Galaxies

10 pages, 1547 KiB

Open AccessReview

A Search for Blazar-Like Radio-Loud Narrow-Line Seyfert 1 Galaxies

by Hugh R. Miller, Jeremy D. Maune, Joseph R. Eggen, Clay S. Turner, Elizabeth C. Ferrara, Diana O. Gudkova and Amy M. Battles

Galaxies 2017, 5(1), 20; https://doi.org/10.3390/galaxies5010020 - 21 Mar 2017

Cited by 6 | Viewed by 3984

Abstract

We report the results of an observational program to investigate the gamma-ray and optical variability properties of the vRL NLSY1 galaxies listed in the Yuan et al. sample. We have identified 17 members of the Yuan et al. sample possibly associated with gamma-ray [...] Read more.

We report the results of an observational program to investigate the gamma-ray and optical variability properties of the vRL NLSY1 galaxies listed in the Yuan et al. sample. We have identified 17 members of the Yuan et al. sample possibly associated with gamma-ray sources based on a combination of their optical polarization and optical variability and their gamma-ray properties. Eight have previously been associated with gamma-ray sources. We find nine additional members that we predict are excellent candidates to be associated with gamma-ray sources in the future. All 17 sources have many properties in common with flat spectrum radio quasars (FSRQs), suggesting that they may, in fact, constitute a new subclass of FSRQs. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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7 pages, 1528 KiB

Open AccessArticle

Gaussian Processes for Blazar Variability Studies

by Vassilis Karamanavis

Galaxies 2017, 5(1), 19; https://doi.org/10.3390/galaxies5010019 - 17 Mar 2017

Cited by 4 | Viewed by 4425

Abstract

This article briefly introduces Gaussian processes as a new approach for modelling time series in the field of blazar physics. In the second part of the paper, recent results from an application of GP modelling to the multi-wavelength light curves of the blazar [...] Read more.

This article briefly introduces Gaussian processes as a new approach for modelling time series in the field of blazar physics. In the second part of the paper, recent results from an application of GP modelling to the multi-wavelength light curves of the blazar PKS 1502+106 are discussed. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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Graphical abstract

8 pages, 2713 KiB

Open AccessArticle

Long-Term Monitoring of Bright Blazars in the Multi-GeV to TeV Range with FACT

by Fabian Temme, Jan Adam, Max L. Ahnen, Dominik Baack, Matteo Balbo, Matthias Bergmann, Adrian Biland, Michael Blank, Thomas Bretz, Kai A. Brügge, Jens Buss, Anton Dmytriiev, Daniela Dorner, Sabrina Einecke, Christina Hempfling, Dorothee Hildebrand, Gareth Hughes, Lena Linhoff, Karl Mannheim, Sebastian Müller, Dominik Neise, Andrii Neronov, Max Nöthe, Aleksander Paravac, Felicitas Pauss, Wolfgang Rhode, Amit Shukla, Julia Thaele and Roland Walter Show full author list Hide full author list

Galaxies 2017, 5(1), 18; https://doi.org/10.3390/galaxies5010018 - 20 Feb 2017

Cited by 4 | Viewed by 6745

Abstract

Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, [...] Read more.

Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, with changing activity levels on timescales between minutes, months, and even years. Several questions are part of the current research, such as the question of the emission regions or the engine of the AGN and the particle acceleration. A dedicated longterm monitoring program is necessary to investigate the properties of blazars in detail. A densely sampled and unbiased light curve allows for observation of both high and low states of the sources, and the combination with multi-wavelength observation could contribute to the answer of several questions mentioned above. FACT (First G-APD Cherenkov Telescope) is the first operational telescope using silicon photomultiplier (SiPM, also known as Geigermode—Avalanche Photo Diode, G-APD) as photon detectors. SiPM have a very homogenous and stable longterm performance, and allow operation even during full moon without any filter, leading to a maximal duty cycle for an Imaging Air Cherenkov Telescope (IACT). Hence, FACT is an ideal device for such a longterm monitoring of bright blazars. A small set of sources (e.g., Markarian 421, Markarian 501, 1ES 1959+650, and 1ES 2344+51.4) is currently being monitored. In this contribution, the FACT telescope and the concept of longterm monitoring of bright blazars will be introduced. The results of the monitoring program will be shown, and the advantages of densely sampled and unbiased light curves will be discussed. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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46 pages, 2624 KiB

Open AccessArticle

Small Scale Problems of the ΛCDM Model: A Short Review

by Antonino Del Popolo and Morgan Le Delliou

Galaxies 2017, 5(1), 17; https://doi.org/10.3390/galaxies5010017 - 20 Feb 2017

Cited by 223 | Viewed by 14952

Abstract

The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems [...] Read more.

