M. Jordan

Detection of the BL Lacertae Object H1426+428 at TeV Gamma-Ray Energies

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.

Discovery of Spectral Variability of Markarian 421 at TeV Energies

The detection of spectral variability of the g-ray blazar Mrk 421 at TeV energies is reported. Observations with the Whipple Observatory 10 m g-ray telescope taken in 2000/2001 revealed exceptionally strong and longlasting flaring activity. Flaring levels of 0.4–13 times that of the Crab Nebula flux provided sufficient statistics for a detailed study of the energy spectrum between 380 GeV and 8.2 TeV as a function of the flux level. These spectra are well described by a power law with an exponential cutoff: m 2 s 1 TeV 1 . There a E/E0 dN/dE ∝ Ee is no evidence for variation in the cutoff energy with flux, and all spectra are consistent with an average value for the cutoff energy of 4.3 TeV. The spectral index varies between in a high flux state 1.89 0.04 0.05 stat syst and in a low state. The correlation between spectral index and flux is tight when averaging 2.72 0.11 0.05 stat syst over the total 2000/2001 data set. Spectral measurements of Mrk 421 from previous years (1995/1996 and 1999) by the Whipple collaboration are consistent with this flux–spectral index correlation, which suggests that this may be a constant or a long-term property of the source. If a similar flux–spectral index correlation were found for other g-ray blazars, this universal property could help disentangle the intrinsic emission mechanism from external absorption effects. Subject headings: BL Lacertae objects: individual (Markarian 421) — gamma rays: observations

CONSTRAINTS ON THE VERY HIGH ENERGY EMISSION FROM BL LACERTAE OBJECTS

We present results from observations of 29 BL Lacertae objects, taken with the Whipple Observatory 10 m gamma-ray telescope between 1995 and 2000. The observed objects are mostly at low redshift (z < 0:2), but observations of objects of up to z ¼ 0:444 are also reported. Five of the objects are EGRET sources and two are unconfirmed TeV sources. Three of the confirmed sources of extragalactic TeV gamma rays were originally observed as part of this survey and have been reported elsewhere. No significant excesses are detected from any of the other objects observed, on timescales of days, months, or years. We report 99.9% confidence level flux upper limits for the objects for each observing season. The flux upper limits are typically 20% of the Crab flux, although for some sources, limits as sensitive as 6% of the Crab flux were derived. The results are consistent with the synchrotron self-Compton model predictions considered in this work. Subject headings: BL Lacertae objects: general — galaxies: jets — gamma rays: observations

The TeV spectrum of H1426+428

The BL Lac object H1426+428 was recently detected as a high-energy γ-ray source by the VERITAS collaboration (Horan et al.). We have reanalyzed the 2001 portion of the data used in the detection in order to examine the spectrum of H1426+428 above 250 GeV. We find that the time-averaged spectrum agrees with a power law of the shape The statistical evidence from our data for emission above 2.5 TeV is 2.6 σ. At the 95% confidence level, the integral flux of H1426+428 above 2.5 TeV is larger than 3% of the corresponding flux from the Crab Nebula. The spectrum is consistent with the (noncontemporaneous) measurement by Aharonian et al. both in shape and in normalization. Below 800 GeV, the data clearly favor a spectrum steeper than that of any other TeV blazar observed so far, indicating a difference in the processes involved either at the source or in the intervening space.

Cutoff in the TeV Energy Spectrum of Markarian 421 During Strong Flares in 2001

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.

Search for high-energy gamma rays from an X-ray-selected blazar sample

Our understanding of blazars has been greatly increased in recent years by extensive multiwavelength observations, particularly in the radio, X-ray, and gamma-ray regions. Over the past decade the Whipple 10 m telescope has contributed to this with the detection of five BL Lacertae objects at very high gamma-ray energies. The combination of multiwavelength data has shown that blazars follow a well-defined sequence in terms of their broadband spectral properties. Together with providing constraints on emission models, this information has yielded a means by which potential sources of TeV emission may be identified and predictions made as to their possible gamma-ray flux. We have used the Whipple telescope to search for TeV gamma-ray emission from eight objects selected from a list of such candidates. No evidence has been found for very high energy emission from the objects in our sample, and upper limits have been derived for the mean gamma-ray flux above 390 GeV. These flux upper limits are compared with the model predictions, and the implications of our results for future observations are discussed.

A survey of unidentified egret sources at very high energies

The Whipple Observatory 10 m ?-ray telescope has been used to survey the error boxes of EGRET unidentified sources in an attempt to find counterparts at energies of 350 GeV and above. Twenty-one unidentified sources detected by EGRET (more than 10% of the total number) have been included in this survey. In no case is a statistically significant signal found in the EGRET error box, which implies that, at least for this sample, the ?-ray spectra of these sources steepen between 100 MeV and 350 GeV. For each EGRET source location, we list candidate associations and derive upper limits on the integral ?-ray flux above 350 GeV.

OBSERVATION OF M87 AT 400 GeV WITH THE WHIPPLE 10 METER TELESCOPE

We present results from observations taken with the Whipple 10 m very high energy γ-ray telescope with maximal sensitivity at 400 GeV during 39 hr between 2000 and 2003 in the direction of the giant radio galaxy M87. Using the entire data set, we derive a 99% confidence level upper limit on the flux of γ-ray emission above 400 GeV from M87 to be ≤6.9 × 10-12 cm-2 s-1. This suggests variability at the 90% confidence level when compared to the flux measured by the HEGRA collaboration in 1999 if the differential spectrum is steeper than a power law of index 3.75. Our search for a correlation between the Rossi X-Ray Timing Explorer all-sky monitor observation and a potential γ-ray signal is inconclusive.

Detection of TeV gamma rays from the BL Lacertae object 1ES 1959+650 with the Whipple 10 meter telescope

We present the first strong detection of very high energy gamma-rays from the close (z=0.048) X-ray selected BL Lacertae object 1ES1959+650. Observations were made with the Whipple 10m telescope on Mt. Hopkins, Arizona, using the atmospheric Cherenkov imaging technique. The flux between May and July 2002 was highly variable, with a mean of 0.64 +/- 0.03 times the steady flux from the Crab Nebula and reaching a maximum of five Crab, with variability on timescales as short as seven hours.

Detection of T[CLC]e[/CLC]V Gamma Rays from the BL Lacertae Object 1ES 1959+650 with the Whipple 10 Meter Telescope

We present the first strong detection of very high energy gamma-rays from the close (z=0.048) X-ray selected BL Lacertae object 1ES1959+650. Observations were made with the Whipple 10m telescope on Mt. Hopkins, Arizona, using the atmospheric Cherenkov imaging technique. The flux between May and July 2002 was highly variable, with a mean of 0.64 +/- 0.03 times the steady flux from the Crab Nebula and reaching a maximum of five Crab, with variability on timescales as short as seven hours.