Pulsed Gamma-ray Emission Research Articles

PULSED VERY HIGH ENERGY γ-RAY EMISSION CONSTRAINTS FOR PSR B1951+32 FROM STACEE OBSERVATIONS

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a ground-based telescope that uses the wave-front-sampling technique to detect very high energy (VHE) gamma rays. STACEE's sensitivity in the energy range near 100 GeV permits useful observations of pulsars with the potential to discriminate between various proposed mechanisms for pulsed gamma-ray emission. Based on the 11.3 hr of data taken during the 2005 and 2006 observing seasons, we derive an upper limit on the pulsed gamma-ray emission from PSR B1951+32 of <6.53 × 10–11 photons cm–2 s–1 above an energy threshold of 117 GeV.

VHE γ‐Ray Observation of the Crab Nebula and its Pulsar with the MAGIC Telescope

We report about very high energy (VHE) gamma-ray observations of the Crab Nebula with the MAGIC telescope. The gamma-ray flux from the nebula was measured between 60 GeV and 9 TeV. The energy spectrum can be described by a curved power law dF/dE = f(0)(E/300 GeV)([a+b log)((E/300 GeV)])(10) with a flux normalization f(0) of (6.0 +/- 0.2(stat)) x 10(-10) cm(-2) s(-1) TeV-1, a = 2.31 +/- 0.06(stat), and b = 0.26 +/- 0.07(stat). The peak in the spectral energy distribution is estimated at 77 +/- 35 GeV. Within the observation time and the experimental resolution of the telescope, the gamma-ray emission is steady and pointlike. The emission's center of gravity coincides with the position of the pulsar. Pulsed gamma-ray emission from the pulsar could not be detected. We constrain the cutoff energy of the pulsed spectrum to be less than 27 GeV, assuming that the differential energy spectrum has an exponential cutoff. For a superexponential shape, the cutoff energy can be as high as 60 GeV.

Pulsars as gamma ray sources: Nebular shocks and magnetospheric gaps

I summarize the results of recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is primarily electrons and positrons with an admixture of heavy ions. Shocks which contain heavy ions that are a minority constituent by number but which carry most of the energy density in the upstream medium put ~ 20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) α E -2, where N(E)dE is the number of particles with energy between E and E + dE. Synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front, provides the mechanism of thermalization and non-thermal particle acceleration. The maximum energy achievable by the pairs is γ±m±c2 = m i c 2 γ 1/Z i , where ; 1 is the Lorentz factor of the upstream flow and Z i is the atomic number of the ions. The shock’s spatial structure contains a series of “overshoots” in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value. These overshoots provide a new interpretation of the structure of the inner regions of the Crab Nebula, in particular of the “wisps”, surface brightness enhancements near the pulsar. The wisps appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar’s wind in the Crab Nebula is spatially resolved, and allows one to measure ; 1~ 4 × 106, the upstream magnetic field B 1 to be ~ 3 × 10-5 Gauss, as well as to show that the total ion flow is~ 3 × 1034 elementary charges/sec, in good agreement with the total current flow predicted by the early Goldreich and Julian (1969) model. The total pair outflow is shown to be about 5 × 1037 pairs per second, in good agreement with the particle flux required to explain the nebular X—ray source. The energetics of particle acceleration within the magnetospheres of rotation powered pulsars and the consequences for pulsed gamma ray emission are also briefly discussed. The gamma ray luminosity above 100 MeV is shown to scale in proportion to Ė1/2 R, as is in accord with some of the simplest ideas about “polar cap” models. Models based on acceleration in the outer magnetosphere are also briefly discussed.

Pulsed gamma-ray emission from short-period pulsars

Pulsed gamma-ray emission from short-period pulsars

HEAO 3 upper limits to pulsed gamma-ray emission from PSR 1509 - 58 and PSR 0833 - 45

view Abstract Citations (12) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS HEAO 3 Upper Limits to Pulsed Gamma-Ray Emission from PSR 1509-58 and PSR 0833-45 Ulmer, M. P. ; Purcell, W. R. ; Wheaton, W. A. ; Mahoney, W. A. Abstract This paper reports upper limits to the 50 keV-10 MeV gamma-ray pulsations from PSR 1509 - 58 and PSR 0833 - 45 (Vela) made with the HEAO 3 gamma-ray spectrometer. The 2 sigam upper limit to the 50 - 300 keV flux from PSR 1509 - 58 is 6.9 x 10 to the -6th photons/sq cm per sec per keV. Combined with the best-fit X-ray spectrum, this limit suggests there is a break in the spectrum below about 100 keV. This upper limit is not stringent enough, however, to distinguish between thermal and nonthermal models for the source of the X-ray emission. The 2 sigma upper limit to the 3.2 - 10 MeV flux from PSR 0833 - 45 is 4.9 x 10 to the -8th photons/sq cm per sec per keV. Publication: The Astrophysical Journal Pub Date: March 1991 DOI: 10.1086/169778 Bibcode: 1991ApJ...369..485U Keywords: Gamma Ray Bursts; Gamma Ray Spectrometers; Pulsars; X Ray Spectra; Ephemerides; Signal To Noise Ratios; Stellar Magnetic Fields; Space Radiation; GAMMA RAYS: BURSTS; PULSARS; STARS: INDIVIDUAL ALPHANUMERIC: PSR 1509-58; STARS: INDIVIDUAL ALPHANUMERIC: PSR 0833-45 full text sources ADS | data products SIMBAD (2)