The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. ΛCDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the ΛDCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the ΛCDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe. Full article

(This article belongs to the Special Issue Dark Matter: Large versus Small Scale Structures)

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9 pages, 3289 KiB

Open AccessArticle

Extended Radio Emission in the Perhipheral Regions of the Shapley Concentration Core

by Tiziana Venturi, Sandro Bardelli, Daniele Dallacasa, Gabriella Di Gennaro, Fabio Gastaldello, Simona Giacintucci and Mariachiara Rossetti

Galaxies 2017, 5(1), 16; https://doi.org/10.3390/galaxies5010016 - 14 Feb 2017

Cited by 4 | Viewed by 5095

Abstract

The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the [...] Read more.

The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the very early stages of merger, fairly massive clusters with ongoing accretion activity, and smaller groups located in filaments. These features make the SC an ideal place to observe the signatures of the formation of large-scale structures in the Universe. As a matter of fact, the SC has been observed over a broad range of frequencies with the most important observatories, allowing for a unique multiband study. In this paper, we will present new results from an ongoing study of the Shapley Concentration Core, which is being carried out with the Giant Metrewave Radio Telescope (GMRT). Our work confirms the role played by radio observations in disentangling the details of the accretion and merging processes, and delivers a wealth of information in regions well outside the cluster cores. In particular, we will report on the discovery of a relic in the region between the two clusters A 3558 and A 3562, and of the radio properties of the brightest galaxy in the peripheral cluster A 3556. Full article

(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)

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11 pages, 1769 KiB

Open AccessArticle

Black Hole Accretion in Gamma Ray Bursts

by Agnieszka Janiuk, Michal Bejger, Petra Sukova and Szymon Charzynski

Galaxies 2017, 5(1), 15; https://doi.org/10.3390/galaxies5010015 - 13 Feb 2017

Cited by 6 | Viewed by 5194

Abstract

We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of [...] Read more.

We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on 14 September 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 s and appeared about 0.4 s after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnant’s matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts: Recent Theoretical Models and Observations)

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6 pages, 474 KiB

Open AccessArticle

The Statistical Study of the Daily Fermi Light Curves of 130 Sources

by Wei Cai, Yi Liu and Jun H. Fan

Galaxies 2017, 5(1), 14; https://doi.org/10.3390/galaxies5010014 - 11 Feb 2017

Cited by 1 | Viewed by 4440

Abstract

Blazars show rapid and high amplitude variability. It is interesting to question what kind of process the variability corresponds to. Maybe it is a result of the instability of the accretion flows. In this work, Fermi daily light curves of 130 sources are [...] Read more.

Blazars show rapid and high amplitude variability. It is interesting to question what kind of process the variability corresponds to. Maybe it is a result of the instability of the accretion flows. In this work, Fermi daily light curves of 130 sources are analyzed, and the distributions of daily variability are compared by using a Kolmogorov–Smirnov (K-S) test. The results can be summarized as follows:(1) in most cases, the distributions are not Gaussian; (2) some pairs of the distributions are similar. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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6 pages, 898 KiB

Open AccessArticle

Study of the Time-Series of Microvariability in Kepler Blazar W2R 1926+42

by Mahito Sasada, Shin Mineshige, Shinya Yamada and Hitoshi Negoro

Galaxies 2017, 5(1), 13; https://doi.org/10.3390/galaxies5010013 - 8 Feb 2017

Viewed by 3307

Abstract

One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than [...] Read more.

