118956204
Observation of Gamma Rays from the Galactic Center with the MAGIC Telescope
Recently, the Galactic center has been reported to be a source of very high energy (VHE) gamma-rays by the CANGAROO, VERITAS, and HESS experiments. The energy spectra as measured by these experiments show substantial differences. In this Letter we present MAGIC observations of the Galactic center, resulting in the detection of a differential gamma-ray flux consistent with a steady, hard-slope power law, described as dN gamma/(dA dt dE) = (2.9 +/- 0.6) x 10(-12) (E/TeV)-2.2 +/- 0.2 cm(-2) s(-1) TeV-1. The gamma-ray source is centered at (R.A., decl.) p (17(h)45(m)20(s), -29 degrees 2'). This result confirms the previous measurements by the HESS experiment and indicates a steady source of TeV gamma-rays. We briefly describe the observational technique used and the procedure implemented for the data analysis, and we discuss the results in the perspective of different models proposed for the acceleration of the VHE gamma-rays.
1226034
Discovery of Very High Energy gamma-rays from 1ES 1011+496 at z=0.212
We report on the discovery of Very High Energy (VHE) gamma-ray emission from the BL Lacertae object 1ES1011+496. The observation was triggered by an optical outburst in March 2007 and the source was observed with the MAGIC telescope from March to May 2007. Observing for 18.7 hr we find an excess of 6.2 sigma with an integrated flux above 200 GeV of (1.58$\pm0.32) 10^{-11}$ photons cm$^{-2}$ s$^{-1}$. The VHE gamma-ray flux is >40% higher than in March-April 2006 (reported elsewhere), indicating that the VHE emission state may be related to the optical emission state. We have also determined the redshift of 1ES1011+496 based on an optical spectrum that reveals the absorption lines of the host galaxy. The redshift of z=0.212 makes 1ES1011+496 the most distant source observed to emit VHE gamma-rays up to date.
118954339
Variable VHE gamma-ray emission from Markarian 501
The blazar Markarian 501 (Mrk 501) was observed at energies above 0.10 TeV with the MAGIC telescope from May through July 2005. The high sensitivity of the instrument enabled the determination of the flux and spectrum of the source on a night-by-night basis. Throughout our observational campaign, the flux from Mrk 501 was found to vary by an order of magnitude. Intra-night flux variability with flux-doubling times down to 2 minutes was observed during the two most active nights, namely June 30 and July 9. These are the fastest flux variations ever observed in Mrk 501. The ~20-minute long flare of July 9 showed an indication of a 4 +/- 1 min time delay between the peaks of F(<0.25 TeV) and F(>1.2 TeV), which may indicate a progressive acceleration of electrons in the emitting plasma blob. The flux variability was quantified for several energy ranges, and found to increase with the energy of the gamma-ray photons. The spectra hardened significantly with increasing flux, and during the two most active nights, a spectral peak was clearly detected at 0.43 +/- 0.06 TeV and 0.25 +/- 0.07 TeV, respectively for June 30 and July 9. There is no evidence of such spectral feature for the other nights at energies down to 0.10 TeV, thus suggesting that the spectral peak is correlated with the source luminosity. These observed characteristics could be accommodated in a Synchrotron-Self-Compton (SSC) framework in which the increase in gamma-ray flux is produced by a freshly injected (high energy) electron population.
