VHE gamma-ray sources and implications

Summary This paper highlights the status of very high-energy (VHE) gamma-ray astronomy with emphasis on the astrophysical significance of the discovery of TeV γ-radiation from four celestial objects, Crab Nebula, PSR B1706-44, Mrk 421 and Mrk 501, detected at very high confidence levels using the atmospheric Cherenkov imaging technique. Paper presented at the Special Session on ground-based gamma-ray astronomy of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Limits on an energy dependence of the speed of light from a flare of the active galaxy PKS 2155-304

In the past few decades, several models have predicted an energy dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this Letter a search for such time lags during the High Energy Stereoscopic System observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.     

Spectrophotometric determination of acetone in acetic acid

A sensitive and selective spectrophotometric method for the determination of acetone in acetic acid has been worked out. It is based on the reaction of acetone with diazotized p-aminobenzoic acid in a strongly alkaline medium to form a purple color with maximum absorption at 540 nm. Beer's law holds up in the range 20–140 μg of acetone in a final volume of 20 ml, with a molar absorptivity of 8.1 × 10³ liters mol^?1 cm^?1, sensitivity index of 0.0072 μg cm^?2, relative error of ?1.9 to +0.6%, and relative standard deviation of 0.7–3.5, depending on the concentration level. Furthermore, the color reaction is fast, and the procedure is simple, and avoids the use of an extraction.     

Participation of the alpha,alpha'-diiminopyridine ligand system in reduction of the metal center during alkylation

The reaction of [[2,6-(i-Pr)(2)PhN=C(Me)](2)(C(5)H(3)N)]MnCl(2) with alkylating agents formed a dinuclear Mn(I) derivative via ligand reductive coupling. In the case of the trivalent Cr analogue, a similar reaction afforded reduction toward Cr(II) but also alkylation at the pyridine ring para position followed by an unprecedented cycloaddition that generated a tricyclic system.     

Synthesis of 2-aryl-6-carbethoxythiazolo[4,5-c]pyridine and 7-chloro-2-phenylthiazolo[5,4-c]pyridine

Starting from readily available 2-aryl-5-formyl-4-mehtylthiazole, 2-aryl-6-carbethoxythiazolo[4,5-c]pyridine was prepared. β-(2-Phenylthiazol-4-yl)acrylic acid was converted to the corresponding azide (VI). Cyclization of compound VI afforded 2-phenylthiazolo[5,4-c]pyridin-7(6H)one. Reaction of the latter with phosphorous oxychloride gave 7-chloro-2-phenylthiazolo[5,4-c]pyridine.     

Fermi bubbles: giant, multibillion-year-old reservoirs of Galactic center cosmic rays

Recently evidence has emerged for enormous features in the γ-ray sky observed by the Fermi-LAT instrument: bilateral "bubbles" of emission centered on the core of the Galaxy and extending to around ± 10 kpc from the Galactic plane. These structures are coincident with a nonthermal microwave "haze" and an extended region of x-ray emission. The bubbles' γ-ray emission is characterized by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity 4×10(37) erg/s, around 1 order of magnitude larger than their microwave luminosity while more than order of magnitude less than their x-ray luminosity. Here we show that the bubbles are naturally explained as due to a population of relic cosmic ray protons and heavier ions injected by processes associated with extremely long time scale (? 8 Gyr) and high areal density star formation in the Galactic center.     

Indirect spectrophotometric determination of cerium(IV)

A simple and very fast photometric method for the determination of trace levels of cerium(IV) has been developed. Use has been made of the reaction in which iron (II), on oxidation with cerium(IV), gives iron(III) which is complexed with tiron, in acidic medium, to form a stable blue color, the intensity of which is in direct proportion to the amount of cerium(IV) originally present in the sample solution. In addition, the method has a wide range of determination and reasonable sensitivity, and it avoids both temperature control and the tedious extraction step.     

Pointlike gamma ray sources as signatures of distant accelerators of ultrahigh energy cosmic rays

We discuss the possibility of observing distant accelerators of ultrahigh energy cosmic rays in synchrotron gamma rays. Protons propagating away from their acceleration sites produce extremely energetic electrons during photopion interactions with cosmic microwave background photons. If the accelerator is embedded in a magnetized region, these electrons will emit high energy synchrotron radiation. The resulting synchrotron source is expected to be pointlike, steady, and detectable in the GeV-TeV energy range if the magnetic field is at the nanoGauss level.