One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than a day is known as “microvariability.” Such short-term variations can provide insights regarding the origin of the variability after they are distinguished from longer-term variational components. We select about 195 microvariability events from the continuous light curve of blazar W2R 1926+42 with 1-min time resolution obtained by the Kepler spacecraft, and estimate the timescale and amplitude of each event. The rise and decay timescales of the events reveal random variations over short timescales less than a day, but they indicate systematic variations on timescales longer than several days. This result implies that the events are not independent, but rather mutually correlated. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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8 pages, 1910 KiB

Open AccessArticle

OJ 287 as a Rotating Helix

by Marshall H. Cohen

Galaxies 2017, 5(1), 12; https://doi.org/10.3390/galaxies5010012 - 8 Feb 2017

Cited by 28 | Viewed by 5848

Abstract

We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of∼30 years. The EVPA of the core rotated by 240° in 2001–2002 and decreased slowly [...] Read more.

We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of∼30 years. The EVPA of the core rotated by 240° in 2001–2002 and decreased slowly after that. The inner jet apparently moved to a new direction after the rotation,as shown by the emergence of a new component at a new PA at 43 GHz, in 2004. This was presaged by a strong rise in the ﬂux density of the core, and then its sudden fall as the new component was identiﬁed. The equivalent sequence of events took place about 5 years later at 15 GHz, but in addition the core EVPA had a step in 2006 and moved to be aligned with the new 43-GHz component. The 15-GHz core became optically thin in 2006, but the angular resolution was insufﬁcient to separate the new component from the core until 2010 Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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18 pages, 466 KiB

Open AccessArticle

Area Products for H^± in AdS Space

by Parthapratim Pradhan

Galaxies 2017, 5(1), 10; https://doi.org/10.3390/galaxies5010010 - 3 Feb 2017

Cited by 1 | Viewed by 3524

Abstract

We derive the thermodynamic products, in particular the area (or entropy) products of

H^{\pm}

for a wide variety of black holes (BHs) in anti-de Sitter (AdS) space. We show by explicit and exact calculations that, for this class of BHs, more complicated [...] Read more.

We derive the thermodynamic products, in particular the area (or entropy) products of

H^{\pm}

for a wide variety of black holes (BHs) in anti-de Sitter (AdS) space. We show by explicit and exact calculations that, for this class of BHs, more complicated functions of the event horizon area and Cauchy horizon area are indeed mass-independent. This mass-independent results indicate that they could turn out to be a “universal” quantity provided that they depend only on the quantized angular momentum, quantized charges, and cosmological constant, etc. Furthermore, these area (or entropy) product relations for several classes of BHs in AdS space gives us strong indication to understanding the nature of non-extremal BH entropy (both inner and outer) at the microscopic level. Moreover, we compute the famous Cosmic Censorship Inequality (which requires Cosmic-Censorship hypothesis) for these classes of BHs in AdS space. Local thermodynamic stability has been discussed for these BHs and under certain conditions, these classes of BHs displayed second order phase transition. The super-entropic BH does not provide any kind of second order phase transition. Full article

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10 pages, 1834 KiB

Open AccessArticle

The Origin and Structure of the Magnetic Fields and Currents of AGN Jets

by Denise Gabuzda

Galaxies 2017, 5(1), 11; https://doi.org/10.3390/galaxies5010011 - 2 Feb 2017

Cited by 8 | Viewed by 6481

Abstract

This paper reviews observational evidence obtained to date about the overall structure of the magnetic ﬁelds in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-ﬁeld component, Faraday rotation observations of AGN jets provide an effective tool [...] Read more.

This paper reviews observational evidence obtained to date about the overall structure of the magnetic ﬁelds in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-ﬁeld component, Faraday rotation observations of AGN jets provide an effective tool for searching for toroidal jet magnetic ﬁelds, whose line-of-sight component changes systematically across the jet. Transverse Faraday rotation measure (RM) gradients providing direct evidence for helical/toroidal magnetic ﬁelds have been reliably detected in nearly 40 AGN on parsec scales. Helical magnetic ﬁelds are believed to form due to the combined action of the rotation of the central black hole and accretion disk, and these observations demonstrate that at least some of this helical ﬁeld survives to distances well beyond the Very Long Baseline Interferometry (VLBI) core. Observations of reversals in the direction of the transverse RM gradients in a number of AGN provide evidence for a“return”magnetic ﬁeld forming a nested helical-ﬁeld structure with oppositely directed azimuthal components in the inner and outer regions of the helical magnetic ﬁeld. The collected data now provide ﬁrm evidence for a predominance of inward jet currents on parsec scales and outward currents on scales greater than a few tens of parsecs. This suggests a global pattern of magnetic ﬁelds and currents with an inward current near the jet axis and an outward current farther from the jet axis, with these currents closing in the accretion disk and far out in the radio lobes, forming a self-consistent set of ﬁelds and currents together with the implied nested helical-ﬁeld structure. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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9 pages, 2675 KiB

