al., GCN 11823) the Fermi spacecraft was operating in pointing mode observing the region of Cyg X-3. 
The ToO was terminated at 15:13 UT (2.25 hours on target after the initial Swift trigger) and the Fermi spacecraft continued in normal rocking mode. 
During the time of Cyg X-3 pointed mode observation the GRB110328A/Swift J164449.3+573451 was at 47 degrees from the LAT boresight.

We report here the 95% confidence upper limits on the flux for different exposures spanning the time period of the bright activity of the source.
Upper limits (in units of ph/cm^2/s) have been computed between 100 MeV and 10 GeV using the standard likelihood tool, publicly available at the Fermi Science Support Center Web site (http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/).

The model assumed in the fit for the source is a power law.  The two sets of ULs were derived by fixing the spectral index to 2 and 2.5, respectively.

We report upper limits on day-long time scales:

| Date            | Flux (Index = 2) |  Flux (Index = 2.5)  |
| 2011-03-26 |     <3.3e-07       |     <6.0e-07            |
| 2011-03-27 |     <2.9e-07       |     <5.2e-07            |
| 2011-03-28 |     <1.8e-07       |     <2.8e-07            |
| 2011-03-29 |     <1.7e-07       |     <2.6e-07            |
| 2011-03-30 |     <2.3e-07       |     <3.9e-07            |
| 2011-03-31 |     <2.2e-07       |     <3.4e-07            |

We also searched over a shorter time window around the time of the first three Swift triggers (during the hour following each trigger). 
For the last Swift trigger (Sakamoto et al., GCN 11842) the source was never in the LAT field of view, therefore we omit it in the following table:

|                  Date             | Flux (Index = 2) |   Flux (Index = 2.5)  |
| 2011-03-28 12:57:45.2 |         <1.5e-06    |     <9.1e-07            |
| 2011-03-28 13:40:41.2 |         <3.0e-06    |     <4.7e-06            |
| 2011-03-29 18:26:25.1 |         <2.1e-06    |     <2.6e-06            |

No significant gamma-ray emission is seen from the direction of GRB110328A/Swift J164449.3+573451 in the full 27 months of Fermi LAT data. 
We report an upper limit of 1.7e-8 ph/cm^2/s (from 100 MeV to 10 GeV) and an upper limit of 1.5e-10 ph/cm^2/s (from 1 GeV to 300 GeV).
Knowing that this object could be highly variable, we investigated possible emission on timescales of 5 and 2 days over the lifetime of the Fermi mission.
No significant emission was seen in any of the time bins in this light curve.
We also report that no significant sources are detected within three degrees from the position of GRB110328A/Swift J164449.3+573451 are present integrating the data over 27 months.

The Fermi LAT point of contact for this source is Nicola Omodei (nicola.omodei@slac.stanford.edu).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. 
It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.