16174766
Detection of VHE radiation from the BL Lac PG 1553+113 with the MAGIC telescope
In 2005 and 2006, the MAGIC telescope has observed very high energy gamma-ray emission from the distant BL Lac object PG 1553+113. The overall significance of the signal is 8.8 σ for 18.8 h observation time. The light curve shows no significant flux variations on a daily time-scale, the flux level during 2005 was, however, significantly higher compared to 2006. The differential energy spectrum between ∼ 90 GeV and 500 GeV is well described by a power law with photon index Γ = 4.2 ± 0.3. The combined 2005 and 2006 energy spectrum provides an upper limit of z = 0.74 on the redshift of the object. Subject headings: PG 1553+113, BL Lac, AGN, VHE gamma-ray, imaging air Cherenkov telescope
15324383
Upper Limit for γ-Ray Emission above 140 GeV from the Dwarf Spheroidal Galaxy Draco
The nearby dwarf spheroidal galaxy Draco, with its high mass to light ratio, is one of the most auspicious targets for indirect dark matter (DM) searches. Annihilation of hypothetical DM particles can result in high-energy gamma-rays, e. g., from neutralino annihilation in the supersymmetric framework. A search for a possible DM signal originating from Draco was performed with the MAGIC telescope during 2007. Analysis of the data results in a flux upper limit (2 sigma) of 1.1 x 10(-11) photons cm(-2) s(-1) for photon energies above 140 GeV, assuming a pointlike source. A comparison with predictions from supersymmetric models is also given. While our results do not constrain the mSUGRA phase parameter space, a very high flux enhancement can be ruled out.
319964
Systematic search for VHE gamma-ray emission from X-ray bright high-frequency BL Lac objects
All but three (M87, BL Lac, and 3C 279) extragalactic sources detected so far at very high energy gamma-rays belong to the class of high-frequency-peaked BL Lac objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the Donato et al. compilation of X-ray blazars. The observations took place from 2004 December to 2006 March and cover northern sky sources visible under small zenith distances zd 30 degrees at culmination, constraining the declination to -2 degrees to +58 degrees. The sensitivity of the search was planned for detecting X-ray-bright [F(1 keV) > 2 mu Jy] sources emitting at least the same energy flux at 200 GeV as at 1 keV. To avoid strong gamma-ray attenuation close to the energy threshold, source redshift was constrained to z < 0.3. Of the 14 sources observed, 1ES 1218+304 (for the first time at VHE) and 1ES 2344+514 (strong detection in a low flux state) were detected in addition to the known bright TeV blazars Mrk 421 and Mrk 501. A marginal excess of 3.5 sigma from the position of 1ES 1011+496 was observed and then confirmed as a VHE gamma-ray source by a second MAGIC observation triggered by a high optical state. For the remaining sources, we present 99% c. l. upper limits on the integral flux greater than or similar to 200 GeV. We characterize the HBL sample (including all HBLs detected at VHE so far) by looking for correlations between their multifrequency spectral indices determined from simultaneous optical, archival X-ray, and radio luminosities, finding that VHE-emitting HBLs do not seem to constitute a unique subclass. The HBLs' absorption-corrected gamma-ray luminosities at 200 GeV are generally not higher than their X-ray luminosities at 1 keV.
14097319
The Stability of the Point Spread Function of the Advanced Camera for Surveys on the Hubble Space Telescope and Implications for Weak Gravitational Lensing
(abridged) We examine the spatial and temporal stability of the HST ACS Wide Field Camera (WFC) point spread function (PSF) using the two square degree COSMOS survey. We show that stochastic aliasing of the PSF necessarily occurs during `drizzling'. This aliasing is maximal if the output pixel scale is equal to the input pixel scale of 0.05''. We show that this source of PSF variation can be significantly reduced by choosing a Gaussian drizzle kernel and by setting the output pixel size to 0.03''. We show that the PSF is temporally unstable, most likely due to thermal fluctuations in the telescope's focus. We find that the primary manifestation of this thermal drift in COSMOS images is an overall slow periodic focus change. Using a modified version of TinyTim, we create undistorted stars in a 30x30 grid across the ACS WFC CCDs. These PSF models are created for telescope focus values in the range -10 microns to +5 microns, thus spanning the allowed range of telescope focus values. We then use the approximately ten well measured stars in each COSMOS field to pick the best-fit focus value for each field. The TinyTim model stars are then used to perform PSF corrections for weak lensing allowing systematics due to incorrectly modeled PSFs to be greatly reduced. We have made the software for PSF modeling using our modified version of TinyTim available to the astronomical community. We show the effects of Charge Transfer Efficiency (CTE) degradation, which distorts the object in the readout direction, mimicking a weak lensing signal. We derive a parametric correction for the effect of CTE on the shapes of objects in the COSMOS field as a function of observation date, position within the ACS WFC field, and object flux.
