- ID:
- ivo://CDS.VizieR/J/A+A/641/A40
- Title:
- S5 0836+710 Ground and Space VLBI images
- Short Name:
- J/A+A/641/A40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure and physics of the relativistic jet in this object. The RadioAstron observations at wavelengths of 18cm, 6cm, and 1.3cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02mas to 200mas. Brightness temperatures in excess of 10^13^K are measured in the jet, requiring Doppler factors of >=100 for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin-Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be Mj~=12 and {eta}~=0.33. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations.
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- ID:
- ivo://CDS.VizieR/J/ApJ/729/87
- Title:
- SiIV absorption systems
- Short Name:
- J/ApJ/729/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identified 24 SiIV absorption systems with z<~1 from a blind survey of 49 low-redshift quasars with archival Hubble Space Telescope ultraviolet spectra. We relied solely on the characteristic wavelength separation of the doublet to automatically detect candidates. After visual inspection, we defined a sample of 20 definite (group G=1) and 4 "highly likely" (G=2) doublets with rest equivalent widths W_r_ for both lines detected at >=3{sigma}_W_r__.
- ID:
- ivo://CDS.VizieR/J/ApJS/243/2
- Title:
- SiIV and NV absorption line SDSS quasar systems
- Short Name:
- J/ApJS/243/2
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using the quasar spectra from the Sloan Digital Sky Survey and with variable C IV absorption line systems, we measure the Si IV and N V absorption line systems. We obtain 50 variable Si IV absorption line systems and 39 variable N V absorption line systems. We find that the variations in most of the C IV, Si IV, and N V absorption lines are correlated with the changes in quasar continuum. In addition, a significant portion of the variable absorption lines are the consistent variations of multiple systems with large velocity separations. Therefore, the variations of the C IV, Si IV, and N V absorption lines could be mainly driven by the changes in quasar radiations, which cause changes in ionization states or column densities of absorbing gas. We also find that the variable C IV, Si IV, and N V absorption line systems can be divided into low-ionization systems and high-ionization systems. The former positively responds to the changes in the quasar's brightness, and the later is the oppositive case.
- ID:
- ivo://CDS.VizieR/J/A+A/545/A65
- Title:
- S5 0836+710 image at 1.6GHz
- Short Name:
- J/A+A/545/A65
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The remarkable stability of extragalactic jets is surprising, given the reasonable possibility of the growth of instabilities. In addition, much work in the literature has invoked this possibility to explain observed jet structures and obtain information about the jet from these structures. For example, it has been shown that the observed helical structures in the jet in S5 0836+710 could be associated with helical pressure waves generated by a Kelvin-Helmholtz instability. Our aim is to resolve the arc-second structure of the jet in the quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found by present observing arrays, as this lack implies a loss of jet collimation before interaction with the intergalactic medium.
- ID:
- ivo://CDS.VizieR/J/ApJ/766/37
- Title:
- Sizes of MRC radio galaxies and QSOs
- Short Name:
- J/ApJ/766/37
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the currently popular orientation-based unified scheme, a radio galaxy appears as a quasar when its principal radio-axis happens to be oriented within a certain cone opening angle around the observer's line of sight. Due to geometrical projection, the observed sizes of quasars should therefore appear smaller than those of radio galaxies. We show that this simple, unambiguous prediction of the unified scheme is not borne out by the actually observed angular sizes of radio galaxies and quasars. Except in the original 3CR sample, based on which the unified scheme was proposed, in other much larger samples no statistically significant difference is apparent in the size distributions of radio galaxies and quasars. The population of low-excitation radio galaxies with apparently no hidden quasars inside, which might explain the observed excess number of radio galaxies at low redshifts, cannot account for the absence of any foreshortening of the sizes of quasars at large redshifts. On the other hand, from infrared and X-ray studies, there is evidence of a hidden quasar within a dusty torus in many radio galaxies, at z>0.5. It is difficult to reconcile this with the absence of foreshortening of quasar sizes at even these redshifts, and perhaps one has to allow that the major radio axis may not have anything to do with the optical axis of the torus. Otherwise, to resolve the dichotomy of radio galaxies and quasars, a scheme quite different from the present might be required.
- ID:
- ivo://CDS.VizieR/J/AJ/121/2308
- Title:
- Sloan Digital Sky Survey quasar photometry
- Short Name:
- J/AJ/121/2308
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an empirical investigation of the colors of quasars in the Sloan Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625 quasars with SDSS photometry: 1759 quasars found during SDSS spectroscopic commissioning and SDSS follow-up observations on other telescopes, 50 matches to FIRST quasars, 573 matches to quasars from the NASA Extragalactic Database, and 243 quasars from two or more of these sources. The quasars are distributed in a 2.5{deg} wide stripe centered on the celestial equator covering ~529deg^2^. Positions (accurate to 0.2") and SDSS magnitudes are given for the 898 quasars known prior to SDSS spectroscopic commissioning.