Search for pulsed gamma-ray emission at MeV energies from 24 radio pulsars

view Abstract Citations (3) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Search for pulsed gamma-ray emission at MeV energies from 24 radio pusars. Graser, U. ; Schoenfelder, V. Abstract A search for pulsed gamma-ray emission of 24 radio pulsars at their extrapolated radio periods was performed from balloon data of the MPI balloon-borne Compton telescope, which is sensitive to gamma-rays in the energy range 1-20 MeV. No clear significant signal was obtained from any of these pulsars. For PSR 0950+8 and PSR 1929+10, which both are nearby and relatively short-period pulsars, the derived upper limits to the pulsed flux are close to the theoretically expected luminosities. In case of PSR 0950+08, there are several hints in the data indicating that pulsed gamma-rays might have been detected already. At preseent, the statistical significance of about 3 sigma is too small to rule out an accidental result. This pulsar is a very promising candidate for further studies. A confirmation of the result would show that older pulsars are more efficient gamma-ray emitters than younger ones, as predicted from theory. Publication: The Astrophysical Journal Pub Date: October 1983 DOI: 10.1086/161402 Bibcode: 1983ApJ...273..681G Keywords: Gamma Ray Astronomy; Pulsars; Radio Sources (Astronomy); Astronomical Catalogs; Balloon-Borne Instruments; Gamma Ray Telescopes; Light Curve; Pulsed Radiation; Astrophysics full text sources ADS | data products SIMBAD (24)

High Energy Gamma Rays from Pulsars

We present here the results from observations of pulsed gamma ray emission from the Crab and Vela pulsars for energies above 100 GeV using the atmospheric Cerenkov technique. The results suggest a very steep energy spectrum for gamma rays emitted from pulsars at high energies. Our observations over the last 4 years suggest also that the flux is highly variable with time.

Production and beaming of pulsar gamma-ray emission

view Abstract Citations (42) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Production and beaming of pulsar gamma -ray emission. Hardee, P. E. Abstract An analysis of pulsar emission models is performed which demonstrates that the relative phasing of the observed radio and gamma-ray emission from the Vela pulsar and PSR 1747-46 can be explained as an effect of rotational aberration, with the gamma rays originating at or near the stellar surface and the radio emission generated at some distance above the surface. The results also indicate that the pulsed gamma-ray emission from PSR 1818-04 is produced similarly, but that the combination of the magnetic-field configuration near the stellar surface and some shearing of the field far from the surface can lead to surface (gamma-ray) emission which precedes the (radio) emission from large radial distances. It is suggested that the very-high-energy pulsed gamma rays from the Crab pulsar cannot be emitted from the stellar surface, but rather are produced at the light cylinder by the small number of particles with energies greater than 10 to the 14th power eV that can reach the light cylinder along the magnetic axis. The observed variable phase of this emission is tentatively attributed to production of very-high-energy gamma rays along strongly sheared magnetic-field lines at the light cylinder. Publication: The Astrophysical Journal Pub Date: September 1977 DOI: 10.1086/155532 Bibcode: 1977ApJ...216..873H Keywords: Gamma Ray Astronomy; Neutron Stars; Pulsars; Radio Sources (Astronomy); Stellar Magnetic Fields; Energy Dissipation; Lines Of Force; Magnetic Field Configurations; Phase Deviation; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (2)

Search for pulsed gamma rays of about 10 to the 13th power eV from NP 0532

Results are reported for five nights of observations of the Crab Nebula with a ground-based Cerenkov light detector, which were made to search for pulsed gamma-ray emission in the region above 10 to the 11th power eV from the Crab Pulsar, NP 0532. The time distribution of 1999 detected gamma-ray events shows no evidence for pulsation at either the main phase or the interpulse phase. A chi-squared test applied separately to the data for each night indicates that NP 0532 was active on one night, with a narrow peak emission about 10.5 ms after the phase of the main peak and possible broader emission elsewhere. It is shown that if the data are interpreted as a positive result, they suggest the presence of a pulsed emission component at an energy of at least 2 by 10 to the 13th power eV which is variable on a timescale of less than one day and does not appear strongly at either the main or the interpulse phase. The pulsed photon intensity averaged over all five nights is determined to be 1.2 by 10 to the -12th power photon/sq cm per sec.

A search of the SAS-2 data for pulsed gamma-ray emission from radio pulsars

Data from the SAS-2 high energy gamma ray experiment were examined for pulsed emission from each of 75 radio pulsars which were viewed by the instrument and which have sufficiently well defined period and period derivative information from radio observations to allow for gamma ray periodicity searches. When gamma ray arrival times were converted to pulsar phase using the radio reference timing information, two pulsars, PSR 1747-46 and PSR 1818-04, showed positive effects, each with a probability less than 0.0001 of being a random fluctuation in the data for that pulsar. These are in addition to PSR 0531+21 and PSR 0833-45, previously reported. The results of this study suggest that gamma-ray astronomy has reached the detection threshold for gamma ray pulsars and that work in the near future should give important information on the nature of pulsars.