Open AccessArticle

The Power of (Near) Simultaneous Multi-Frequency Observations for mm-VLBI and Astrometry

by María J. Rioja, Richard Dodson, José L. Gómez, Sol N. Molina, Taehyun Jung and Bong Won Sohn

Galaxies 2017, 5(1), 9; https://doi.org/10.3390/galaxies5010009 - 24 Jan 2017

Cited by 7 | Viewed by 4227

Abstract

Simultaneous or near-simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the [...] Read more.

Simultaneous or near-simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the signals can be coherently combined, and preventing the use of phase referencing and astrometric measurements. We present two demonstrations of the power of (near) simultaneous multi-frequency observations with the KVN and VLBA, and our recently developed analysis strategies to enable new measurements at mm-VLBI. The first case comprises simultaneous observations at 22, 43, 87 and 130 GHz of a group of five AGNs, the weakest of which is ∼200 mJy at 130 GHz, with angular separations ranging from 3.6 to 11 degrees, using the KVN. We analysed this data using the Frequency Phase Transfer (FPT) and the Source Frequency Phase Referencing (SFPR) techniques, which use the observations at a lower frequency to correct those at a higher frequency. The results of the analysis provide an empirical demonstration of the increase in the coherence times at 130 GHz from a few tens of seconds to about twenty minutes, with FPT, and up to many hours with SFPR. Moreover the astrometric analysis provides high precision relative position measurements between two frequencies, including, for the first time, astrometry at 130 GHz. The second case is a variation of the above, whereby adding dedicated wide-band cm-wavelength observations to measure the ionosphere eliminates the need for a second, calibrator, source. This addresses the scarcity of calibrators at mm-VLBI. We dubbed this technique Multi Frequency Phase Referencing (MFPR). We present bona fide astrometrically aligned VLBA images of BL Lacertae at 22 and 43 GHz using MFPR, which, combined with results from conventional phase referencing at cm-wavelengths, suggests the VLBI core has a recollimation shock that is revealed at mm-wavelengths. These shocks could be responsible for the γ-ray emission in blazar jets. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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9 pages, 3063 KiB

Open AccessArticle

Proper Motions of Jets on the Kiloparsec Scale: New Results with HST

by Eileen T. Meyer, William B. Sparks, Markos Georganopoulos, Roeland Van der Marel, Jay Anderson, Sangmo T. Sohn, John Biretta, Colin Norman, Marco Chiaberge and Eric Perlman

Galaxies 2017, 5(1), 8; https://doi.org/10.3390/galaxies5010008 - 20 Jan 2017

Cited by 19 | Viewed by 4437

Abstract

The Hubble Space Telescope recently celebrated 25 years of operation. Some of the first images of extragalactic optical jets were taken by HST in the mid-1990s; with time baselines on the order of 20 years and state-of-the-art astrometry techniques, we are now able [...] Read more.

The Hubble Space Telescope recently celebrated 25 years of operation. Some of the first images of extragalactic optical jets were taken by HST in the mid-1990s; with time baselines on the order of 20 years and state-of-the-art astrometry techniques, we are now able to reach accuracies in proper-motion measurements on the order of a tenth of a milliarcsecond per year. We present the results of a recent HST program to measure the kiloparsec-scale proper motions of eleven nearby optical jets with Hubble, the first sample of its kind. When paired with VLBI proper-motion measurements on the parsec scale, we are now able to map the full velocity profile of these jets from near the black hole to the final deceleration as they extend out into and beyond the host galaxy. We see convincing evidence that weak-flavor jets (i.e., FR Is) have a slowly increasing jet speed up to 100 pc from the core, where superluminal components are first seen. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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4 pages, 885 KiB

Open AccessArticle

Optical and Gamma-Ray Variability of the vRL NLSy1 Galaxy, 1H 0323+342

by Hugh R. Miller, Clay. S. Turner, Jeremy D. Maune and Joseph R. Eggen

Galaxies 2017, 5(1), 7; https://doi.org/10.3390/galaxies5010007 - 20 Jan 2017

Cited by 3 | Viewed by 3626

Abstract

1H 0323+342 was one of the first vRLNLSy1 galaxies detected at gamma-rays with the Fermi-LAT and is one of the brightest of this class observed at optical wavelengths. We report the results of monitoring the optical flux, polarization and the gamma-ray flux of [...] Read more.

