Fermi-LAT TOO observations of PSR J1119-6127 following its 2016 magnetar-like outburst

ATel #9378; G. Younes (GWU), R. Archibald (McGill U.), C. Kouveliotou (GWU), V. Kaspi (McGill U.), and P. S. Ray (NRL), J. McEnery (NASA/GSFC) for the Fermi LAT Collaboration
on 16 Aug 2016; 14:36 UT
Credential Certification: George Younes (gyounes@email.gwu.edu)

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We requested and obtained Fermi-LAT target of opportunity (TOO) observations of the high-B gamma-ray pulsar PSR J1119-6127 between 2016 August 1 - 8. This pulsar recently showed magnetar-like bursts on 2016 July 27 and 28 (GCNs #19735, #19736, and ATel #9274).

We performed a preliminary analysis of all LAT data (survey and pointed mode) from 2016 July 27 until 2016 August 12. We do not detect a source at the position of PSR J1119-6127. Using a binned likelihood analysis, and assuming a power-law spectral model with a photon index of 2.35 (Acero et al. 2015 ApJS, 218, 23), we derive a 95% confidence level upper limit of 2.3E-7 ph/s/cm^2 (1.3E-10 erg/s/cm^2) on the flux in the 0.1-300 GeV range. Assuming an exponentially-cutoff power law model with parameters from Parent et al. (2011, ApJ, 743, 170P), we derive a 95% confidence level upper limit of 1.1E-7 ph/s/cm^2 (7.6E-11 erg/s/cm^2) 0.1-300 GeV range. These limits are consistent with the pre-outburst detection of the source at the flux level of 9.3E-8 ph/s/cm^2 (6.4E-11 erg/s/cm^2) reported in Parent et al. (2011, ApJ, 743, 170) and ATel #9365.

Extrapolating the X-ray spectrum in Archibald et al. 2016 (power law with photon index of 1.2) to gamma-ray energies, we predict an energy flux in the LAT band of about 6.0E-7 erg/s/cm^2. We conclude that the power-law X-ray component in this pulsar must break in the MeV range, similar to other magnetars (e.g., Kuiper et al. 2006 ApJ, 645, 556).

We also searched the post-burst LAT data for pulsations from the source using an updated timing solution from Archibald et al. 2016 (arXiv:1608.01007). We do not detect significant pulsations thus far.

Finally, we analyzed all available Swift XRT pointings during the same time-span as the LAT data. The spectra are well fit with an absorbed black-body model (see also Archibald et al. 2016, arXiv:1608.01007). We derive an average X-ray flux in the energy range 0.5-10 keV of about 2.0E-11 erg/s/cm^2. This flux level is 100 times larger than the quiescent flux from the source (Ng et al. 2012 ApJ, 761, 65).

Continued LAT analysis is ongoing, as more data come in, aiming at a detection of gamma-ray pulsations.

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.