[show abstract][hide abstract] ABSTRACT: Exceptionally strong and long-lasting flaring activity of the blazar Mrk 421 occurred between 2001 January and March. Based on the excellent signal-to-noise ratio of the data, we derive the energy spectrum between 260 GeV and 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead, the data can be described by a power law with exponential cutoff: dN/dE Ee m-2 s-1 TeV-1 with E0 = 4.3 ± 0.3stat TeV. Mrk 421 is the second γ-ray blazar that unambiguously exhibits an absorption-like feature in its spectral energy distribution at 3-6 TeV.
The Astrophysical Journal 12/2008; 560(1):L45. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: A survey of binary systems containing pulsars was conducted, with the intention of detecting Galactic sources of very high energy γ-ray emission. Observations were carried out with the Whipple 10 m imaging atmospheric Cerenkov telescope. Standard analysis techniques were applied to these sources to search for steady, unpulsed emission. Periodic tests were also performed to search for emission correlated with both the orbital and spin phases, where appropriate. Analyses indicate that the binaries in this study do not emit detectable levels of very high energy photons within the sensitivity of our instrument. The flux upper limits presented here fail to seriously constrain emission models.
The Astrophysical Journal 12/2008; 583(2):853. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: A very high energy γ-ray signal has been detected at the 5.5 σ level from H1426+428, an X-ray-selected BL Lacertae object at a redshift of 0.129. The object was monitored from 1995 to 1998 with the Whipple 10 m imaging atmospheric Cerenkov telescope as part of a general blazar survey; the results of these observations, although not statistically significant, were consistently positive. X-ray observations of H1426+428 during 1999 with the BeppoSAX instrument revealed that the peak of its synchrotron spectrum occurs at greater than 100 keV, leading to the prediction of observable TeV emission from this object. H1426+428 was monitored extensively at the Whipple Observatory during the 1999, 2000, and 2001 observing seasons. The strongest TeV signals were detected in 2000 and 2001. During 2001, an integral flux of 2.04 ± 0.35 × 10-11 cm-2 s-1 above 280 GeV was recorded from H1426+428. The detection of H1426+428 supports the idea that, as also seen in Mrk 501 and 1ES 2344+514, BL Lacertae objects with extremely high synchrotron peak frequencies produce γ-rays in the TeV range.
The Astrophysical Journal 12/2008; 571(2):753. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: In 1998 and 1999 the Whipple Observatory 10 m telescope was used to search for diffuse γ-ray emission from the Galactic plane. At this time, the telescope was equipped with a large (48) field of view camera, well suited to detect diffuse γ-ray emission. No significant evidence of emission was found. Assuming the TeV emission profile matches EGRET observations above 1 GeV with a differential spectral index of 2.4, we derive an upper limit of 3.0 × 10-8 cm-2 s-1 sr-1 for the average diffuse emission above 500 GeV in the Galactic latitude range from -2° to +2° at Galactic longitude 40°. Comparisons with EGRET observations provide a lower limit of 2.31 for the differential spectral index of the diffuse emission, assuming there is no break in the spectrum between 30 and 500 GeV. This constrains models for diffuse emission with a significant inverse Compton contribution.
The Astrophysical Journal 12/2008; 539(1):209. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Kernel multivariate analysis technique is optimised to select
γ-ray events from ON/OFF observations of the Crab Nebula recorded
by the Whipple 10 m Imaging Atmospheric Cherenkov Telescope in January
and February 2000. Results are compared with the conventional Supercuts
analysis and with a Neural Network analysis. The technique is also
applied to ON/OFF data taken on Markarian 421 during spring of 2000. A
method to estimate the energy of γ-ray primaries is examined, and
a TeV spectrum of the Crab Nebula extracted on this basis.
[show abstract][hide abstract] ABSTRACT: We report on a periodicity analysis of gamma-ray observations of very high energy (VHE, E>250 GeV) gamma-rays from Markarian 501 (Mrk 501) taken with the Whipple telescope between March and July of 1997. The data correspond to a period of bright flaring of the source, quasi-periodic signals were detected by the Telescope Array Project (TAP) and HEGRA collaboration. Periodic analysis of these data is complicated because the source observations are not continuous and the reported period (~24 days) is very close to the sampling period. We use the Lomb method, a technique for extracting spectral information from unevenly sampled data, in an attempt to overcome this difficulty. Similar results are shown to exist in data from the strongly correlated X-ray band sampled by the RXTE ASM instrument. No periodicity in the data could be identified with a source frequency. .
[show abstract][hide abstract] ABSTRACT: Markarian 421 was observed for about four days with BeppoSAX and the Whipple Observatory gamma-ray telescope in April 1998. A pronounced, well-defined, flare with hour-scale variability was observed simultaneously in X-rays and very high energy gamma-rays. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours. While the rise of the flare occurred on a similar time-scale in the two wavebands, the decay of the flare was much more rapid in gamma rays, providing the first clear indication that the X-ray and gamma-ray emission may not be completely correlated in Markarian 421.
[show abstract][hide abstract] ABSTRACT: Markarian 501, a nearby (z=0.033) X-ray selected BL Lacertae object, is a well established source of Very High Energy (VHE, E>=300 GeV) gamma rays. Dramatic variability in its gamma-ray emission on time-scales from years to as short as two hours has been detected. Multiwavelength observations have also revealed evidence that the VHE gamma-ray and hard X-ray fluxes may be correlated. Here we present results of observations made with the Whipple Collaboration's 10 m Atmospheric Cerenkov Imaging Telescope during 1999 and discuss them in the context of observations made on Markarian 501 during the period from 1996-1998.
[show abstract][hide abstract] ABSTRACT: If cosmic rays with energies <100 TeV originate in the galaxy and are accelerated in shock waves in shell-type supernova remnants (SNRs), gamma-rays will be produced as the result of proton and electron interactions with the local interstellar medium, and by inverse Compton emission from electrons scattering soft photon fields. We report on observations of two supernova remnants with the Whipple Observatory's 10 m gamma-ray telescope. No significant detections have been made and upper limits on the >500 GeV flux are reported. Non-thermal X-ray emission detected from one of these remnants (Cassiopeia A) has been interpreted as synchrotron emission from electrons in the ambient magnetic fields. Gamma-ray emission detected from the Monoceros/Rosette Nebula region has been interpreted as evidence of cosmic-ray acceleration. We interpret our results in the context of these observations.
[show abstract][hide abstract] ABSTRACT: The Whipple Collaboration has been carrying out an upgrade program, GRANITE III, on the 10m telescope atop Mt. Hopkins. The 3 year program, 1996 through 1999, has involved a large field-of-view camera, a hardware pattern recognition trigger, and finally a small pixel camera. Results of the first and second stage of the program indicate that a large field-of-view yield an increased collection area for cosmic-ray triggers and the hardware pattern trigger reduces the energy threshold of the telescope by greatly suppressing night sky accidental triggers over a simple multiplicity coincidence trigger. The final phase of the program, being completed during the summer and fall of 1999, will be the installation of a small pixel camera. The pixel size of ~0°.12 will yield the lowest threshold energy at which the 10 m Whipple telescope has ever operated. .