We propose a scenario where blazars are classified into flat-spectrum radio quasars (FSRQs), BL Lacertae (BL Lac) objects, low-synchrotron, or high-synchrotron peaked objects according to a varying ...mix of the Doppler-boosted radiation from the jet, the emission from the accretion disc, the broad-line region, and the light from the host galaxy. In this framework, the peak energy of the synchrotron power (
) in blazars is independent of source type and radio luminosity. We test this new approach, which builds upon unified schemes, using extensive Monte Carlo simulations, and show that it can provide simple answers to a number of long-standing issues, including, amongst others, the different cosmological evolution of BL Lac objects selected in the radio and X-ray bands, the larger
values observed in BL Lac objects, the fact that high-synchrotron peaked blazars are always of BL Lac type, and the existence of FSRQ-BL Lac transition objects. Objects so far classified as BL Lac objects on the basis of their observed weak, or undetectable, emission lines are of two physically different classes: intrinsically weak lined objects, more common in X-ray-selected samples, and heavily diluted broad-lined sources, more frequent in radio-selected samples, which explains some of the confusion in the literature. We also show that strong selection effects are the main cause of the diversity observed in radio and X-ray samples, and that the correlation between luminosity and
, which led to the proposal of the 'blazar sequence', is also a selection effect arising from the comparison between shallow radio and X-ray surveys, and to the fact that high-
-high-radio-power objects have never been considered because their redshift is not measurable.
ABSTRACT
Blazars research is one of the hot topics of contemporary extragalactic astrophysics. That is because these sources are the most abundant type of extragalactic γ-ray sources and are ...suspected to play a central role in multimessenger astrophysics. We have used Swift$\_$xrtproc, a tool to carry out an accurate spectral and photometric analysis of the Swift-XRT data of all blazars observed by Swift at least 50 times between December 2004 and the end of 2020. We present a database of X-ray spectra, best-fit parameter values, count rates and flux estimations in several energy bands of over 31 000 X-ray observations and single snapshots of 65 blazars. The results of the X-ray analysis have been combined with other multifrequency archival data to assemble the broad-band Spectral Energy Distributions (SEDs) and the long-term light curves of all sources in the sample. Our study shows that large X-ray luminosity variability on different time-scales is present in all objects. Spectral changes are also frequently observed with a ‘harder-when-brighter’ or ‘softer-when-brighter’ behaviour depending on the SED type of the blazars. The peak energy of the synchrotron component (νpeak) in the SED of HBL blazars, estimated from the log-parabolic shape of their X-ray spectra, also exhibits very large changes in the same source, spanning a range of over two orders of magnitude in Mrk421 and Mrk501, the objects with the best data sets in our sample.
ABSTRACT
Blazars are a peculiar class of active galactic nuclei that enlighten the sky at all wavelengths. The electromagnetic emission of these sources is jet-dominated, resulting in a spectral ...energy distribution (SED) that has a typical double-humped shape. X-ray photons provide a wealth of information on the physics of each source as in the X-ray band, we can observe the tail of SED first peak, the rise of the second one or the transition between the two. NuSTAR, thanks to its capability of focusing X-rays up to 79 keV provides broad-band data particularly suitable to compute SEDs in a still poorly explored part of the spectrum. In the context of the Open Universe initiative, we developed a dedicated pipeline, NuSTAR_Spectra, a shell-script that automatically downloads data from the archive, generates scientific products and carries out a complete spectral analysis. The script homogeneously extracts high level scientific products for both NuSTAR’s telescopes and the spectral characterization is performed testing two phenomenological models. The corresponding X-ray properties are derived from the data best fit, and the SEDs are also computed. The systematic processing of all blazar observations of the NuSTAR public archive allowed us to release the first hard X-ray spectroscopic catalogue of blazars (NuBlazar). The catalogue, updated to 2021 September 30, includes 253 observations of 126 distinct blazars, 30 of which have been multiply observed.
