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I. Agudo

Publications and source records attributed to I. Agudo.

At least 109 records · Page 6Linked to original sources
arXiv2018

AGILE, Fermi, Swift, and GASP-WEBT multi-wavelength observations of the high-redshift blazar 4C $+$71.07 in outburst

The flat-spectrum radio quasar 4C $+$71.07 is a high-redshift ($z=2.172$), $γ$-loud blazar whose optical emission is dominated by the thermal radiation from accretion disc. 4C $+$71.07 has been detected in outburst twice by the AGILE $γ$-ray satellite during the period end of October - mid November 2015, when it reached a $γ$-ray flux of the order of $F_{\rm E>100\,MeV} = (1.2 \pm 0.3)\times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$ and $F_{\rm E>100\,MeV} = (3.1 \pm 0.6)\times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$, respectively, allowing us to investigate the properties of the jet and of the emission region. We investigated its spectral energy distribution by means of almost simultaneous observations covering the cm, mm, near-infrared, optical, ultra-violet, X-ray and $γ$-ray energy bands obtained by the GASP-WEBT Consortium, the Swift and the AGILE and Fermi satellites. The spectral energy distribution of the second $γ$-ray flare (the one whose energy coverage is more dense) can be modelled by means of a one-zone leptonic model, yielding a total jet power of about $4\times10^{47}$ erg s$^{-1}$. During the most prominent $γ$-ray flaring period our model is consistent with a dissipation region within the broad-line region. Moreover, this class of high-redshift, large-mass black-hole flat-spectrum radio quasars might be good targets for future $γ$-ray satellites such as e-ASTROGAM.

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arXiv2018

Spatially resolved origin of mm-wave linear polarization in the nuclear region of 3C 84

We report results from a deep polarization imaging of the nearby radio galaxy 3C$\,$84 (NGC$\,$1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86$\,$GHz at an ultra-high angular resolution of $50μ$as (corresponding to 250$R_{s}$). We also add complementary multi-wavelength data from the Very Long Baseline Array (VLBA; 15 & 43$\,$GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0, and 343.5$\,$GHz). At 86$\,$GHz, we measure a fractional linear polarization of $\sim2$% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15$\,$GHz ($\sim0.3-0.7$% and $<0.1$%, respectively). This suggests an increasing linear polarization degree towards shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of $\sim10^{5-6}~{\rm rad/m^{2}}$ in the core at $\gtrsim$43$\,$GHz. Moreover, the VLBA 43$\,$GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution, and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.

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arXiv2018

Exploring the connection between parsec-scale jet activity and broadband outbursts in 3C 279

We use a combination of high-resolution very long baseline interferometry (VLBI) radio and multi-wavelength flux density and polarization observations to constrain the physics of the dissipation mechanism powering the broadband flares in 3C 279 during an episode of extreme flaring activity in 2013-2014. Six bright flares superimposed on a long-term outburst are detected at $γ$-ray energies. Four of the flares have optical and radio counterparts. The two modes of flaring activity (faster flares sitting on top of a long term outburst) present at radio, optical, $γ$-ray frequencies are missing in X-rays. X-ray counterparts are only observed for two flares. The first three flares are accompanied by ejection of a new VLBI component (NC2) suggesting the 43 GHz VLBI core as the site of energy dissipation. Another new component, NC3, is ejected after the last three flares, which suggests that the emission is produced upstream from the core (closer to the black hole). The study therefore indicates multiple sites of energy dissipation in the source. An anti-correlation is detected between the optical percentage polarization (PP) and optical/$γ$-ray flux variations, while the PP has a positive correlation with optical/$γ$-rays spectral indices. Given that the mean polarization is inversely proportional to the number of cells in the emission region, the PP vs. optical/$γ$-ray anti-correlation could be due to more active cells during the outburst than at other times. In addition to the turbulent component, our analysis suggests the presence of a combined turbulent and ordered magnetic field, with the ordered component transverse to the jet axis.

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arXiv2018

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare

Combined with very-long-baseline interferometry measurements, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL~Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL~Lacertae observed by VERITAS, with a rise time of $\sim$2.3~hr and a decay time of $\sim$36~min. The peak flux above 200 GeV is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photon} \;\text{m}^{-2}\; \text{s}^{-1}$ measured with a 4-minute-binned light curve, corresponding to $\sim$180\% of the flux which is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in VLBA observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models which invoke relativistic plasma passing stationary shocks.

