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The Whipple Observatory 10 m reflector, operating as a 37 pixel camera, has been used to observe the Crab Nebula in TeV gamma rays. By selecting gamma-ray images based on their predicted properties, more than 98% of the background is rejected; a detection is reported at the 9.0 a level, corresponding to a flux of 1.8 x 10 11 photons cm 2 s 1 above 0.7 TeV (with a factor of 1.5 uncertainty in both flux and energy). Less than 25% of the observed flux is pulsed at the period of PSR 0531. There is no evidence for variability on time scales from months to years. Although continuum emission from the pulsar cannot be ruled out, it seems more likely that the observed flux comes from the hard Compton synchrotron spectrum of the nebula.

We present multiwavelength observations of the BL Lacertae object Markarian 501 (Mrk 501) in 1997 between April 8 and April 19. Evidence of correlated variability is seen in very high energy (VHE; E տ 350 GeV) gray observations taken with the Whipple Observatory g-ray telescope, data from the Oriented Scintillation Spectrometer Experiment of the Compton Gamma Ray Observatory, and quick-look results from the All-Sky Monitor of the Rossi X-Ray Timing Explorer, while EGRET did not detect Mrk 501. Short-term optical correlations are not conclusive, but the U-band flux observed with the 1.2 m telescope of the Whipple Observatory was 10% higher than in March. The average energy output of Mrk 501 appears to peak in the 2-100 keV range, which suggests an extension of the synchrotron emission to at least 100 keV, the highest observed in a blazar and ∼100 times higher than that seen in the other TeV-emitting BL Lac object, Mrk 421. The VHE g-ray flux observed during this period is the highest ever detected from this object. The VHE g-ray energy output is somewhat lower than the 2-100 keV range, but the variability amplitude is larger. The correlations seen here do not require relativistic beaming of the emission unless the VHE spectrum extends to տ5 TeV.

A catalog of c-ray sources based on photons with energies greater than 1 GeV has been developed from observations taken by the EGRET instrument of the Compton Gamma Ray Observatory. The data are taken from the 4.5 yr of observation available in the public data archives. We emphasize sources that are detected using the entire database, without regard to any possible transient or variable behavior. Ten of the 57 sources reported here have not previously been reported in the catalogs developed using photons above 100 MeV in energy. Twenty-seven sources have identiÐcations with objects seen at other wavelengths : the Large Magellanic Cloud, Ðve pulsars, and 21 blazars. The remaining 30 sources are classiÐed as unidentiÐed ; however, seven may be associated with Galactic supernova remnants and one source may be a Galactic X-ray binary (LSI 61 303). The 30 unidentiÐed sources are distributed nearly uniformly along the Galactic plane and are symmetric about it. Only one of the unidentiÐed sources has a Galactic latitude in excess of 30¡, whereas, if the sources were distributed uniformly, D12 would be expected on the basis of the combined EGRET exposure. A scatter plot of the Ñux from the unidentiÐed sources versus Galactic latitude reveals two rather distinct categories of source : "" bright ÏÏ sources with Ñuxes greater than or equal to 4.0 ] 10~8 photons cm~2 s~1 and "" dim ÏÏ sources with Ñuxes of less than 4.0 ] 10~8 photons cm~2 s~1. The absence of high-latitude bright sources is striking. The bright unidentiÐed sources have an average Galactic latitude of which is consistent with a Population I 2¡ .7, distribution at distances of 1È5 kpc. The dim unidentiÐed sources have a broader latitude distribution with an average indicating that if they are at the same average distance from the Galactic o b o \ 13¡ .8, plane as the bright sources, they are paradoxically approximately 5 times closer than the bright objects on average and therefore roughly 2 orders of magnitude less luminous.