Context. The X-ray variability of the active galactic nuclei (AGN) has been most often investigated with studies of individual, nearby sources, and only a few ensemble analyses have been applied to ...large samples in wide ranges of luminosity and redshift. Aims. We aim to determine the ensemble variability properties of two serendipitously selected AGN samples extracted from the catalogues of XMM-Newton and Swift, with redshift between ~0.2 and ~4.5, and X-ray luminosities, in the 0.5–4.5 keV band, between ~1043 erg/s and ~1046 erg/s. Methods. We used the structure function (SF), which operates in the time domain, and allows for an ensemble analysis even when only a few observations are available for individual sources and the power spectral density (PSD) cannot be derived. The SF is also more appropriate than fractional variability and excess variance, because these parameters are biased by the duration of the monitoring time interval in the rest-frame, and therefore by cosmological time dilation. Results. We find statistically consistent results for the two samples, with the SF described by a power law of the time lag, approximately as SF ∝ τ0.1. We do not find evidence of the break in the SF, at variance with the case of lower luminosity AGNs. We confirm a strong anti-correlation of the variability with X-ray luminosity, accompanied by a change of the slope of the SF. We find evidence in support of a weak, intrinsic, average increase of X-ray variability with redshift. Conclusions. The change of amplitude and slope of the SF with X-ray luminosity provides new constraints on both single oscillator models and multiple subunit models of variability.
Abstract
Mini-EUSO is a telescope observing the Earth in the ultraviolet band from the International Space Station. It is a part of the JEM-EUSO program, paving the way to future larger missions, ...such as K-EUSO and POEMMA, devoted primarily to the observation of ultrahigh-energy cosmic rays from space. Mini-EUSO is capable of observing extensive air showers generated by ultrahigh-energy cosmic rays with an energy above 10
21
eV and to detect artificial showers generated with lasers from the ground. Other main scientific objectives of the mission are the search for nuclearites and strange quark matter, the study of atmospheric phenomena such as transient luminous events, meteors, and meteoroids, the observation of sea bioluminescence and of artificial satellites and man-made space debris. Mini-EUSO will map the nighttime Earth in the UV range (290–430 nm), with a spatial resolution of about 6.3 km and a temporal resolution of 2.5
μ
s, through a nadir-facing UV-transparent window in the Russian Zvezda module. The instrument, launched on 2019 August 22, from the Baikonur Cosmodrome, is based on an optical system employing two Fresnel lenses and a focal surface composed of 36 multianode photomultiplier tubes, 64 channels each, for a total of 2304 channels with single-photon counting sensitivity and an overall field of view of 44°. Mini-EUSO also contains two ancillary cameras to complement measurements in the near-infrared and visible ranges. In this paper, we describe the detector and present the various phenomena observed in the first months of operations.
Context. The observed relation between the X-ray radiation from active galactic nuclei, originating in the corona, and the optical/UV radiation from the disk is usually described by the ...anticorrelation between the UV to X-ray slope $\alpha_{\rm ox}$ and the UV luminosity. Many factors can affect this relation, including: i) enhanced X-ray emission associated with the jets of radio-loud AGNs, ii) X-ray absorption associated with the UV broad absorption line (BAL) outflows, iii) other X-ray absorption not associated with BALs, iv) intrinsic X-ray weakness, v) UV and X-ray variability, and non-simultaneity of UV and X-ray observations. The separation of these effects provides information about the intrinsic $\alpha_{\rm ox}$ - LUV relation and its dispersion, constraining models of disk-corona coupling. Aims. We use simultaneous UV/X-ray observations to remove the influence of non-simultaneous measurements from the $\alpha_{\rm ox}$ - LUV relation. Methods. We extract simultaneous data from the second XMM-Newton serendipitous source catalogue (XMMSSC) and the XMM-Newton Optical Monitor Serendipitous UV Source Survey catalogue (XMMOMSUSS), and derive the single-epoch $\alpha_{\rm ox}$ indices. We use ensemble structure functions to analyse multi-epoch data. Results. We confirm the anticorrelation of $\alpha_{\rm ox}$ with LUV, and do not find any evidence of a dependence of $\alpha_{\rm ox}$ on z. The dispersion in our simultaneous data (σ~0.12) is not significantly smaller than in previous non-simultaneous studies, suggesting that “artificial $\alpha_{\rm ox}$ variability” introduced by non-simultaneity is not the main cause of dispersion. “Intrinsic $\alpha_{\rm ox}$ variability”, i.e., the true variability of the X-ray to optical ratio, is instead important, and accounts for ~30% of the total variance, or more. “Inter-source dispersion”, due to intrinsic differences in the average $\alpha_{\rm ox}$ values from source to source, is also important. The dispersion introduced by variability is mostly caused by the long timescale variations, which are expected to be driven by the optical variations.
Mini-EUSO is a space experiment selected to be installed inside the International Space Station. It has a compact telescope with a large field of view (44 × 44 sq. deg.) focusing light on an array of ...photo-multipliers tubes in order to observe UV emission coming from Earth’s atmosphere. Observations will be complemented with data recorded by some ancillary detectors. In particular, the Mini-EUSO Additional Data Acquisition System (ADS) is composed by two cameras, which will allow us to obtain data in the near infrared, and in the visible range. These will be used to monitor the observation conditions, and to acquire useful information on several scientific topics to be studied with the main instrument, such as the physics of atmosphere, meteors, and strange quark matter. Here we present the ADS control software developed to stream cameras together with the UV main instrument, in order to grab images in an automated and independent way, and we also describe the calibration activities performed on these two ancillary cameras before flight.