55970305
Satellite-Mounted Light Sources as Photometric Calibration Standards
A significant and growing portion of systematic error on a number of fundamental parameters in astrophysics and cosmology is due to uncertainties from absolute photometric and flux standards. A path toward achieving major reduction in such uncertainties may be provided by satellite-mounted light sources, resulting in improvement in the ability to precisely characterize atmospheric extinction, and thus helping to usher in the coming generation of precision results in astronomy. Toward this end, we have performed a campaign of observations of the 532 nm pulsed laser aboard the CALIPSO satellite, using a portable network of cameras and photodiodes, to precisely measure atmospheric extinction.
2823949
Modeling and Correcting the Time-Dependent ACS PSF
The ability to accurately measure the shapes of faint objects in images taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) depends upon detailed knowledge of the Point Spread Function (PSF). We show that thermal fluctuations cause the PSF of the ACS Wide Field Camera (WFC) to vary over time. We describe a modified version of the TinyTim PSF modeling software to create artificial grids of stars across the ACS field of view at a range of telescope focus values. These models closely resemble the stars in real ACS images. Using � 10 bright stars in a real image, we have been able to measure HST's apparent focus at the time of the exposure. TinyTim can then be used to model the PSF at any position on the ACS field of view. This obviates the need for images of dense stellar fields at different focus values, or interpolation between the few observed stars. We show that residual differences between our TinyTim models and real data are likely due to the effects of Charge Transfer Efficiency (CTE) degradation. Furthermore, we discuss stochastic noise that is added to the shape of point sources when distortion is removed, and we present MultiDrizzle parameters that are optimal for weak lensing science. Specifically, we find that reducing the MultiDrizzle output pixel scale and choosing a Gaussian kernel significantly stabilizes the resulting PSF after image combination, while still eliminating cosmic rays/bad pixels, and correcting the large geometric distortion in the ACS. We discuss future plans, which include more detailed study of the effects of CTE degradation on object shapes and releasing our TinyTim models to the astronomical community.
53581559
Telescope Spectrophotometric and Absolute Flux Calibration, and National Security Applications, Using a Tunable Laser on a Satellite
We propose a tunable laser-based satellite-mounted spectrophotometric and absolute flux calibration system, to be utilized by ground- and space-based telescopes. As spectrophotometric calibration may play a significant role in the accuracy of photometric redshift measurement, and photometric redshift accuracy is important for measuring dark energy using SNIa, weak gravitational lensing, and baryon oscillations, a method for reducing such uncertainties is needed. We propose to improve spectrophotometric calibration, currently obtained using standard stars, by placing a tunable laser and a wide-angle light source on a satellite by early next decade (perhaps included in the upgrade to the GPS satellite network) to improve absolute flux calibration and relative spectrophotometric calibration across the visible and near-infrared spectrum. As well as fundamental astrophysical applications, the system proposed here potentially has broad utility for defense and national security applications such as ground target illumination and space communication.
36960262
Weak Lensing from Space. III. Cosmological Parameters
Weak gravitational lensing provides a unique method to directly map the dark matter in the universe and measure cosmological parameters. Current weak-lensing surveys are limited by the atmospheric seeing from the ground and by the small fields of view of existing space telescopes. We study how a future wide-field space telescope can measure the lensing power spectrum and skewness and thus set constraints on cosmological parameters. The lensing sensitivity was calculated using detailed image simulations and instrumental specifications studied in earlier papers in this series. For instance, the planned SuperNova/Acceleration Probe (SNAP) mission will be able to measure the matter density parameter Ωm and the dark energy equation-of-state parameter w with precisions comparable and nearly orthogonal to those derived with SNAP from supernovae. The constraints degrade by a factor of about 2 if redshift tomography is not used but are little affected if only the skewness is dropped. We also study how the constraints on these parameters depend on the survey geometry and define an optimal observing strategy.