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arXiv2018

Science with the Cherenkov Telescope Array

The Cherenkov Telescope Array, CTA, will be the major global observatory for very high energy gamma-ray astronomy over the next decade and beyond. The scientific potential of CTA is extremely broad: from understanding the role of relativistic cosmic particles to the search for dark matter. CTA is an explorer of the extreme universe, probing environments from the immediate neighbourhood of black holes to cosmic voids on the largest scales. Covering a huge range in photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of performance with respect to current instruments. The observatory will operate arrays on sites in both hemispheres to provide full sky coverage and will hence maximize the potential for the rarest phenomena such as very nearby supernovae, gamma-ray bursts or gravitational wave transients. With 99 telescopes on the southern site and 19 telescopes on the northern site, flexible operation will be possible, with sub-arrays available for specific tasks. CTA will have important synergies with many of the new generation of major astronomical and astroparticle observatories. Multi-wavelength and multi-messenger approaches combining CTA data with those from other instruments will lead to a deeper understanding of the broad-band non-thermal properties of target sources. The CTA Observatory will be operated as an open, proposal-driven observatory, with all data available on a public archive after a pre-defined proprietary period. Scientists from institutions worldwide have combined together to form the CTA Consortium. This Consortium has prepared a proposal for a Core Programme of highly motivated observations. The programme, encompassing approximately 40% of the available observing time over the first ten years of CTA operation, is made up of individual Key Science Projects (KSPs), which are presented in this document.

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arXiv2017

VLBA polarimetric monitoring of 3C 111

We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes. To do so, we analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. We additionally consider data from the IRAM 30-m Telescope as well as the SMA. Jet properties such as the electric vectors, the (polarized) flux density, feature size, and brightness temperature, describe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large and smooth rotation of $\gtrsim 180^{\circ}$ across a distance of about 20 pc. In contrast, the EVPAs are strongly variable within the first parsecs of the jet. We find a tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The transverse extent of the flow decreases coincident with a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradients of the feature size and particle density with distance steepen in that region. We interpret the propagating polarized features with shocks and the observed local anomalies with the interaction of these shocks with a recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region.

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arXiv2017

The MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey

The MIGHTEE large survey project will survey four of the most well-studied extragalactic deep fields, totalling 20 square degrees to $μ$Jy sensitivity at Giga-Hertz frequencies, as well as an ultra-deep image of a single ~1 square degree MeerKAT pointing. The observations will provide radio continuum, spectral line and polarisation information. As such, MIGHTEE, along with the excellent multi-wavelength data already available in these deep fields, will allow a range of science to be achieved. Specifically, MIGHTEE is designed to significantly enhance our understanding of, (i) the evolution of AGN and star-formation activity over cosmic time, as a function of stellar mass and environment, free of dust obscuration; (ii) the evolution of neutral hydrogen in the Universe and how this neutral gas eventually turns into stars after moving through the molecular phase, and how efficiently this can fuel AGN activity; (iii) the properties of cosmic magnetic fields and how they evolve in clusters, filaments and galaxies. MIGHTEE will reach similar depth to the planned SKA all-sky survey, and thus will provide a pilot to the cosmology experiments that will be carried out by the SKA over a much larger survey volume.

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arXiv2017

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV $γ$-ray flare

Observations of fast TeV $γ$-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV $γ$-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at $>$200 GeV measured with the 4-minute binned light curve is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photons} \;\text{m}^{-2}\, \text{s}^{-1}$, or $\sim$180% the Crab Nebula flux. Variability in GeV $γ$-ray, X-ray, and optical flux, as well as in optical and radio polarization was observed around the time of the TeV $γ$-ray flare. A possible superluminal knot was identified in the VLBA observations at 43 GHz. The flare constrains the size of the emitting region, and is consistent with several theoretical models with stationary shocks.

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arXiv2016

Polarization angle swings in blazars: The case of 3C 279

Over the past few years, several occasions of large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy gamma-ray flares and they have attracted considerable attention, as they could allow one to probe the magnetic field structure in the gamma-ray emitting region of the jet. The flat-spectrum radio quasar 3C279 is one of the most prominent examples showing this behaviour. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008-2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. 3C279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth approx. 360 degrees rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. We conclude that during the two different optical flux states, two different processes govern the polarization variation, possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.

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arXiv2016

Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT campaign and its extension

After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and gamma-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray emission regions. The relation between the gamma-ray and optical fluxes is consistent with the SSC mechanism, with a quadratic dependence of the SSC gamma-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the gamma-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in gamma-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.