1H 0323+342 was one of the first vRLNLSy1 galaxies detected at gamma-rays with the Fermi-LAT and is one of the brightest of this class observed at optical wavelengths. We report the results of monitoring the optical flux, polarization and the gamma-ray flux of 1H 0323+342 during the past ~5 years. In some cases, the optical flux has been monitored on timescales as short as ~minutes simultaneously with two telescopes, demonstrating, for the first time, the reality of microvariability events with durations as short as ~15 min for this object. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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7 pages, 432 KiB

Open AccessArticle

Challenging the Forward Shock Model with the 80 Ms Follow up of the X-ray Afterglow of Gamma-Ray Burst 130427A

by Massimiliano De Pasquale, Mathew Page, David Alexander Kann, Samantha R. Oates, Steve Schulze, Bing Zhang, Zach Cano, Bruce Gendre, Daniele Malesani, Andrea Rossi, Neil Gehrels, Eleonora Troja, Luigi Piro, Michel Boër and Giulia Stratta

Galaxies 2017, 5(1), 6; https://doi.org/10.3390/galaxies5010006 - 16 Jan 2017

Cited by 3 | Viewed by 4597

Abstract

GRB 130427A was the most luminous gamma-ray burst detected in the last 30 years. With an isotropic energy output of

8.5 \times 10^{53}

erg and redshift of 0.34, it combined very high energetics with a relative proximity to Earth in an unprecedented [...] Read more.

GRB 130427A was the most luminous gamma-ray burst detected in the last 30 years. With an isotropic energy output of

8.5 \times 10^{53}

erg and redshift of 0.34, it combined very high energetics with a relative proximity to Earth in an unprecedented way. Sensitive X-ray observatories such as XMM-Newton and Chandra have detected the afterglow of this event for a record-breaking baseline longer than 80 million seconds. The light curve displays a simple power-law over more than three decades in time. In this presentation, we explore the consequences of this result for a few models put forward so far to interpret GRB 130427A, and more in general the implication of this outcome in the context of the standard forward shock model. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts: Recent Theoretical Models and Observations)

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2 pages, 162 KiB

Open AccessEditorial

Acknowledgement to Reviewers of Galaxies in 2016

by Galaxies Editorial Office

Galaxies 2017, 5(1), 5; https://doi.org/10.3390/galaxies5010005 - 10 Jan 2017

Viewed by 2555

Abstract

The editors of Galaxies would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2016.[...] Full article

8 pages, 2000 KiB

Open AccessArticle

Exploring the Behaviour of Long Gamma-Ray Bursts with Intrinsic Afterglow Correlations: log L_200s−α_>200s

by Samantha R. Oates, Judith L. Racusin, Massimilliano De Pasquale, Daniel Kocevski, Mathew J. Page, Alberto J. Castro-Tirado, Javier Gorosabel, Philip J. Smith, Alice A. Breeveld and Paul M. Kuin

Galaxies 2017, 5(1), 4; https://doi.org/10.3390/galaxies5010004 - 10 Jan 2017

Cited by 2 | Viewed by 3684

Abstract

In these proceedings, we summarise the exploration so far of the relationship between the afterglow luminosity (measured at rest frame 200s;

\log L_{200 s}

) and average afterglow decay rate (measured from rest frame 200s onwards,

α_{> 200 s}

) of [...] Read more.

