We analyze the multi-frequency behavior of the quasar 3C 454.3 during three prominent γ -ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a triple flare structure, in the properties of each γ -ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the γ -ray outbursts, although the γ -ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a single synchrotron component whose properties do not change significantly over the different outbursts. Despite a general increase in the degree of optical linear polarization during an outburst, the polarization drops significantly at the peak of the γ -ray event, which suggests that both shocks and turbulent processes are involved. We detect two disturbances (knots) with superluminal apparent speeds in the parsec-scale jet associated with the outbursts in 2009 Autumn and 2010 Autumn. The kinematic properties of the knots can explain the difference in amplitudes of the γ -ray events, while their millimeterwave polarization is related to the optical polarization during the outbursts. We interpret the multi-frequency behavior within models involving either a system of standing conical shocks or magnetic reconnection events located in the parsec-scale millimeter-wave core of the jet. We argue that γ -ray outbursts with variability timescales as short as ∼3 hr can occur on parsec scales if flares take place in localized regions such as turbulent cells.

The discovery of the first naturally occurring high gain hydrogen recombination line (HRL) maser, in the millimeter and submillimeter spectrum of the emission line star MWC349, requires an expansion of current paradigms about HRLs. In this paper we re-examine in general the physics of non-LTE populations in recombining hydrogen and specify the conditions necessary for high-gain masing and lasing in HRLs. To do so we use the extensive new results on hydrogen level populations produced by Storey and Hummer (1995), and our calculations for the net (that is, line plus continuum) absorption coefficient for the hydrogen, and we present results for the α-and β-lines whose principal quantum numbers n are between 5 and 100, for gas whose electron number density 3 ≤ log N e (cm −3 ) ≤ 11, at two electron temperatures, T e = 5, 000 and 10,000 K. We show that the unsaturated maser gain in an HRL is a sharp function of N e , and thus to achieve high-gain masing each line requires a sufficiently extended region over which the density is rather closely specified.Saturation of masing recombination lines is a critical consideration. We derive a simple equation for estimating the degree of saturation from the observed flux density and the interferometric and/or model information about the amplification path length, avoiding the vague issue of the solid angle of masing. We also present a qualitative way to approach the effects of saturation on adjacent emission lines, although the detailed modeling is highly case-specific.We draw attention to another non-LTE phenomenon active in hydrogen: the overcooling of populations. This occurs for HRLs with n > ∼ 20, in gas where N e < ∼ 10 5 cm −3 . Observationally the HRL overcooling might manifest itself as an anomalously weak emission recombination line, or as a "dasar," that is, an anomalously strong absorption line. In the simplest case of a homogeneous HII region the absorption can be observed on the proper free-free continuum of the region, if some conditions for the line or/and continuum optical depths are satisfied.We briefly discuss the prospects of detecting hydrogen masers, lasers and dasars in several classes of galactic and extragalactic objects, including compact HII regions, Be or Wolf-Rayet stars, and AGNs.

Since 2005, the blazar 3C 454.3 has shown remarkable flaring activity at all frequencies, and during the last four years it has exhibited more than one γ -ray flare per year, becoming the most active γ -ray blazar in the sky. We present for the first time the multi-wavelength AGILE, Swift, INTEGRAL, and GASP-WEBT data collected in order to explain the extraordinary γ -ray flare of 3C 454.3 which occurred in 2010 November. On 2010 November 20 (MJD 55520), 3C 454.3 reached a peak flux (E >100 MeV) of F p γ = (6.8 ± 1.0) × 10 −5 photons cm −2 s −1 on a timescale of about 12 hr, more than a factor of six higher than the flux of the brightest steady γ -ray source, the Vela pulsar, and more than a factor of three brighter than its previous super-flare on 2009 December 2-3. The multi-wavelength data make possible a thorough study of the present event: the comparison with the previous

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Class I methanol masers are believed to be produced in the shock-excited environment around star-forming regions. Many authors have argued that the appearance of various subsets of class I masers may be indicative of specific evolutionary stages of star formation or excitation conditions. Until recently, however, no major interferometer was capable of imaging the important 36 GHz transition. We report on Expanded Very Large Array observations of the 36 GHz methanol masers and Submillimeter Array observations of the 229 GHz methanol masers in DR21(OH), DR21N, and DR21W. The distribution of 36 GHz masers in the outflow of DR21(OH) is similar to that of the other class I methanol transitions, with numerous multitransition spatial overlaps. At the site of the main continuum source in DR21(OH), class I masers at 36 and 229 GHz are found in virtual overlap with class II 6.7 GHz masers. To the south of the outflow, the 36 GHz masers are scattered over a large region but usually do not appear coincident with 44 GHz masers. In DR21W, we detect an "S-curve" signature in Stokes V that implies a large value of the magnetic field strength if interpreted as due to Zeeman splitting, suggesting either that class I masers may exist at higher densities than previously believed or that the direct Zeeman interpretation of S-curve Stokes V profiles in class I masers may be incorrect. We find a diverse variety of different maser phenomena in these sources, suggestive of differing physical conditions among them.