Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userNo precise localization for FRB 150418: claimed radio transient is AGN variability
52
0
0.0
(
0
)
Authors
P. K. G. Williams
Ask ChatGPT about the research
No Arabic abstract
Keane et al. have recently claimed to have obtained the first precise localization for a Fast Radio Burst (FRB) thanks to the identification of a contemporaneous fading slow (~week-timescale) radio transient. They use this localization to pinpoint the FRB to a galaxy at z ~ 0.49 that exhibits no discernable star formation activity. We argue that the transient is not genuine and that the host candidate, WISE J071634.59-190039.2, is instead a radio variable: the available data did not exclude this possibility; a random radio variable consistent with the observations is not unlikely to have a redshift compatible with the FRB dispersion measure; and the proposed transient light curve is better explained as a scintillating steady source, perhaps also showing an active galactic nucleus (AGN) flare, than a synchrotron-emitting blastwave. The radio luminosity of the host candidate implies that it is an AGN and we present new late-time Very Large Array observations showing that the galaxy is indeed variable at a level consistent with the claimed transient. Therefore the claimed precise localization and redshift determination for FRB 150418 cannot be justified.
rate research
Read More
8.4-GHZ radio imaging study of an optically selected sample of early type Seyfert galaxies; comparison of images taken at two epochs reveals possible variation in the nuclear radio flux density in five of them over a seven year period. It is shown that there is a possible correlation between the presence of nuclear radio variability and the absence of hundred parsec-scale radio emission, analogous with radio-loud AGN. Our results suggest that all Seyferts may exhibit variation in their nuclear radio flux density at 8.4 GHz, but that variability is more easily recognized in compact sources in which emission from the variable nucleus is not diluted by unresolved, constant flux density radio jet emission within the central 50 pc. Taken in combination with other Seyfert properties, these results suggest a paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population and demonstrate the importance of investigating the temporal domain at radio wavelengths, which remains completely unexplored for faint radio-quiet AGN. Discovery of intermittent activity and radio flares has important implications for the AGN duty cycles and the origin of Ultra-High Energy Cosmic Rays. New radio facilities, such as the EVLA/VLBA, eMERLIN, LOFAR and eVLBI on the EVN, will revolutionise the study of radio-quiet AGN; in particular, the combination of increased sensitivity and sampling rate with high-angular resolution and automatic data reduction will open up the transient radio sky and bring major future breakthroughs.
We study variability of active galactic nuclei (AGNs) by using the deep optical multiband photometry data obtained from the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) survey in the COSMOS field. The images analyzed here were taken with 8, 10, 13, and 15 epochs over three years in the $g$, $r$, $i$, and $z$ bands, respectively. We identified 491 robust variable AGN candidates, down to $i=25$ mag and with redshift up to $4.26$. Ninety percent of the variability-selected AGNs are individually identified with the X-ray sources detected in the Chandra COSMOS Legacy survey. We investigate their properties in variability by using structure function analysis and find that the structure function for low-luminosity AGNs ($L_{mathrm{bol}}lesssim10^{45}$ erg s$^{-1}$) shows a positive correlation with luminosity, which is the opposite trend for the luminous quasars. This trend is likely to be caused by larger contribution of the host galaxy light for lower-luminosity AGNs. Using the model templates of galaxy spectra, we evaluate the amount of host galaxy contribution to the structure function analysis and find that dominance of the young stellar population is needed to explain the observed luminosity dependence. This suggests that low-luminosity AGNs at $0.8lesssim zlesssim1.8$ are predominantly hosted in star-forming galaxies. The X-ray stacking analysis reveals the significant emission from the individually X-ray undetected AGNs in our variability-selected sample. The stacked samples show very large hardness ratios in their stacked X-ray spectrum, which suggests that these optically variable sources have large soft X-ray absorption by dust-free gas.
Comparison of 8.4-GHz radio images of a sample of 11 early-type Seyfert galaxies with previous observations reveals possible variation in the nuclear radio flux density in 5 of them over a 7-yr period. We find no correlation between radio variability and nuclear radio luminosity or Seyfert nuclear type, although the sample is small and dominated by type 2 Seyferts. Instead, a possible correlation between the presence of nuclear radio variability and the absence of ~100-pc-scale radio emission is seen. NGC2110 is the only source with significant extended radio structure and strong nuclear variability (>38% nuclear decline over seven years). Our results suggest that all Seyferts may exhibit variation in their nuclear radio flux density at 8.4 GHz, but that variability is more easily recognised in compact sources in which emission from the variable nucleus is not diluted by unresolved, constant flux density radio-jet emission within the central ~50 pc. If flares in radio light curves correspond to ejection of new relativistic components or emergence of shocks in the underlying flow, we suggest that radio jets may be intrinsically non-relativistic during quiescence, but that Seyferts, as black-hole driven AGN, have the capacity to accelerate relativistic jets during radio flares. Taken together with the increased detection rate of flat spectrum radio nuclei in Seyferts imaged at VLBI resolutions and the detection of variable water megamaser emission, our results support the paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population. (Abridged).
We present new Jansky Very Large Array (VLA) radio images of the Crab Nebula at 5.5 GHz, taken at two epochs separated by 6 days about two months after a gamma-ray flare in 2012 July. We find no significant change in the Crabs radio emission localized to a region of <2 light-months in radius, either over the 6-day interval between our present observations or between the present observations and ones from 2001. Any radio counterpart to the flare has a radio luminosity of <~ $2 times 10^{-4}$ times that of the nebula. Comparing our images to one from 2001, we do however find changes in radio brightness, up to 10% in amplitude, which occur on decade timescales throughout the nebula. The morphology of the changes is complex suggesting both filamentary and knotty structures. The variability is stronger, and the timescales likely somewhat shorter, nearer the centre of the nebula. We further find that even with the excellent uv~coverage and signal-to-noise of the VLA, deconvolution errors are much larger than the noise, being up to 1.2% of peak brightness of the nebula in this particular case.
To gain insights into long-term Active Galactic Nuclei (AGN) variability, we analyze an AGN sample from the Sloan Digital Sky Survey (SDSS) and compare their photometry with observations from the Hyper Suprime-Cam survey (HSC) observed $langle 14.85 rangle$ years after SDSS. On average, the AGN are fainter in HSC than SDSS. We demonstrate that the difference is not due to subtle differences in the SDSS versus HSC filters or photometry. The decrease in mean brightness is redshift dependent, consistent with expectations for a change that is a function of the rest-frame time separation between observations. At a given redshift, the mean decrease in brightness is stronger for more luminous AGN and for objects with longer time separation between measurements. We demonstrate that the dependence on redshift and luminosity of measured mean brightness decrease is consistent with simple models of Eddington ratio variability in AGN on long (Myr, Gyr) timescales. We show how our results can be used to constrain the variability and demographic properties of AGN populations.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