Following the recent discovery of γ rays from the radio-loud narrow-line Seyfert 1 galaxy PMN J0948+0022 (z = 0.5846), we started a multiwavelength campaign from radio to γ rays, which was carried out between the end of 2009 March and the beginning of July. The source displayed activity at all the observed wavelengths: a general decreasing trend from optical to γ -ray frequencies was followed by an increase of radio emission after less than two months from the peak of the γ -ray emission. The largest flux change, about a factor of about 4, occurred in the X-ray band. The smallest was at ultraviolet and near-infrared frequencies, where the rate of the detected photons dropped by a factor 1.6-1.9. At optical wavelengths, where the sampling rate was the highest, it was possible to observe day scale variability, with flux variations up to a factor of about 3. The behavior of PMN J0948+0022 observed in this campaign and the calculated power carried out by its jet in the form of protons, electrons, radiation, and magnetic field are quite similar to that of blazars, specifically of flat-spectrum radio quasars. These results confirm the idea that radio-loud narrow-line Seyfert 1 galaxies host relativistic jets with power similar to that of average blazars.

One of the great mysteries of the high-energy gamma-ray sky is the group of approximately 170 unidentified point sources found along the Galactic plane. They are more numerous than all other high-energy gamma-ray sources combined and, despite 20 years of effort, no clear counterparts have been found at other wavelengths. Here we report a new population of such objects. A cluster of approximately 20 faint sources appears north of the Galactic Centre, which is part of a broader class of faint objects at mid-latitudes. In addition, we show in a model-independent way that the mid-latitude sources are distinct from the population of bright unidentified sources along the Galactic plane. The distribution on the sky indicates that the faint mid-latitude sources are associated with the Gould belt of massive stars and gas clouds at approximately 600 light years distance, as has been previously suggested.

We present extensive multiwavelength (radio to X-ray) observations of the Type Ib/c SN 2013ge from −13 to +457 days relative to maximum light, including a series of optical spectra and Swift UVoptical photometry beginning 2 − 4 days post-explosion. This data set makes SN 2013ge one of the best observed normal Type Ib/c SN at early times-when the light curve is particularly sensitive to the progenitor configuration and mixing of radioactive elements-and reveals two distinct light curve components in the UV bands. The first component rises over 4 − 5 days and is visible for the first week post-explosion. Spectra of the first component have blue continua and show a plethora of moderately high-velocity (∼15,000 km s −1 ) but narrow (∼3500 km s −1 ) spectroscopic features, indicating that the line-forming region is restricted. The explosion parameters estimated for the bulk explosion (M ej ∼ 2 − 3 M ⊙ ; E K ∼ 1 − 2 ×10 51 erg) are standard for Type Ib/c SN, and there is evidence for weak He features at early times-in an object which would have otherwise been classified as Type Ic. In addition, SN 2013ge exploded in a low metallicity environment (∼0.5 Z ⊙ ) and we have obtained some of the deepest radio and X-ray limits for a Type Ib/c SN to date, which constrain the progenitor mass-loss rate to beṀ < 4 × 10 −6 M ⊙ yr −1 . We are left with two distinct progenitor scenarios for SN 2013ge, depending on our interpretation of the early emission. If the first component is cooling envelope emission, then the progenitor of SN 2013ge either possessed an extended ( 30 R ⊙ ) envelope or ejected a portion of its envelope in the final 1 year before core-collapse. Alternatively, if the first component is due to outwardly mixed 56 Ni, then our observations are consistent with the asymmetric ejection of a distinct clump of nickel-rich material at high velocities. Current models for the collision of a SN shock with a binary companion cannot reproduce both the timescale and luminosity of the early emission in SN 2013ge. Finally, the spectra of the first component of SN 2013ge are similar to those of the rapidly-declining SN 2002bj.

We report on the ␥-ray variability of Mrk 421 at E ␥ Ͼ 300 GeV during the 1995 season, and concentrate on the results of an intense multiwavelength observing campaign in the period April 20 to May 5, which included Ͼ100 MeV ␥-ray, X-ray, extreme-ultraviolet, optical, and radio observations, some of which show evidence for correlated behavior. Rapid variations in the TeV ␥-ray light curve with doubling and decay times of =1 day require a compact emission region and significant Doppler boosting. The TeV data reveal that the ␥-ray emission is best characterized by a succession of rapid flares with a relatively low baseline level of steady emission.