Aims. To provide a significantly improved probability distribution for the H-test for periodicity in X-ray and γ-ray arrival times, which is already extensively used by the γ-ray pulsar community. ...Also, to obtain an analytical probability distribution for stacked test statistics in the case of a search for pulsed emission from an ensemble of pulsars where the significance per pulsar is relatively low, making individual detections insignificant on their own. This information is timely given the recent rapid discovery of new pulsars with the Fermi-LAT t γ-ray telescope. Methods. Approximately 1014 realisations of the H-statistic (H) for random (white) noise is calculated from a random number generator for which the repetition cycle is ≫1014. From these numbers the probability distribution P(>H) is calculated. Results. The distribution of H is found to be exponential with parameter λ = 0.4 so that the cumulative probability distribution P(>H)= $\exp$(-λ H). If we stack independent values for H, the sum of K such values would follow the Erlang-K distribution with parameter λ for which the cumulative probability distribution is also a simple analytical expression. Conclusions. Searches for weak pulsars with unknown pulse profile shapes in the Fermi-LAT, Agile or other X-ray data bases should benefit from the H-test since it is known to be powerful against a broad range of pulse profiles, which introduces only a single statistical trial if only the H-test is used. The new probability distribution presented here favours the detection of weaker pulsars in terms of an improved sensitivity relative to the previously known distribution.
The recent Fermi detection of the globular cluster (GC) 47 Tucanae highlighted the importance of modeling collective gamma-ray emission of millisecond pulsars (MSPs) in GCs. Steady flux from such ...populations is also expected in the very high energy domain covered by ground-based Cherenkov telescopes. We present pulsed curvature radiation (CR) as well as unpulsed inverse Compton (IC) calculations for an ensemble of MSPs in the GCs 47 Tucanae and Terzan 5. We demonstrate that the CR from these GCs should be easily detectable for Fermi, while constraints on the total number of MSPs and the nebular B-field may be derived using the IC flux components.
The source of Galactic cosmic rays (with energies up to 1015 eV) remains unclear, although it is widely believed that they originate in the shock waves of expanding supernova remnants. At present the ...best way to investigate their acceleration and propagation is by observing the γ-rays produced when cosmic rays interact with interstellar gas. Here we report observations of an extended region of very-high-energy (> 1011 eV) γ-ray emission correlated spatially with a complex of giant molecular clouds in the central 200 parsecs of the Milky Way. The hardness of the γ-ray spectrum and the conditions in those molecular clouds indicate that the cosmic rays giving rise to the γ-rays are likely to be protons and nuclei rather than electrons. The energy associated with the cosmic rays could have come from a single supernova explosion around 104 years ago.
Curvature radiation (CR) is believed to be a dominant mechanism for creating gamma-ray emission from pulsars and is emitted by relativistic particles that are constrained to move along curved ...magnetic field lines. Additionally, synchrotron radiation (SR) is expected to be radiated by both relativistic primaries (involving cyclotron resonant absorption of radio photons and re-emission of SR photons), or secondary electron-positron pairs (created by magnetic or photon-photon pair production processes involving CR gamma rays in the pulsar magnetosphere). When calculating these high-energy spectra, especially in the context of pulsar population studies where several millions of CR and SR spectra have to be generated, it is profitable to consider approximations that would save computational time without sacrificing too much accuracy. This paper focuses on one such approximation technique, and we show that one may gain significantly in computational speed while preserving the accuracy of the spectral results.
The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the ...‘first stars’, which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. An alternative approach is to study the absorption features imprinted on the γ-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons. Here we report the discovery of γ-ray emission from the blazars H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources—in particular from the first stars formed. This result also indicates that intergalactic space is more transparent to γ-rays than previously thought.
One of the most fundamental questions in pulsar physics is the generation of pair cascading to explain radio through g-ray pulsed emission. In this paper we derive the pair production multiplicity ...for pulsars from the very high energy (VHE) g-ray spectra of their pulsar wind nebulae (PWNs), as measured by H.E.S.S., since the IC-generated g-ray spectra are free from assumptions of the associated nebular magnetic field strength. Most VHE PWNs have been resolved by H.E.S.S., and since the VHE size is already consistent with the maximal expected PWN size of the sources considered in this paper, the age of the VHE emitting electrons must at least be comparable to the age of the pulsar. This allows us to integrate the Goldreich-Julian current over the lifetime of the pulsar, resulting in time averages super( )greater that 2000 and 500 for PSR B1509-58 and PSR B1823-13, respectively, whereas the H.E.S.S. data alone for Vela X indicate an anomalously low value, less than unity. The extreme brightness of Vela X in radio, however, indicates that most of the electrons from the pulsar have been reprocessed into the radio synchrotron domain, leading to an upper limit of super( )< 4 x 10 super(5), if we assume the minimal field strength of 1.7 kG derived from a comparison of H.E.S.S. and ASCA/ROSAT data of Vela X. Similar (large) numbers are, however, derived for PSR B1823-13 and PSR B1509-58 if we extrapolate the H.E.S.S.-derived electron spectra into the uncooled domain toward the unseen radio nebular domain. Such divergent limits on can be resolved by adding forthcoming GLAST data to H.E.S.S. information.
Geminga and B0656+14 are the closest pulsars with characteristic ages in the range of 100 kyr to 1 Myr. They both have spin-down powers of the order 3 x 10 super(34) ergs s super(-1) at present. The ...winds of these pulsars had most probably powered pulsar wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of the pulsars. Assuming that leptonic particles accelerated by the pulsars were confined in the PWNe and were released into the interstellar medium (ISM) on breakup of the PWNe, we show that, depending on the pulsar parameters, both pulsars make a nonnegligible contribution to the local cosmic ray (CR) positron spectrum, and they may be the main contributors above several GeV. The relatively small angular distance between Geminga and B0656+14 thus implies an anisotropy in the local CR positron flux at these energies. We calculate the contribution of these pulsars to the locally observed CR electron and positron spectra depending on the pulsar birth period and the magnitude of the local CR diffusion coefficient. We further give an estimate of the expected anisotropy in the local CR positron flux. Our calculations show that within the framework of our model, the local CR positron spectrum imposes constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar period at birth, and also the local interstellar diffusion coefficient for CR leptons.
We show that the high-energy gamma -ray flux in the GeV domain from mature pulsar wind nebulae (PWNe) scales as the change in rotational kinetic energy l( Omega super(2) sub(0) - Omega super(2))/2 ...since birth, rather than the present-day spin-down power / Omega Omega . This finding holds as long as the lifetime of inverse Compton emitting electrons exceeds the age of the system. For a typical gamma super(-2) electron spectrum, the predicted flux depends mostly on the pulsar birth period, conversion efficiency of spin-down power to relativistic electrons, and distance to the PWN, so that first-order analytic estimates of the birth period can be assessed from GLAST LAT observations of PWNe. The associated ("uncooled") photon spectral index in the GeV domain is expected to cluster around similar to 1.5, which is bounded at low energies by an intrinsic spectral break, and at higher energies by a second spectral break where the photon index steepens to similar to 2 due to radiation losses. However, if the spectral parameters deviate from the above-mentioned assumptions, we can combine GLAST LAT observations with multiwavelength data from radio to TeV gamma -ray observations to improve such birth period estimates. From the ATNF pulsar catalog we expect more than 50 Galactic PWNe to be detectable if the ratio of birth period to current period is <0.5. However, many of them may remain unidentified during evolved stages characterized by low spin-down power and low synchrotron brightness, while only this relic GeV component is visible.
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