We report on the search for 0.1-10 GeV emission from magnetars in 17 months of Fermi Large Area Telescope (LAT) observations. No significant evidence for gamma-ray emission from any of the currently-known magnetars is found. The most stringent upper limits to date on their persistent emission in the Fermi-LAT energy range are estimated between ~10^{-12}-10^{-10} erg/s/cm2, depending on the source. We also searched for gamma-ray pulsations and possible outbursts, also with no significant detection. The upper limits derived support the presence of a cut-off at an energy below a few MeV in the persistent emission of magnetars. They also show the likely need for a revision of current models of outer gap emission from strongly magnetized pulsars, which, in some realizations, predict detectable GeV emission from magnetars at flux levels exceeding the upper limits identified here using the Fermi-LAT observations.
Magnetars are isolated neutron stars whose emission is thought to be powered by their magnetic energy. They are discovered either through their bursting activity (and in this case named Soft Gamma Repeaters; SGRs; Kouveliotou et al. 1998) or by their strong persistent soft X-ray emission (then named Anomalous X-ray Pulsars; AXPs; Mereghetti & Stella 1995). In recent years SGRs and AXPs have been recognized as part of the magnetar class, with the discovery of many AXPs and SGRs showing common characteristics and properties (e.g. Kaspi et al. 2003;Rea et al. 2009;Mereghetti et al. 2009;Israel et al. 2010).
Their X-ray luminosities are typically 10 33 -10 36 erg s -1 . They have spin periods between 2-12 s and period derivatives in the 10 -13 -10 -11 ßrange. In most of the cases, the magnetic fields derived from their spin periods and period derivatives, assuming only magnetic dipolar losses as is usually done for normal pulsars, are inferred to be ∼ 10 14 -10 15 Gauss. These high B fields, in particular their toroidal components, impose tremendous stresses on neutron star crusts, and thereby are believed to be the ultimate energy source of magnetar emission (Duncan & Thompson 1992;Thompson & Duncan 1993). The strong soft X-ray emission can be empirically modeled with a black body (kT∼0.3-0.7 keV), plus a power law (Γ ∼1.5-4; see e.g. Mereghetti 2008 for a review), although recently more physically based models have been developed to account for their X-ray spectra (e.g. Lyutikov & Gavrill 2006;Fernandez & Thompson 2007;Rea et al. 2008;Zane et al. 2009). In the past years, magnetars have been discovered as persistent hard non-thermal X-ray sources, emitting up to ∼250 keV (e.g. Kuiper et al. 2004Kuiper et al. , 2006;;Götz et al. 2006;den Hartog et al. 2008;Rea et al. 2009;Enoto et al. 2010).
Our current knowledge of their spectra at much higher energies (>0.5 MeV) is very limited. Archival studies of COMPTEL observations were used to place upper limits on the emission of a few magnetars in the 0.75-30 MeV range, of ∼ 10 -10 erg s -1 cm -2 at a 2σ level (Kuiper et al. 2006). Very poor so far is the knowledge concerning their behavior at energies >30 MeV (Heyl & Hernquist 2005), a band of interest given model predictions of measurable synchrotron/curvature emission (Chang & Zheng 2001;Zhang & Cheng 2002).
The Large Area Telescope (LAT), the main instrument on board the Fermi Gamma-ray Space Telescope, launched on 2008 June 11th, is the most sensitive telescope to date in the GeV energy range. We present in this Letter results of a search for emission in the GeV domain from the first 17 months of Fermi-LAT observations of magnetars 1 .
The data analyzed here were taken in survey mode with the Fermi Large Area Telescope, from 4 August 2008 until 1 January 2010 . The Fermi-LAT telescope is sensitive to photons with energies from about 20 MeV to more than 300 GeV and uses the pair conversion technique. The direction of an incident photon is derived by tracking the electron-positron pair in a high-resolution converter tracker, and the energy of the pair is measured with a CsI(Tl) crystal calorimeter. The Fermi-LAT has an on-axis effective area of 8000 cm 2 , a 2.4 sr field of view, and an angular resolution of ∼0.6 • at 1 GeV (for events converting in the front section of the tracker). Furthermore, an anti-coincidence detector identifies the background of charged particles (Atwood et al. 2009).
We analyzed the data using the Fermi Science Tools v9r15 package. Events from the “Pass 6 Diffuse” event class are selected, i.e. the event class with the greatest Note.
-Properties of the magnetars studied in this work ordered by the measured TS values derived from the binned analysis (for further info on the first 4 columns see Mereghetti (2008) and reference therein; Rea et al. (2009Rea et al. ( , 2010) ) for the newly discovered SGR 0501+4516 and SGR 0418+5729, respectively). The GeV upper limits are reported at 95% confidence level (see Sect. 5 for details). Fluxes are in units of 10 -11 erg s -1 cm -2 (or 10 -8 photons cm -2 s -1 for numbers in brackets). The last 4 sources and 1E 1841-045 are discussed in detail in the text. * Note that most of the sources have very variable X-ray luminosities, and very uncertain distances, hence those values should be taken as indicative.
purity of gamma rays, having the most stringent background rejection (Atwood et al. 2009). The “Pass 6 v3 Diffuse” instrument response functions (IRFs) are applied in the analysis. For each analyzed source we select events with energy E>100 MeV in a circular region of interest (ROI) of 10 • radius. The good time intervals are defined such that the ROI does not go below the gammaray-bright Earth limb (defined at 105 • from the Zenith angle), and that the source is always inside the LAT field of view, namely in a cone angle of 66 • .
Gamma-ray emission was analyzed at the positions of all the magnetars known to date, excluding yet unconfirmed ca
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