13,977 research outputs found
How many metals does it take to fix N2? A mechanistic overview of biological nitrogen fixation
During the process of biological nitrogen fixation, the enzyme nitrogenase catalyzes the ATP-dependent reduction of dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the iron (Fe) protein and the molybdenum-iron (MoFe) protein; the Fe protein mediates the coupling of ATP hydrolysis to interprotein electron transfer, whereas the active site of the MoFe protein contains the polynuclear FeMo cofactor, a species composed of seven iron atoms, one molybdenum atom, nine sulfur atoms, an interstitial light atom, and one homocitrate molecule. This Perspective provides an overview of biological nitrogen fixation and introduces three contributions to this special feature that address central aspects of the mechanism and assembly of nitrogenase
Implications of very rapid TeV variability in blazars
We discuss the implications of rapid (few-minute) variability in the TeV flux
of blazars, which has been observed recently with the HESS and MAGIC
telescopes. The variability timescales seen in PKS 2155-304 and Mrk 501 are
much shorter than inferred light-crossing times at the black hole horizon,
suggesting that the variability involves enhanced emission in a small region
within an outflowing jet. The enhancement could be triggered by dissipation in
part of the black hole's magnetosphere at the base of the outflow, or else by
instabilities in the jet itself. By considering the energetics of the observed
flares, along with the requirement that TeV photons escape without producing
pairs, we deduce that the bulk Lorentz factors in the jets must be >50. The
distance of the emission region from the central black hole is less
well-constrained. We discuss possible consequences for multi-wavelength
observations.Comment: 5 pages, no figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Letter
Limits from rapid TeV variability of Mrk 421
The extreme variability event in the TeV emission of Mrk 421, recently
reported by the Whipple team, imposes the tightest limits on the typical size
of the TeV emitting regions in Active Galactic Nuclei (AGN). We examine the
consequences that this imposes on the bulk Lorentz factor of the emitting
plasma and on the radiation fields present in the central region of this Active
Nucleus. No strong evidence is found for extreme Lorentz factors. However,
energetics arguments suggest that any accretion in Mrk 421 has to take place at
small rates, compatible with an advection-dominated regime.Comment: 5 pages (Latex MNRAS style), revised version, submitted to MNRA
The matter content of the jet in M87: evidence for an electron-positron jet
Recent observations have allowed the geometry and kinematics of the M87 jet
to be tightly constrained. We combine these constraints with historical Very
Long Baseline Interferometry (VLBI) results and the theory of synchrotron
self-absorbed radio cores in order to investigate the physical properties of
the jet. Our results strongly suggest the jet to be dominated by an
electron-positron (pair) plasma. Although our conservative constraints cannot
conclusively dismiss an electron-proton plasma, the viability of this solution
is extremely vulnerable to further tightening of VLBI surface brightness
limits. The arguments presented, coupled with future high-resolution
multi-frequency VLBI studies of the jet core, will be able to firmly
distinguish these two possibilities.Comment: 8 pages, 1 ps figure. Revised and accepted for publication in MNRA
Fitting Pulsar Wind Tori. II. Error Analysis and Applications
We have applied the torus fitting procedure described in Ng & Romani (2004)
to PWNe observations in the Chandra data archive. This study provides
quantitative measurement of the PWN geometry and we characterize the
uncertainties in the fits, with statistical errors coming from the fit
uncertainties and systematic errors estimated by varying the assumed fitting
model. The symmetry axis of the PWN are generally well determined, and
highly model-independent. We often derive a robust value for the spin
inclination . We briefly discuss the utility of these results in
comparison with new radio and high energy pulse measurementsComment: 15 pages, 3 figures, ApJ in pres
Steep Slopes and Preferred Breaks in GRB Spectra: the Role of Photospheres and Comptonization
The role of a photospheric component and of pair breakdown is examined in the
internal shock model of gamma-ray bursts. We discuss some of the mechanisms by
which they would produce anomalously steep low energy slopes, X-ray excesses
and preferred energy breaks. Sub-relativistic comptonization should dominate in
high comoving luminosity bursts with high baryon load, while synchrotron
radiation dominates the power law component in bursts which have lower comoving
luminosity or have moderate to low baryon loads. A photosphere leading to steep
low energy spectral slopes should be prominent in the lowest baryon loadComment: ApJ'00, in press; minor revs. 10/5/99; (uses aaspp4.sty), 15 pages, 3
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