Wednesday Colloquium


"M1-92 revisited: the chemistry of an ejected common envelope ?"

Javier Alcolea (Observatorio Astronomico Nacional, Madrid)

The formation of axisymmetric planetary nebulae from the evolution of isotropic AGB circumstellar envelopes has been a matter of debate since more than two decades. A good deal of information on this process has been obtained from the observations of in between nebulae, which have been termed pre-planetary nebulae (pPNe). Going beyond the usual CO observation, which provide the main parameters of these post-AGB envelopes, we report on observations of other species in the bipolar pPN M1–92. The new IRAM 30m-MRT and NOEMA data show the presence of shock induced chemistry in the nebula. In addition, from the derived [17O]/[18O] ratio, we suggest that the sudden mass loss event responsible for the formation of the nebula 1200 yr ago may also resulted in the premature end of the AGB phase of the central star. Depending on the prevalence of such sudden mass losses, this may have and impact of the yields of AGB and the chemical evolution of the galaxy.


"Extremes of Power in the Universe"

Andrew Fabian (Institute of Astronomy, Cambridge, UK)


"Stochastic blazar variability"

Arti Goyal (Astronomical Observatory, Jagiellonian University)

Blazars are a rare class of active galactic nuclei (AGN) whose total radiative energy output is dominated by non-thermal processes; synchrotron (radio–to–optical frequencies) and inverse-Compton (IC; ∼X-ray–to–γ−ray frequencies) in a relativistic, non-stationary jet. Besides showing extreme luminosities up to ∼ 1047−48 ergs s−1 , blazars are also extremely variable on timescales ranging from decades to hours and even down to minutes. The power-law form of variability power spectral densities, P(νk) = A ν −β k , where νk is the temporal frequency, A is the normalization and β is the slope, indicate that the variability is generated by the underlying stochastic processes which is of colored noise type (i.e., β ' 1−3). In this talk, I will summarize the results of our analysis using multiwavelength data sets at TeV (HESS and VERITAS), GeV (Fermi-LAT), X-ray (Swift-XRT and RXTE– PCA), multiband optical/infrared and radio (GHz band from MRO, UMRAO and OVRO programmes) frequencies covering a few decades to minutes timescales. The novelty of this study is that at optical frequency, by combining long-term (historical optical light curves) and densely sampled intra-night lightcurves, the PSD characteristics are investigated for temporal frequencies ranging over 7 orders of magnitude. Our main results are: (1) nature of processes generating flux variability at synchrotron frequencies is different from those at IC frequencies (β ∼ 2 and 1, respectively); this could imply, that γ−ray variability, unlike the Synchrotron (radio-to-optical) one, is generated by the superposition of two stochastic processes with different relaxation timescales, (2) the main driver behind the optical variability is the same on years, months, days, and hours timescales (β ∼ 2), which argues against the scenario where different drivers behind the long-term flux changes and intra-night flux changes are considered, such as internal shocks due to the jet bulk velocity fluctuation (long-term flux changes) versus small-scale magnetic reconnection events taking place at the jet base (intra-night flux changes). Implications of these results are discussed in the context of commonly employed blazar emission models.


"First results from the Event Horizon Telescope"

Maciej Wielgus (Harvard-Smithsonian Center for Astrophysics, Cambridge, MA)

Event Horizon Telescope (EHT) is a global very long baseline interferometry array, capable of performing observations in 1 millimeter wavelength. After over a decade of tests and technical developments, in April 2017 EHT has performed first observations as a mature instrument, with sensitivity, resolution and coverage unparalleled in the history of the millimeter wavelength radioastronomy, for the first time expected to allow for the imaging of the observed sources. Observations of black hole candidates in the centers of Milky Way and M87 were conducted with a nominal resolution better than the diameter of a 'shadow of a black hole' expected from general relativity. The data set was processed and analyzed since then, finally leading to the announcement of the results on April 10th 2019.