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Register a new userMeasurement of the absolute Crab flux with NuSTAR
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Added by
Kristin Kruse Madsen
Publication date
2017
fields
Physics
and research's language is
English
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We present results from a NuSTAR observation of the Crab made at a large off-axis angle of 1.5degree. At these angles X-rays do not pass through the optics, but rather illuminate the detectors directly due to incomplete baffling. Due to the simplicity of the instrument response in this configuration and the good absolute calibration of the detectors, we are able to measure the absolute intrinsic flux of the Crab to better than 4%. We find the spectral parameters of the powerlaw to be Gamma=2.106 +/- 0.006, N=9.71 +/-0.16, in agreement with the values measured 42 years ago by Toor & Seward (1974). This suggests that the observed variability of the Crab is not part of a long term trend, but instead results from fluctuations around a steady mean. The NuSTAR observation also enabled improved measurement of the detector absorption parameters without the added complications of the mirror response.
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We present TeV gamma-ray observations of the Crab Nebula, the standard reference source in ground-based gamma-ray astronomy, using data from the High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory. In this analysis we use two independent energy-estimation methods that utilize extensive air shower variables such as the core position, shower angle, and shower lateral energy distribution. In contrast, the previously published HAWC energy spectrum roughly estimated the shower energy with only the number of photomultipliers triggered. This new methodology yields a much improved energy resolution over the previous analysis and extends HAWCs ability to accurately measure gamma-ray energies well beyond 100 TeV. The energy spectrum of the Crab Nebula is well fit to a log parabola shape $left(frac{dN}{dE} = phi_0 left(E/textrm{7 TeV}right)^{-alpha-betalnleft(E/textrm{7 TeV}right)}right)$ with emission up to at least 100 TeV. For the first estimator, a ground parameter that utilizes fits to the lateral distribution function to measure the charge density 40 meters from the shower axis, the best-fit values are $phi_o$=(2.35$pm$0.04$^{+0.20}_{-0.21}$)$times$10$^{-13}$ (TeV cm$^2$ s)$^{-1}$, $alpha$=2.79$pm$0.02$^{+0.01}_{-0.03}$, and $beta$=0.10$pm$0.01$^{+0.01}_{-0.03}$. For the second estimator, a neural network which uses the charge distribution in annuli around the core and other variables, these values are $phi_o$=(2.31$pm$0.02$^{+0.32}_{-0.17}$)$times$10$^{-13}$ (TeV cm$^2$ s)$^{-1}$, $alpha$=2.73$pm$0.02$^{+0.03}_{-0.02}$, and $beta$=0.06$pm$0.01$pm$0.02. The first set of uncertainties are statistical; the second set are systematic. Both methods yield compatible results. These measurements are the highest-energy observation of a gamma-ray source to date.
We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of the Crab nebula and pulsar. We show that while the phase-averaged and spatially integrated nebula + pulsar spectrum is a power-law in this energy band, spatially resolved spectroscopy of the nebula finds a break at $sim$9 keV in the spectral photon index of the torus structure with a steepening characterized by $DeltaGammasim0.25$. We also confirm a previously reported steepening in the pulsed spectrum, and quantify it with a broken power-law with break energy at $sim$12 keV and $DeltaGammasim0.27$. We present spectral maps of the inner 100as of the remnant and measure the size of the nebula as a function of energy in seven bands. These results find that the rate of shrinkage with energy of the torus size can be fitted by a power-law with an index of $gamma = 0.094pm 0.018$, consistent with the predictions of Kennel and Coroniti (1984). The change in size is more rapid in the NW direction, coinciding with the counter-jet where we find the index to be a factor of two larger. NuSTAR observed the Crab during the latter part of a $gamma$-ray flare, but found no increase in flux in the 3 - 78 keV energy band.
Strong magnetic fields, synchrotron emission, and Compton scattering are omnipresent in compact celestial X-ray sources. Emissions in the X-ray energy band are consequently expected to be linearly polarized. X-ray polarimetry provides a unique diagnostic to study the location and fundamental mechanisms behind emission processes. The polarization of emissions from a bright celestial X-ray source, the Crab, is reported here for the first time in the hard X-ray band (~20-160 keV). The Crab is a complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures in the inner nebula including a jet and torus. Measurements are made by a purpose-built and calibrated polarimeter, PoGO+. The polarization vector is found to be aligned with the spin axis of the pulsar for a polarization fraction, PF = (20.9 $pm$ 5.0)%. This is higher than that of the optical diffuse nebula, implying a more compact emission site, though not as compact as, e.g., the synchrotron knot. Contrary to measurements at higher energies, no significant temporal evolution of phase-integrated polarisation parameters is observed. The polarization parameters for the pulsar itself are measured for the first time in the X-ray energy band and are consistent with observations at optical wavelengths.
We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cherenkov detector called CELESTE, which records gamma rays above 30 GeV (7E24 Hz). Here we present the first sub-100 GeV detection by a ground based telescope of a gamma ray source, the Crab nebula, in the energy region between satellite measurements and imaging atmospheric Cherenkov telescopes. At our analysis threshold energy of 60 +/- 20 GeV we measure a gamma ray rate of 6.1 +/- 0.8 per minute. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma ray flux of I(E>60 GeV) = 6.2^{+5.3}_{-2.3} E-6 photons m^-2 s^-1. The analysis methods used to obtain the gamma ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be <12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e^{-E/E_0} then E_0 < 26 GeV. This is the lowest energy probed by a Cherenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.
Results of investigations of the near-horizontal muons in the range of zenith angles of 85-95 degrees are presented. In this range, so-called albedo muons (atmospheric muons scattered in the ground into the upper hemisphere) are detected. Albedo muons are one of the main sources of the background in neutrino experiments. Experimental data of two series of measurements conducted at the experimental complex NEVOD-DECOR with the duration of about 30 thousand hours live time are analyzed. The results of measurements of the muon flux intensity are compared with simulation results using Monte-Carlo on the basis of two multiple Coulomb scattering models: model of point-like nuclei and model taking into account finite size of nuclei.
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