We use the COBE/DIRBE (1.2, 2.2, 60, 100, 140, and 240 µm) maps and the COBE/FIRAS spectra (for the wavelength range 100−1000 µm) to constrain a model for the spatial distribution of the dust, the stars, and the gas in the Milky Way. By assuming exponential axisymmetric distributions for the dust and the stars and by performing the corresponding radiative transfer calculations we closely (given the simple geometry of the model) reproduce the FIR and NIR maps of the Milky Way. Similar distributions for the atomic and molecular hydrogen in the disk are used (with an inner cut-off radius for the atomic hydrogen) to fit the gas data. The star formation rate as a function of the Galactic radius is derived from the FIR emission and is well in agreement with existing estimates from various star formation tracers. The gas surface density is plotted against the star formation rate density and an "intrinsic" Galactic Schmidt law is derived with excellent agreement with the "external" Schmidt law found for spiral galaxies. The Milky Way is found to consume ∼1% and ∼10% of its gas in the outer and inner regions respectively (for a period of 0.1 Gyr) to make stars. The dust-induced B − V color excess observed in various directions and distances (up to ∼6.5 kpc) with well-studied Cepheid stars is compared with the model predictions showing a good agreement. The simple assumption of exponential distributions of stars and dust in the Galaxy is found to be quite instructive and adequate in modeling all the available data sets from 0.45 µm (B-band) to 1000 µm.

Results from regular monitoring of relativistic compact binaries like PSR 1913+16 are consistent with the dominant (quadrupole) order emission of gravitational waves (GWs). We show that observations associated with the binary black hole (BBH) central engine of blazar OJ287 demand the inclusion of gravitational radiation reaction effects beyond the quadrupolar order. It turns out that even the effects of certain hereditary contributions to GW emission are required to predict impact flare timings of OJ287. We develop an approach that incorporates this effect into the BBH model for OJ287. This allows us to demonstrate an excellent agreement between the observed impact flare timings and those predicted from ten orbital cycles of the BBH central engine model. The deduced rate of orbital period decay is nine orders of magnitude higher than the observed rate in PSR 1913+16, demonstrating again the relativistic nature of OJ287ʼs central engine. Finally, we argue that precise timing of the predicted 2019 impact flare should allow a test of the celebrated black hole "no-hair theorem" at the 10% level.

Abstract.We have observed BL Lacertae in the B, R and I bands for 2 nights in July, 1999, and 3 nights in July, 2001. The observations resulted in almost evenly sampled light curves, with an average sampling interval of ∼5 min. Because of the dense sampling and the availability of light curves in three bands we are able to study the intra-night flux and spectral variability of the source in detail. The source is significantly variable in all bands, showing variations on different time scales. On average, the variability amplitude increases from ∼5% in the I band, to ∼5.5% in the R and ∼6.5% in the B band light curves.The rising and decaying time scales are comparable within each band, but they increase from the B, to R and I band light curves. The optical power spectrum shows a red noise component with a slope of ∼−2. Cross-correlation analysis shows that in most cases the delay between the variations in the B and I band light curves is less than ∼±0.4 hrs. However, the cross-correlation functions are asymmetric, implying complex delays of the I band variations with respect to the B band variations. Furthermore, in one case we find that the I band variations are significantly delayed (by ∼0.2 hrs) with respect to the B band variations. We also detect significant spectral variations. We find that the spectrum hardens, (i.e. it gets flatter) as the flux increases, and the flattest spectral index corresponds to the maximum B band flux. The rate of the spectral variations does not remain the same during the observations. Our results imply that the fast, intra-night variations of the source correspond to perturbations of different regions in the jet which cause localized injections of relativistic particles on time scales much shorter that the average sampling interval of the light curves. The variations are controlled by the cooling and light crossing time scales, which are probably comparable.

We present results from a study of optically emitting Supernova Remnants (SNRs) in six nearby galaxies (NGC 2403, NGC 3077, NGC 4214, NGC 4395, NGC 4449 and NGC 5204) based on deep narrow band Hα and [S II] images as well as spectroscopic observations. The SNR classification was based on the detected sources that fulfill the well-established emission line flux criterion of [S II]/Hα> 0.4. This study revealed ∼400 photometric SNRs down to a limiting Hα flux of 10 −15 erg sec −1 cm −2 . Spectroscopic observations confirmed the shockexcited nature of 56 out of the 96 sources with ([S II]/Hα) phot > 0.3 (our limit for an SNR classification) for which we obtained spectra. 11 more sources were spectroscopically identified as SNRs although their photometric [S II]/Hα ratio was below 0.3. We discuss the properties of the optically-detected SNRs in our sample for different types of galaxies and hence different environments, in order to address their connection with the surrounding interstellar medium. We find that there is a difference in [N II]/Hα line ratios of the SNR populations between different types of galaxies which indicates that this happens due to metallicity. We cross-correlate parameters of the optically detected SNRs ([S II]/Hα ratio, luminosity) with parameters of coincident X-ray emitting SNRs, resulted from our previous studies in the same sample of galaxies, in order to understand their evolution and investigate possible selection effects. We do not find a correlation between their Hα and X-ray luminosities, which we attribute to the presence of material in a wide range of temperatures. We also find evidence for a linear relation between the number of luminous optical SNRs (10 37 erg sec −1 ) and SFR in our sample of galaxies.

OpenBU http://open.bu.edu Astronomy BU Open Access Articles 2018-08-20 Stochastic modeling of multiwavelength variability of the classical BL Lac Object OJ287...