The Fermi LAT/GBM detection of pulsed gamma-ray emission from PSR J1846-0258 up to 100 MeV

ATel #9077; Lucien Kuiper (SRON), Ariane Dekker (SRON, UU)
on 24 May 2016; 13:57 UT
Credential Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

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Applying phase coherent timing models, created using RXTE PCA and Swift XRT monitoring data of PSR J1846-0258 covering the period August 4, 2008 - March 11, 2016 (MJD 54682 - 57458), in timing analyses of Fermi LAT (PASS8) and Fermi GBM (TTE) data yielded for the first time the detection of pulsed gamma-ray emission from PSR J1846-0258 up to 100 MeV.

Phase folding the barycentered Fermi LAT events (period MJD 56185-56338, i.e. Sept. 15, 2012 - Feb. 15, 2013 has been excluded because of the loss of phase coherence), falling within the energy dependent 68%-cone, resulted in a 30-100 MeV pulse profile, which deviates more than 3.7 sigma (single trial) from uniformity applying a Z ₁²-test with its single -slightly asymmetric- pulse aligned with the Swift XRT 2.5-10 keV profile (note: a slightly tighter cone selection of 85% of the 68%-cone yielded a 4.4 sigma signal). For energies above 100 MeV no significant pulsed emission has been detected indicating the softness of the spectrum at these energies. Assuming a similar spectral shape for the pulsed spectrum as the 'canonical' soft gamma-ray pulsar PSR B1509-58 (see Kuiper & Hermsen 2015; MNRAS 449, p. 3839) the measured 30-100 MeV pulsed count rates for PSR J1846-0258 and PSR B1509-58 translate in a 30-100 MeV pulsed flux of (1.41+/- 0.43)E-11 erg/cm²s for PSR J1846-0258, about 4-5 times weaker than that observed for PSR B1509-58.

We also phase-folded the time-tagged events (TTE - the default operation mode since Nov. 26, 2012; 2 us precision; 128 spectral channels) from the 12 NaI (8-2000 keV) detectors of the Fermi GBM collected during Fermi mission weeks 246-404 (i.e.from Feb. 14, 2013 - March 2, 2016). We detected pulsed emission at 6.2 sigma confidence for the 20-300 keV band, applying a Z₁²-test, with the single asymmetric profile aligned with the 2.5-10 keV Swift XRT profile. For the bands 20-100 keV and 100-300 keV we found 5.2 sigma and 3.2 sigma, respectively. The mean (screened) exposure per NaI detector was 16.86 Ms. A pulse profile collage showing the contemporaneous Swift XRT (2.5-10 keV), Fermi GBM (20-300 keV) and Fermi LAT (30-100 MeV) profiles with superposed the best fit RXTE PCA 2-20 keV pulsed profile is shown at PSR J1846-0258 profile .

Our results confirm that the shape of the pulsed spectrum of PSR J1846-0258 mimics that of PSR B1509-58, and that its spectrum extends now up to 100 MeV. Future Fermi observations of PSR J1846-0258, assuming a continuation of the Swift XRT monitoring program of PSR J1846-0258, will improve the knowledge of the soft gamma-ray spectral characteristics considerably. We acknowledge the use of public Fermi GBM and LAT as well as Swift XRT data.