Anisotropies in the gamma-ray sky from millisecond pulsars

Mon. Not. R. Astron. Soc. 415, 1074–1082 (2011) doi:10.1111/j.1365-2966.2011.18672.x Anisotropies in the gamma-ray sky from millisecond pulsars Jennifer M. Siegal-Gaskins,1 Rebecca Reesman,1 Vasiliki Pavlidou,2 † Stefano Profumo3 and Terry P. Walker1 1 Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA 2 Astronomy Department, California Institute of Technology, Pasadena, CA 91125, USA 3 Department of Physics and Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA Accepted 2011 March 8. Received 2011 February 28; in original form 2010 November 24 ABSTRACT Key words: methods: statistical – pulsars: general – gamma-rays: diffuse background. 1 I N T RO D U C T I O N In the era of precision gamma-ray astronomy, with data of unprecedented quality from the Fermi-Large Area Telescope (FermiLAT; Atwood et al. 2009) and ground-based atmospheric Cherenkov telescopes, including HESS, VERITAS and MAGIC, longstanding questions about the high-energy Universe might soon be successfully addressed. One of these is the detailed nature and origin of the diffuse gamma-ray emission. The gamma-ray sky is dominated at low Galactic latitudes by a bright diffuse Galactic component, stemming dominantly from processes involving cosmic rays such as inelastic hadronic collisions producing neutral pions, and inverse Compton and bremsstrahlung emission from relativistic cosmic ray electrons and positrons (see e.g. Strong, Moskalenko & Reimer 2000). At high latitudes, the diffuse gamma-ray background is customarily attributed to extragalactic gamma-ray emitters, such as blazars (e.g. Stecker & Salamon 1996). Recent Fermi-LAT results, however, indicate that unresolved blazars only contribute a small fraction of the observed emission (Abdo et al. 2010a; however, see also Abazajian, Blanchet & Harding 2010; Stecker & Venters 2010), in contrast to, for example, the diffuse X-ray background (Brandt & Hasinger 2005; Hickox & Markevitch 2006, 2007).  E-mail: †Einstein (GLAST) Fellow. Since the discovery of periodic gamma-ray emission from pulsars (Browning, Ramsden & Wright 1971), the possibility that this source class contributes non-negligibly to the diffuse gamma-ray emission has been considered (Bhattacharya & Srinivasan 1991; Bailes & Kniffen 1992; Bhatia, Misra & Panchapakesan 1997). Some of the brightest emitters in the Fermi-LAT gamma-ray sky are in fact associated with pulsating objects, often corresponding to pulsars observed at radio and X-ray frequencies (Abdo et al. 2009b). Compared to its predecessor the Energetic Gamma-Ray Experiment Telescope (EGRET) aboard the Compton Gamma-ray Observatory, Fermi is shedding light not only on young, powerful ‘ordinary’ pulsars (with typical rotation periods of the order of 0.01–1 s and ages ranging between 103 and 106 yr), but also on a distinct class of periodic gamma-ray emitters with much shorter pulsating periods (on the order of a few milliseconds), i.e. millisecond pulsars (MSPs). The characteristic age τ c of MSPs, extrapolated from their period and period derivative, indicates that these objects are much older than ordinary pulsars, with τ c ∼ 1010 yr (Abdo et al. 2009a). MSPs are thought to be associated with binary systems, the spin-up of the pulsar period being fuelled by accretion of mass and angular momentum from the neutron star companion (Phinney & Kulkarni 1994; Lorimer 2001). While the determination of the age of MSPs is a debated matter given the highly non-trivial nature of their evolutionary history (see e.g. Kiziltan & Thorsett 2010), the significantly longer lifetime of these objects compared to that of ordinary pulsars might offset a birthrate that is necessarily lower (given the binary  C 2011 The Authors C 2011 RAS Monthly Notices of the Royal Astronomical Society  Pulsars emerge in the Fermi era as a sizable population of gamma-ray sources. Millisecond pulsars (MSPs) constitute an older subpopulation whose sky distribution extends to high Galactic latitudes, and it has been suggested that unresolved members of this class may contribute a significant fraction of the measured large-scale isotropic gamma-ray background (IGRB). We investigate the possible energy-dependent contribution of unresolved MSPs to the anisotropy of the Fermi-measured IGRB. For observationally motivated MSP population models, we show that the preliminary Fermi anisotropy measurement places an interesting constraint on the abundance of MSPs in the Galaxy and the typical MSP flux, about an order of magnitude stronger than constraints on this population derived from the intensity of the IGRB alone. We also examine the possibility of an MSP component in the IGRB mimicking a dark matter signal in anisotropy-based searches, and conclude that the energy dependence of an anisotropy signature would distinguish MSPs from all but very light dark matter candidates. Millisecond pulsar gamma-ray anisotropy  C 2011 The Authors, MNRAS 415, 1074–1082 C 2011 RAS Monthly Notices of the Royal Astronomical Society  dark matter in subhaloes leads to small-scale anisotropies. Consequently, these structures may provide a substantial contribution to anisotropies in the IGRB (Siegal-Gaskins 2008; Ando 2009; Fornasa et al. 2009; Ibarra et al. 2010). The combined use of spectral and anisotropy information in the IGRB (the anisotropy energy spectrum) could conceivably help reveal the presence of even a subdominant component in the diffuse emission (Siegal-Gaskins & Pavlidou 2009). In particular, it has been shown that the anisotropy energy spectrum could be a sensitive probe of the presence of a dark matter component in the IGRB (Hensley, Siegal-Gaskins & Pavlidou 2010; Cuoco et al. 2011). This technique is also promising for detecting a subdominant MSP contribution to the IGRB, since the emission from unresolved MSPs is expected to feature much stronger anisotropy than the extragalactic component, due to the fact that MSPs are relatively few and nearby, compared to cosmological populations that may constitute the dominant contributors to the IGRB intensity. Additional motivation to study the gamma-ray anisotropy properties of MSPs is provided by the potential interference of MSPs with anisotropy-based searches for dark matter. Fermi data (Abdo et al. 2009a) indicate that the typical gamma-ray MSP spectrum is, in fact, uncomfortably similar in its overall features to what is expected for the annihilation or decay of certain particle dark matter candidates, especially if the dark matter is light (mDM  few tens of GeV). Furthermore, although the amplitude of anisotropies from dark matter annihilation is uncertain, in some scenarios it is expected to be quite large, and thus it is conceivable that an MSP-induced modulation in the anisotropy energy spectrum of the IGRB could be confused with a similar modulation induced by dark matter. In this paper, we explore the potential of an (...truncated)