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arXiv2015

A Variable-Density Absorption Event in NGC 3227 mapped with Suzaku and Swift

The morphology of the circumnuclear gas accreting onto supermassive black holes in Seyfert galaxies remains a topic of much debate. As the innermost regions of Active Galactic Nuclei (AGN) are spatially unresolved, X-ray spectroscopy, and in particular line-of-sight absorption variability, is a key diagnostic to map out the distribution of gas. Observations of variable X-ray absorption in multiple Seyferts and over a wide range of timescales indicate the presence of clumps/clouds of gas within the circumnuclear material. Eclipse events by clumps transiting the line of sight allow us to explore the properties of the clumps over a wide range of radial distances from the optical/UV Broad Line Region (BLR) to beyond the dust sublimation radius. Time-resolved absorption events have been extremely rare so far, but suggest a range of density profiles across Seyferts. We resolve a weeks-long absorption event in the Seyfert NGC 3227. We examine six Suzaku and twelve Swift observations from a 2008 campaign spanning 5 weeks. We use a model accounting for the complex spectral interplay of three differently-ionized absorbers. We perform time-resolved spectroscopy to discern the absorption variability behavior. We also examine the IR-to-X-ray spectral energy distribution (SED) to test for reddening by dust. The 2008 absorption event is due to moderately-ionized ($\log ξ\sim 1.2-1.4$) gas covering 90% of the line of sight. We resolve the density profile to be highly irregular, in contrast to a previous symmetric and centrally-peaked event mapped with RXTE in the same object. The UV data do not show significant reddening, suggesting that the cloud is dust-free. The 2008 campaign has revealed a transit by a filamentary, moderately-ionized cloud of variable density that is likely located in the BLR, and possibly part of a disk wind.

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arXiv2015

Optical and Radio Variability of BL Lacertae

We observed the prototype blazar, BL Lacertae, extensively in optical and radio bands during an active phase in the period 2010--2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multifrequency data to learn about the mechanisms producing variability. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multifrequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands. We performed optical observations in B, V, R and I bands from seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in Ukraine, Finland and USA. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical ones of ~250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical 'mini-flares' on short time-scales. Variations on longer term timescales are mildly chromatic with superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source.

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arXiv2015

The Spanish Square Kilometre Array White Book

The Square Kilometre Array (SKA) is called to revolutionise essentially all areas of Astrophysics. With a collecting area of about a square kilometre, the SKA will be a transformational instrument, and its scientific potential will go beyond the interests of astronomers. Its technological challenges and huge cost requires a multinational effort, and Europe has recognised this by putting the SKA on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI). The Spanish SKA White Book is the result of the coordinated effort of 120 astronomers from 40 different research centers. The book shows the enormous scientific interest of the Spanish astronomical community in the SKA and warrants an optimum scientific exploitation of the SKA by Spanish researchers, if Spain enters the SKA project.

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arXiv2015

The connection between radio and high energy emission in black hole powered systems in the SKA era

Strong evidence exists for a highly significant correlation between the radio flux density and gamma-ray energy flux in blazars revealed by Fermi. However, there are central issues that need to be clarified in this field: what are the counterparts of the about 30% of gamma-ray sources that are as yet unidentified? Are they just blazars in disguise or they are something more exotic, possibly associated with dark matter? How would they fit in the radio-gamma ray connection studied so far? With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all of these questions. Even more, while the radio-MeV/GeV connection has been firmly established, a radio-VHE connection has been entirely elusive so far. The advent of CTA in the next few years and the expected CTA-SKA1 synergy will offer the chance to explore this connection, even more intriguing as it involves the opposite ends of the electromagnetic spectrum and the acceleration of particles up to the highest energies. We are already preparing to address these questions by exploiting data from the various SKA pathfinders and precursors. We have obtained 18 cm European VLBI Network observations of E>10 GeV sources, with a detection rate of 83%. Moreover, we are cross correlating the Fermi catalogs with the MWA commissioning survey: when faint gamma-ray sources are considered, pure positional coincidence is not significant enough for selecting counterparts and we need an additional physical criterion to pinpoint the right object. It can be radio spectral index, variability, polarization, or compactness, needing high angular resolution in SKA1-MID; timing studies can also reveal pulsars, which are often found from dedicated searches of unidentified gamma-ray sources. SKA will be the ideal instrument for investigating these characteristics in conjunction with CTA. (abridged)

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arXiv2014

Future mmVLBI Research with ALMA: A European vision

Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants

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arXiv2014

The Outburst of the Blazar S40954+658 in March-April 2011

We present the results of optical (R band) photometric and polarimetric monitoring and Very Long Baseline Array (VLBA) imaging of the blazar S4 0954+658, along with Fermi and gamma;-ray data during a multi-waveband outburst in 2011 March-April. After a faint state with a brightness level R ~17.6 mag registered in the first half of January 2011, the optical brightness of the source started to rise and reached ~14.8 mag during the middle of March, showing flare-like behavior. The most spectacular case of intranight variability was observed during the night of 2011 March 9, when the blazar brightened by ~0.7 mag within ~7 hours. During the rise of the flux the position angle of optical polarization rotated smoothly over more than 300$°$. At the same time, within 1$σ$ uncertainty a new superluminal knot appeared with an apparent speed of 19.0$\pm$0.3 c. We have very strong evidence for association of this knot with the multi-waveband outburst in 2011 March-April. We also analyze the multi-frequency behavior of S4 0954+658 during a number of minor outbursts from August 2008 to April 2012. We find some evidence of connections between at least two more superluminal ejecta and near-simultaneous optical flares.

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