Sub-luminous gamma-Ray pulsars

📝 Abstract

Most pulsars observed by the Fermi LAT have gamma-ray luminosities scaling with spindown power Edot as L_gamma (Edot x 10^33 erg/s)^{1/2}. However, there exist one detection and several upper limits an order of magnitude or more fainter than this trend. We describe these `sub-luminous’ gamma-ray pulsars, and discuss the case for this being an orientation effect. Of the 12 known young radio pulsars with Edot>10^34 erg/s and d<2kpc several are substantially sub-luminous. The limited available geometrical constraints favor aligned geometries for these pulsars, although no one case for alignment is compelling. In this scenario GeV emission detected from such sub-luminous pulsars can be due to a lower altitude, lower-power accelerator gap.

💡 Analysis

Most pulsars observed by the Fermi LAT have gamma-ray luminosities scaling with spindown power Edot as L_gamma (Edot x 10^33 erg/s)^{1/2}. However, there exist one detection and several upper limits an order of magnitude or more fainter than this trend. We describe these `sub-luminous’ gamma-ray pulsars, and discuss the case for this being an orientation effect. Of the 12 known young radio pulsars with Edot>10^34 erg/s and d<2kpc several are substantially sub-luminous. The limited available geometrical constraints favor aligned geometries for these pulsars, although no one case for alignment is compelling. In this scenario GeV emission detected from such sub-luminous pulsars can be due to a lower altitude, lower-power accelerator gap.

📄 Content

arXiv:1106.5762v1 [astro-ph.HE] 28 Jun 2011 TO APPEAR IN THE ASTROPHYSICAL JOURNAL Preprint typeset using LATEX style emulateapj v. 11/10/09 SUB-LUMINOUS γ-RAY PULSARS R. W. ROMANI, M. KERR1 AND H. A. CRAIG Department of Physics, Stanford University, Stanford, CA 94305 S. JOHNSTON Australia Telescope National Facility, CSIRO, Epping, NSW 1710, Australia I. COGNARD Laboratoire de Physique et Chimie de l’Environnement, LPCE UMR 6115 CNRS, 45071 Orl´eans Cedex 02, and Station de radioastronomie de Nanc¸ay, Observatoire de Paris, CNRS/INSU, 18330 Nanc¸ay, France D.A. SMITH Universit´e Bordeaux 1, CNRS/IN2p3, Centre d’Etudes Nucl´eaires de Bordeaux Gradignan, 33175 Gradignan, France To appear in the Astrophysical Journal ABSTRACT Most pulsars observed by the Fermi LAT have γ-ray luminosities scaling with spindown power ˙E as Lγ ≈ ( ˙E · 1033erg/s)1/2. However, there exist one detection and several upper limits an order of magnitude or more fainter than this trend. We describe these ‘sub-luminous’ γ-ray pulsars, and discuss the case for this being an orientation effect. Of the 12 known young radio pulsars with ˙E > 1034erg s−1 and d ≤2 kpc several are substantially sub-luminous. The limited available geometrical constraints favor aligned geometries for these pulsars, although no one case for alignment is compelling. In this scenario GeV emission detected from such sub-luminous pulsars can be due to a lower altitude, lower-power accelerator gap. Subject headings: gamma rays: stars - pulsars: general

1. INTRODUCTION The Large Area Telescope (LAT) on the Fermi satellite has now detected over 75 spin-powered pulsars (Abdo et al. 2010a; Romani 2011). Among the ≈50 non-recycled en- ergetic pulsars there is a clear trend for γ-ray ‘efficiency’ to increase with decreasing spin-down power ˙E, giving a heuris- tic γ-ray luminosity Lγ,heu ≈( ˙E × 1033erg/s)1/2. (1) This is a natural result for models where the emission is produced by a Goldreich-Julian current of charges passing through a characteristic potential drop (Harding 1981; Arons 2006). Of course, energy conservation limits Lγ < ˙E, and as ˙E decreases, the star is unable to maintain the potential drop, leading to a ‘death zone’ below ˙E ≈1033−1034erg s−1 where this process starts to turn off. This is portrayed in fig- ure 5 of Abdo et al. (2010a), where most energetic pulsars lie between Eq (1) and unit efficiency. Only two young pul- sars in that plot lie significantly below the Lγ,heu line: PSR J0205+6449, where a small inferred distance places it just be- low this value, and PSR J0659+1414 (to be discussed in this paper) which is ∼20× less luminous. Thus, independent of its physical validity, Eq. (1) forms an effective lower lumi- nosity envelope to the bulk of the observed pulsar sample. Estimates of Lγ suffer two complications. The first is the source distance; for most LAT pulsars we have only distance estimates based on the pulsar Dispersion Measure (DM). DM modeling (Cordes & Lazio 2002, hereafter CL02) is believed rwr@astro.stanford.edu 1 Einstein Fellow to provide statistically useful estimates of pulsar distances, with a scatter of ≈30% about independent distance estimates, although typical errors for nearby pulsars may be as large as 60% (see Deller 2009). DM distances are certainly not reli- able for individual objects, and it appears (Abdo et al. 2010a) that they may be especially poor for the young, energetic LAT pulsars. This is likely since the sample is nearby and asso- ciated with regions of active star formation where the excess ionized gas may significantly perturb the dispersion measures. About a third of the LAT pulsars are found directly in the γ- ray data through so-called ‘blind’ searches (Abdo et al. 2009; Saz Parkinson et al. 2010); most of these lack radio detections and so do not even have DM distance estimates. The second complication is the conversion from the observed energy flux FE along the Earth line-of-sight to the true sky averaged lu- minosity Lγ = 4πfΩFED2. (2) Watters et al. (2009) and Romani & Watters (2010, RW10) have estimated ‘flux conversion factors’ fΩfor this correc- tion for a variety of pulsar models and viewing geometries. For most of the observed pulsars, fΩshould be in the range 0.7 −1.3, although some lower ˙E pulsars, especially γ- selected objects (Watters & Romani 2011), may have fΩas small as 0.1 for ‘outer gap’ (OG) geometries. However, there are a handful of pulsars whose observed lu- minosity or limit fall an order of magnitude or more below Lγ,heu. In spite of the uncertainties just discussed we can make a case that they are truly sub-luminous. There are three possible interpretations. The first is that the γ-ray radiation is beamed away from the Earth line-of-sight (or equivalently fΩ> 10). The second is that some particular physical prop- 2 Romani et al. FIG. 1.— The spin-down-luminosity plane for energetic pulsars, with the heuristic luminosity trend, which saturates somewhere in the ‘death zone’ (shaded).

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