PSRs J0248+6021 and J2240+5832: young pulsars in the northern Galactic plane

Discovery, timing, and gamma-ray observations

¹ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, LPC2E UMR 6115 CNRS, 45071 Orléans Cedex 02, and Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, 18330 Nançay, France
e-mail: theureau@cnrs-orleans.fr
² Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
e-mail: dparent@ssd5.nrl.navy.mil
³ George Mason University, Fairfax, VA 22030, USA
⁴ Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux, Gradignan, CENBG, Chemin du Solarium, BP 120, 33175 Gradignan, France
e-mail: smith@cenbg.in2p3.fr
⁵ Department of Astronomy and Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
⁶ Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁷ National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA
⁸ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
⁹ Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹¹ High Performance Technologies, Inc., 11955 Freedom Drive, Reston, VA 20190-5673, USA
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁴ Observatoire de Paris-CNRS/LERMA, 77 Av. Denfert Rochereau, 75014 Paris, France
¹⁵ Department of Physics and Astronomy, University of Calgary, Calgary, AB, T2N 1N4, Canada
¹⁶ National Astronomical Observatories of China, Beijing 100012, PR China

Received: 1 July 2010
Accepted: 7 October 2010

Abstract

Context. Pulsars PSR J0248+6021 (with a rotation period P = 217 ms and spin-down power Ė = 2.13  ×  10³⁵ erg s^-1) and PSR J2240+5832 (P = 140 ms, Ė = 2.12  ×  10³⁵ erg s^-1) were discovered in 1997 with the Nançay radio telescope during a northern Galactic plane survey, using the Navy-Berkeley Pulsar Processor (NBPP) filter bank. The GeV gamma-ray pulsations from both were discovered using the Fermi Large Area Telescope.

Aims. We characterize the neutron star emission using radio and gamma-ray observations, and explore the rich environment of PSR J0248+6021.

Methods. Twelve years of radio timing data, including glitches, with steadily improved instrumentation, such as the Berkeley-Orleans-Nançay pulsar backend, and a gamma-ray data set 2.6 times larger than previously published allow detailed investigations of these pulsars. Radio polarization data allow comparison with the geometry inferred from gamma-ray emission models.

Results. The two pulsars resemble each other in both radio and gamma-ray data. Both are rare in having a single gamma-ray pulse offset far from the radio peak. The anomalously high dispersion measure for PSR J0248+6021 (DM  =  370 pc cm^-3) is most likely due to its being within the dense, giant HII region W5 in the Perseus arm at a distance of 2 kpc, as opposed to being beyond the edge of the Galaxy as obtained from models of average electron distributions. Its large transverse velocity and the low magnetic field along the line-of-sight favor this small distance. Neither gamma-ray, X-ray, nor optical data yield evidence of a pulsar wind nebula surrounding PSR J0248+6021. We report the discovery of gamma-ray pulsations from PSR J2240+5832. We argue that it could be in the outer arm, although slightly nearer than its DM-derived distance, but that it may be in the Perseus arm at half the distance.

Conclusions. The energy flux and distance yield a gamma-ray luminosity for PSR J0248+6021 of L_γ = (1.4 ± 0.3)  ×  10³⁴ erg s^-1. For PSR J2240+5832, we find either L_γ = (7.9 ± 5.2)  ×  10³⁴ erg s^-1 if the pulsar is in the outer arm, or L_γ = (2.2 ± 1.7)  ×  10³⁴ erg s^-1 for the Perseus arm. These luminosities are consistent with an ${\mathit{L}}_{\mathit{\gamma }}\mathrm{\propto }\sqrt{\mathit{Ė}}$ rule. Comparison of the gamma-ray pulse profiles with model predictions, including the constraints obtained from radio polarization data, implies outer magnetosphere emission. These two pulsars differ mainly in terms of their inclination angles and acceleration gap widths, which in turn explain the observed differences in the gamma-ray peak widths.