- ID:
- ivo://CDS.VizieR/J/A+A/622/A29
- Title:
- SMC AGN in XMM-Newton
- Short Name:
- J/A+A/622/A29
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Finding active galactic nuclei (AGN) behind the Magellanic Clouds (MCs) is difficult because of the high stellar density in these fields. Although the first AGN behind the Small Magellanic Cloud (SMC) were reported in the 1980s, it is only recently that the number of AGN known behind the SMC has increased by several orders of magnitude. The mid-infrared colour selection technique has proven to be an efficient means of identifying AGN, especially obscured sources. The X-ray regime is complementary in this regard and we use XMM-Newton observations to support the identification of AGN behind the SMC. We present a catalogue of AGN behind the SMC by correlating an updated X-ray point-source catalogue from our XMM-Newton survey of the SMC with previously identified AGN from the literature as well as a list of candidates obtained from the ALLWISE mid-infrared colour-selection criterion. We studied the properties of the sample with respect to their redshifts, luminosities, and X-ray spectral characteristics. We also identified the near-infrared counterpart of the sources from the VISTA observations. The redshift and luminosity distributions of the sample (where known) indicate that we detect sources ranging from nearby Seyfert galaxies to distant and obscured quasars. The X-ray hardness ratios are compatible with those typically expected for AGN, and the VISTA colours and variability are also consistent with AGN. A positive correlation was observed between the integrated X-ray flux (0.2-12keV) and the ALLWISE and VISTA magnitudes. We further present a sample of new candidate AGN and candidates for obscured AGN. Together these make an interesting subset for further follow-up studies. An initial spectroscopic follow-up of 6 out of the 81 new candidates showed that all six sources are active galaxies, although two have narrow emission lines.
- ID:
- ivo://CDS.VizieR/J/ApJS/175/116
- Title:
- SNU bright quasar survey (SNUQSO)
- Short Name:
- J/ApJS/175/116
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from the first phase of the Seoul National University Bright Quasar Survey in Optical (SNUQSO) as well as its basic observational setup. Previous and current large-area surveys have been successful in identifying many quasars, but they could have missed bright quasars due to their survey design. In order to help complete the census of bright quasars, we have performed spectroscopic observations of new bright quasar candidates selected from various methods based on optical colors, near-infrared colors, radio, and X-ray data. In 2005/2006, we observed 55 bright quasar candidates using the Bohyunsan Optical Echelle Spectrograph (BOES) on the 1.8m telescope at the Bohyunsan Optical Astronomy Observatory in Korea. We identify 14 quasars/Seyferts from our observation, including an optically bright quasar with i=14.98mag at z=0.092 (SDSS J003236.59-091026.2). Non-quasar/Seyfert objects are found to be mostly stars, among which there are five M-type stars and one cataclysmic variable. Our result shows that there still exist bright quasars to be discovered. However, at the same time, we conclude that finding new bright quasars in high Galactic latitude regions is very challenging and that the existing compilation of optically bright quasars is nearly complete in the northern hemisphere.
- ID:
- ivo://CDS.VizieR/J/AJ/127/156
- Title:
- Soft X-Ray-Selected AGNs complete sample
- Short Name:
- J/AJ/127/156
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the optical spectra and simple statistical analysis for a complete sample of 110 soft X-ray-selected AGNs. About half of the sources are narrow-line Seyfert 1 galaxies (NLS1's), which have the steepest X-ray spectra, the strongest Fe II emission, and slightly weaker [O III] {lambda}5007 emission than broad-line Seyfert 1's (BLS1's). Kolmogorov-Smirnov tests show that NLS1's and BLS1's have clearly different distributions of the X-ray spectral slope {alpha}_X_, X-ray short-term variability, and Fe II equivalent widths and luminosity and Fe II/H{beta} ratios.
- ID:
- ivo://CDS.VizieR/J/MNRAS/422/3370
- Title:
- Spatial variation in fine-structure constant
- Short Name:
- J/MNRAS/422/3370
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Quasar absorption lines provide a precise test of whether the fine-structure constant, {alpha}, is the same in different places and through cosmological time. We present a new analysis of a large sample of quasar absorption-line spectra obtained using the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) in Chile. We apply the many-multiplet method to derive values of {DELTA}{alpha}/{alpha}==({alpha}z-{alpha}0)/{alpha}0 from 154 absorbers, and combine these values with 141 values from previous observations at the Keck Observatory in Hawaii.