Aims. Open Universe for Blazars is a set of high-transparency multi-frequency data products for blazar science, and the tools designed to generate them. Blazars are drawing growing interest following ...the consolidation of their position as the most abundant type of source in the extragalactic very high-energy γ-ray sky, and because of their status as prime candidate sources in the nascent field of multi-messenger astrophysics. As such, blazar astrophysics is becoming increasingly data driven, depending on the integration and combined analysis of large quantities of data from the entire span of observational astrophysics techniques. The project was therefore chosen as one of the pilot activities within the United Nations Open Universe Initiative, whose objective is to stimulate a large increase in the accessibility and ease of utilisation of space science data for the worldwide benefit of scientific research, education, capacity building, and citizen science. Methods. Our aim is to deliver innovative data science tools for multi-messenger astrophysics. In this work we report on a data analysis pipeline called Swift-DeepSky based on the Swift XRTDAS software and the XIMAGE package, encapsulated into a Docker container. Swift-DeepSky downloads and reads low-level data, generates higher level products, detects X-ray sources, and estimates several intensity and spectral parameters for each detection, thus facilitating the generation of complete and up-to-date science-ready catalogues from an entire space-mission data set. Results. As a first application of our innovative approach, we present the results of a detailed X-ray image analysis based on Swift-DeepSky that was run on all Swift-XRT observations including a known blazar, carried out during the first 14 years of operations of the Neil Gehrels Swift Observatory. Short exposures executed within one week of each other have been added to increase sensitivity, which ranges between ∼1 × 10−12 and ∼1 × 10−14 erg cm−2 s−1 (0.3–10.0 keV). After cleaning for problematic fields, the resulting database includes over 27 000 images integrated in different X-ray bands, and a catalogue, called 1OUSXB, that provides intensity and spectral information for 33 396 X-ray sources, 8896 of which are single or multiple detections of 2308 distinct blazars. All the results can be accessed online in a variety of ways, from the Open Universe portal through Virtual Observatory services, via the VOU-Blazar tool and the SSDC SED builder. One of the most innovative aspects of this work is that the results can be easily reproduced and extended by anyone using the Docker version of the Swift-DeepSky pipeline, which runs on Linux, Mac, and Windows machines, and does not require any specific experience in X-ray data analysis.
We show that a blazar classification in BL Lacs and flat spectrum radio quasars (FSRQs) may not be adequate when it relies solely on the equivalent widths (EWs) of optical lines. In fact, depending ...on redshift, some strong emission lines can fall in the infrared window and be missed. We selected a sample of BL Lacs with firm redshift identification and good visibility from Paranal. We targeted with the X-shooter spectrograph the five BL Lacs with z > 0.7, i.e. those for which the Hα line, one of the strongest among blazars, falls outside the optical window and determined the EW of emission lines in both the infrared and optical bands. Two out of five sources show an observed Hα EW >5 Å (one has rest frame EW > 5 Å) and could be classified as FSRQs by one of the classification schemes used in the literature. A third object is border-line with an observed EW of 4.4 ± 0.5 Å. In all these cases, Hα is the strongest emission line detected. The Hα line of the other two blazars is not detected, but in one case it falls in a region strongly contaminated by sky lines and in the other one the spectrum is featureless. We conclude that a blazar classification based on EW width only can be inaccurate and may lead to an erroneous determination of blazar evolution. This effect is more severe for the BL Lac class, since FSRQs can be misclassified as BL Lacs especially at high redshifts (z > 0.7), where the latter are extremely rare.
Broad absorption lines indicate gas outflows with velocities from thousands of km ssup -1 to about 0.2 the speed of light, which may be present in all quasars and may play a major role in the ...evolution of the host galaxy. The variability of absorption patterns can provide information on changes in the density and velocity distributions of the absorbing gas and its ionisation status. We want to accurately follow the evolution in time of the luminosity and both the broad and narrow CIV absorption features of an individual object, the quasar APM 08279+5255, and analyse the correlations among these quantities. The broad absorption behaviour suggests changes in the ionisation status as the main cause of variability. We show for the first time a correlation of this variability with the R band flux. The different behaviour of the narrow absorption system might be due to recombination time delay.
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