In these proceedings, we summarise the exploration so far of the relationship between the afterglow luminosity (measured at rest frame 200s;

\log L_{200 s}

) and average afterglow decay rate (measured from rest frame 200s onwards,

α_{> 200 s}

) of long duration Gamma-ray Bursts (GRBs), first reported in the optical/UV light curves of GRB afterglows. We show that this correlation is also present in the X-ray afterglows of GRBs as observed by Swift-XRT. We explore how the parameters of the correlation observed in both the X-ray and optical/UV light curves relate to each other and the prompt emission phase and whether these correlations are consistent with predictions of the standard afterglow model. We find that the observed correlations are consistent with a common underlying physical mechanism producing GRBs and their afterglows regardless of the detailed temporal behaviour. However, a basic afterglow model has difficulty explaining all the observed correlations. This leads us to briefly discuss alternative more complex models. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts: Recent Theoretical Models and Observations)

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6 pages, 234 KiB

Open AccessArticle

Multifrequency Study of the Blazar 3C 454.3

by Ilhuiyolitzin Villicaña-Pedraza, Francisco Carreto-Parra, Alberto Carramiñana and Julio Saucedo-Morales

Galaxies 2017, 5(1), 3; https://doi.org/10.3390/galaxies5010003 - 30 Dec 2016

Cited by 2 | Viewed by 4562

Abstract

This work is devoted to multi-frequency studying of the blazar 3C 454.3. The study includes spectroscopic and photometric observations in the optical, IR, and gamma-ray bands. We investigate whether a correlation exists in the light curves at different wavelengths. We have carried out [...] Read more.

This work is devoted to multi-frequency studying of the blazar 3C 454.3. The study includes spectroscopic and photometric observations in the optical, IR, and gamma-ray bands. We investigate whether a correlation exists in the light curves at different wavelengths. We have carried out observations of the optical spectrum (from 4000 to 7000 Angstroms) between 2007 and 2009, and identified MgII [2800 Angstroms] and FeII emission line features. We have obtained optical magnitudes and color indices of the quasar and performed a correlation between the optical, IR, and gamma-ray light curves. We have found statistically significant correlations between the light curves at different wavelengths. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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7 pages, 1402 KiB

Open AccessArticle

A New Statistical Approach to the Optical Spectral Variability in Blazars

by Jose A. Acosta-Pulido, Noel Castro Segura, Maria Isabel Carnerero and Claudia M. Raiteri

Galaxies 2017, 5(1), 1; https://doi.org/10.3390/galaxies5010001 - 28 Dec 2016

Cited by 5 | Viewed by 4396

Abstract

We present a spectral variability study of a sample of about 25 bright blazars, based on optical spectroscopy. Observations cover the period from the end of 2008 to mid 2015, with an approximately monthly cadence. Emission lines have been identified and measured in [...] Read more.

We present a spectral variability study of a sample of about 25 bright blazars, based on optical spectroscopy. Observations cover the period from the end of 2008 to mid 2015, with an approximately monthly cadence. Emission lines have been identified and measured in the spectra, which permits us to classify the sources into BL Lac-type or FSRQs, according to the commonly used EW limit. We have obtained synthetic photometry and produced colour-magnitude diagrams which show different trends associated with the object classes: generally, BL Lacs tend to become bluer when brighter and FSRQs become redder when brighter, although several objects exhibit both trends, depending on brightness. We have also applied a pattern recognition algorithm to obtain the minimum number of physical components which can explain the variability of the optical spectrum. We have used NMF (Non-Negative Matrix Factorization) instead of PCA (Principal Component Analysis) to avoid un-realistic negative components. For most targets we found that 2 or 3 meta-components are enough to explain the observed spectral variability. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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8 pages, 822 KiB

Open AccessArticle

The Structure and Propagation of the Misaligned Jet M87

by Kazuhiro Hada

Galaxies 2017, 5(1), 2; https://doi.org/10.3390/galaxies5010002 - 26 Dec 2016

Cited by 28 | Viewed by 7259

Abstract

Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI [...] Read more.

Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI imaging and monitoring at lower frequencies play an important role in complementing the high-frequency data. Here, we present our recent and ongoing observational studies of the M87 jet on pc-to-subpc scales based on ultra-deep VLBI imaging programs at 86 GHz and 15 GHz. The high-dynamic-range images have allowed us to obtain some remarkably improved views on this jet. We also introduce the KVN and VERA Array (KaVA), a new regularly-operating VLBI network in East Asia, which is quite suitable for studying the structure and propagation of relativistic jets. Some early results from our pilot study for M87—including the detection of superluminal motions near the jet base—implying an efficient magnetic-to-kinetic conversion at these scales, are reported. Full article

(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

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Galaxies, Volume 5, Issue 1 (March 2017) – 20 articles

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