Interpretation of the pulsed gamma-ray emission from VELA

A model is proposed for the Vela pulsar in which the radio emission originates near the surface of the neutron star while the pulsed gamma ray emission is produced by synchrotron radiation near the speed of light cylinder. This model can explain the energy flux, double pulse structure, and phase shift with respect to the radio of the gamma ray emission and offers approximate quantitative predictions for other X-ray and gamma-ray fluxes.

SAS-2 high-energy gamma-ray observations of the VELA pulsar

The second Small Astronomy Satellite high-energy (35 MeV) gamma-ray telescope detected pulsed gamma-ray emission at the radio period from PSR 0833-45, the Vela pulsar, as well as an unpulsed flux from the Vela region. The pulsed emission consists of two peaks, one following the radio peak by about 13 msec, and the other 0.4 period after the first. The luminosity of the pulsed emission above 100 MeV from Vela is about 0.1 that of the pulsar NP0532 in the Crab nebula, whereas the pulsed emission from Vela at optical wavelengths is less than 0.0004 that from the Crab. The relatively high intensity of the pulsed gamma-ray emission and the double peak structure, compared to the single pulse in the radio emission, suggests that the high energy gamma-ray pulsar emission may be produced under different conditions from those found at lower energies.

Pulsed gamma-ray emission from neutron and collapsing stars and supernovae

Three mechanisms generating pulsed γ-ray emission at late stages of stellar evolution are investigated: (1) γ-ray burst produced by the absorption of neutrino emission of a collapsing star in its envelope; (2) γ-ray burst of thermal emission when the outer layers of a compact star (R=0.01–0.1R⊙) are heated up by a powerful shock wave; and (3) γ-ray emission due to ejection of matter from neutron stars at an active stage of their existence. In case (1) the expected flux of γ-quanta with the energy 0.1 MeV amounts to about 10−4 cm−2 which is considerably less than the observed flux. However, observation of such γ-ray pulses would be an important supplement to the direct observations of neutrino emission of collapsing stars. In case (2) the outer layers of the star are heated up to the temperature of about 108 K. This results in a short burst with the emission energy ∼ 1042–43 erg whose main part is concentrated in the X-ray range (∼ 25 keV). In this case approximately 10% of the energy is emitted in the γ-ray range (≳ 0.1 MeV). Generally speaking, this mechanism is sufficient as to the energy for accounting for the observed bursts; however, probably under the condition that supernovae are exploding in our Galaxy. This involves difficulties concerning the frequency of bursts and the spectrum of emission. In case (3) ejection of chemically non-equilibrium matter from the neutron star leads to an intensive emission which is produced due to fission of superheavy nuclei, β-decay of radioactive elements and radiative capture of free neutrons. Ejection of matter from the neutron stars may be related to the observed jumps of periods of pulsars. From the observed gain of kinetic energy of the filaments of the Crab Nebula (∼ 2×1041 erg) the mass of the ejected material may be estimated (∼ 1021 g). This leads to energies of the γ-ray bursts of the order of 1038–1039 erg, which agrees fully with observations at the mean distance up to the sources 0.25 kpc. As distinct from the outbursts of supernovae it would seem that no difficulties concerning the burst frequency and the spectrum of emission are encountered. From the mechanisms of γ-ray emission examined here the mechanism connected with the ejection of matter from neutron stars seems to be the most promising as far as the interpretation of observations is concerned.

Search for Pulsed Gamma Ray Emission above 50 MeV from NP 0532

OBSERVATIONS of the pulsar NP 0532 in the gamma ray region have yielded evidence of pulsed gamma ray emission for energies up to 10 MeV (refs. 1, 2 and Kurfess, to be published). Above this energy, upper limits between 10 MeV and 150 MeV (refs. 3, 4) and above 50 MeV (ref. 5) are indicated. A result above 70 MeV with phase error of 3 ms with respect to optical signal has been given by a counter experiment6.

Possible Pulsed Gamma Ray Emission from the Crab Nebula Pulsar

VASSEUR et al.1 have recently presented results which, they suggest, appear to show that the Crab pulsar NP 0531 + 21 emits a flux of ∼ 10−5 photons cm−2 s−1 at a photon energy >50 MeV. Their results, based on balloon-borne spark chamber observations, would, if confirmed, be of great significance not only for the Crab pulsar but also for the field of gamma ray astronomy as a whole, where the evidence for point sources of gamma rays remains marginal2. We feel, however, that the statistical weight of the data in the report is much weaker than the authors suggest.

Possible Pulsed Gamma Ray Emission above 50 MeV from the Crab Pulsar

A SEARCH for pulsed gamma ray emission from NP 0532 has been carried out with a balloon-borne experiment using a spark chamber as the central detector. This pulsar, situated in the Crab Nebula, is already known to pulsate in the radio, optical and X-ray range1–5.