Related papers: Are quasars reliable standard candles?
We put constraints on the main cosmological parameters of different spatially flat cosmological models by combining the recent clustering results of XMM-Newton soft (0.5-2 keV) X-ray sources, which have a redshift distribution with median…
Strong outflows from active galactic nuclei are frequently observed in objects with lower coronal X-ray luminosity. This intrinsic X-ray weakness is considered a requirement for the formation of radiatively driven winds. To obtain an…
The Cosmic Distance Duality Relation (CDDR) is a basic relation of standard cosmology. This work tests the CDDR and cosmic transparency using angular diameter distances from DESI DR2, luminosity distances from Pantheon+, and direct…
The magnitude-redshift relation is one of the tools for a direct observational approach to cosmology. The discovery of high redshift Type Ia supernovae (SNIa) and their use as ``standard candles'' has resurrected interest in this approach.…
We use supernovae measurements, calibrated by the local determination of the Hubble constant $H_0$ by SH0ES, to interpolate the distance-redshift relation using Gaussian process regression. We then predict, independent of the cosmological…
Risaliti and Lusso have compiled X-ray and UV flux measurements of 1598 quasars (QSOs) in the redshift range $0.036 \leq z \leq 5.1003$, part of which, $z \sim 2.4 - 5.1$, is largely cosmologically unprobed. In this paper we use these QSO…
We combine a high-resolution hydro-simulation of the LambdaCDM cosmology with two radiative transfer schemes (for continuum and line radiation) to predict the properties, spectra and spatial distribution of fluorescent Ly-alpha emission at…
With the many ambitious proposals afoot for new generations of very large telescopes, along with spectrographs of unprecedented resolution, there arises the real possibility that the time evolution of the cosmological redshift may, in the…
Distant supernovae can now be detected routinely. To date 34 supernovae at $z > 0.1$ have been discovered. Among them are 12 Type~Ia supernovae confirmed spectroscopically and suited to measure the cosmic deceleration when appropriately…
We present photometric selection of type 1 quasars in the $\approx5.3~{\rm deg}^{2}$ XMM-Large Scale Structure (XMM-LSS) survey field with machine learning. We constructed our training and \hbox{blind-test} samples using spectroscopically…
The relation between redshift and the CMB temperature, $T_{CMB}(z)=T_0(1+z)$ is a key prediction of standard cosmology, but is violated in many non-standard models. Constraining possible deviations to this law is an effective way to test…
Line-of-sight interactions of cosmic rays provide a natural explanation of the hard gamma-ray spectra of distant blazars, which are believed to be capable of producing both gamma rays and cosmic rays. For sources with redshifts z> 0.1,…
We present a systematic analysis of X-ray archival data of all the 29 quasars (QSOs) at $z$ > 5.5 observed so far with Chandra, XMM-Newton and Swift-XRT, including the most-distant quasar ever discovered, ULAS J1120+0641 ($z$ = 7.08). This…
Gamma-ray bursts (GRBs) are the most luminous explosions and can be detectable out to the edge of Universe. It has long been thought they can extend the Hubble diagram to very high redshifts. Several correlations between temporal or…
We measure long (2200-4000 ang) and short (1450-2200 ang) wavelength spectral slopes \alpha (F_\nu proportional to \nu^\alpha) for quasar spectra from the Sloan Digital Sky Survey. The long and short wavelength slopes are computed from 3646…
The use of Type~Ia SNe has thus far produced the most reliable measurement of the expansion history of the Universe, suggesting that $\Lambda$CDM offers the best explanation for the redshift--luminosity distribution observed in these…
We study the implications of a combined analysis of cosmic standard candles and standard rulers on the viability of cosmological models beyond the cosmological concordance model. To this end, we employ well-established data in the form of…
The radio emission in radio-quiet quasars (RQQs) has been a long mystery and its physical origin remains unclear. In a previous work we find UV/optical more variable quasars have stronger X-ray emission, indicating a link between disc…
The issue of the X-ray to optical luminosity relationship ($L_o-L_x$) is addressed for both optically and X-ray selected quasar samples. We have applied a generalized regression algorithm for the case of samples involving censored data,…
For directions of sufficcient reddening (E(B-V) >~ 0.25), there is a simple relation between the slope of the extinction curve in the far-UV and E(B-V). Regardless of direction, the far-UV extinction curve is proportional to 1/lambda^n…