arXiv subjects

Margo Aller

Publications and source records attributed to Margo Aller.

9 recordsLinked to original sources

arXiv2026

A Multiwavelength Study of a Long-Duration VHE Flare from BL Lacertae with VERITAS

We report the first observations of a long-duration very-high-energy (VHE; $E > 100$ GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On October 15, 2022, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observations from VERITAS, Swift, NuSTAR, and select optical and radio observatories. VERITAS observed the source for a total of $\sim 9.8$ hours between September 1, 2022 and December 1, 2022. An analysis of these data yields a $\sim 28 \sigma$ detection of the source. While previously observed VHE flares from BL Lacertae have lasted on time-scales of minutes to days, VERITAS continued to detect flaring activity from the source for over a month ($\sim 40$ days) after the original flaring activity was detected with Fermi-LAT. Broadband spectral modeling shows that a synchrotron self-Compton (SSC) model with an external inverse-Compton (EC) component is preferred over a one-zone SSC model.

astro-ph.HE↗

arXiv2019

Opacity, variability and kinematics of AGN jets

Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests itself as a time delay between flares observed at high and low radio frequencies. It is also responsible for the observing frequency dependent change in size and position of the apparent base of the jet, aka the core shift effect, detected with very long baseline interferometry (VLBI). We measure the time delays and the core shifts in 11 radio-loud AGN to estimate the speed of their jets without relying on multi-epoch VLBI kinematics analysis. The 15$-$8 GHz total flux density time lags are obtained using Gaussian process regression, the core shift values are measured using VLBI observations and adopted from the literature. A strong correlation is found between the apparent core shift and the observed time delay. Our estimate of the jet speed is higher than the apparent speed of the fastest VLBI components by the median coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to 20. We derive Doppler factors, Lorentz factors and viewing angles of the jets, as well as the corresponding de-projected distance from the jet base to the core. The results support evidence for acceleration of the jets with bulk motion Lorentz factor $Γ\propto R^{0.52\pm0.03}$ on de-projected scales $R$ of 0.5$-$500 parsecs.

astro-ph.GA↗

arXiv2012

Multiwavelength Spectral Study of 3C 279 in the Internal Shock Scenario

We have observed 3C~279 in a gamma-ray flaring state in November 2008. We construct quasi-simultaneous spectral energy distributions (SEDs) of the source for the flaring period of 2008 and during a quiescent period in May 2010. Data have been compiled from observations with Fermi, Swift, RXTE, the VLBA, and various ground-based optical and radio telescopes. The objective is to comprehend the correspondence between the flux and polarization variations observed during these two time periods by carrying out a detailed spectral analyses of 3C~279 in the internal shock scenario, and gain insights into the role of intrinsic parameters and interplay of synchrotron and inverse Compton radiation processes responsible for the two states. As a first step, we have used a multi-slice time-dependent leptonic jet model, in the framework of the internal shock scenario, with radiation feedback to simulate the SED of 3C~279 observed in an optical high state in early 2006. We have used physical jet parameters obtained from the VLBA monitoring to guide our modeling efforts. We briefly discuss the effects of intrinsic parameters and various radiation processes in producing the resultant SED.

astro-ph.HE↗

arXiv2011

Multiwavelength Spectral Studies Of Fermi-LAT Blazars

We present multiwavelength spectral analyses of two Fermi-LAT blazars, OJ 287 and 3C 279, that are part of the Boston University multiwaveband polarization program. The data have been compiled from observations with Fermi, RXTE, the VLBA, and various ground-based optical and radio telescopes. We simulate the dynamic spectral energy distributions (SEDs) within the framework of a multi-slice, time-dependent leptonic jet model for blazars, with radiation feedback, in the internal shock scenario. We use the physical jet parameters obtained from the VLBA monitoring to guide our modeling efforts. We discuss the role of intrinsic parameters and the interplay between synchrotron and inverse Compton radiation processes responsible for producing the resultant SEDs.

astro-ph.HE↗

arXiv2009

Simultaneous Multiwavelength and Optical Microvariability Observations of CTA 102 (PKS J2232+1143)

We present analysis of both the short-term optical and long-term multiwavelength variability of CTA 102. In 2004, this object was observed in an intense optical flaring state. Extensive R-band microvariability observations were carried out during this high state. In 2005, we obtained several weeks of contemporaneous radio, optical, and X-ray observations of CTA 102. These observations recorded distinct flaring activity in all three wavebands. Subsequent analysis revealed that this object may appear redder when in a brighter optical state, and that the X-ray, optical, and radio activity do not appear to be correlated. The shape of the observed spectral energy distributions suggests that both synchrotron-related and external inverse Compton processes may contribute to the X-ray emission. Our results are also compared with other results on this object and archival microvariability observations. It appears that more rapid, dramatic microvariability events occur when CTA 102 is in an elevated optical flux state.

astro-ph.CO↗

arXiv2007

New Multiwavelength Observations of PKS 2155-304 and Implications for the Coordinated Variability Patterns of Blazars

The TeV blazar PKS 2155--304 was the subject of an intensive 2 week optical and near-infrared observing campaign in 2004 August with the CTIO 0.9m telescope. During this time, simultaneous X-ray data from RXTE were also obtained. We compare the results of our observations to the results from two previous simultaneous multiwavelength campaigns on PKS 2155-304. We conclude that the correlation between the X-ray and UV/optical variability is strongest and the time lag is shortest (only a few hours) when the object is brightest. As the object becomes fainter, the correlations are weaker and the lags longer, increasing to a few days. Based on the results of four campaigns, we find evidence for a linear relationship between the mean optical brightness and lag time of X-ray and UV/optical events. Furthermore, we assert that this behavior, along with the different multiwavelength flare lag times across different flux states is consistent with a highly relativistic shock propagating down the jet producing the flares observed during a high state. In a quiescent state, the variability is likely to be due to a number of factors including both the jet and contributions outside of the jet, such as the accretion disk.

astro-ph↗

arXiv2006

Multiwavelength Observations of the Extreme X-Ray Selected BL Lac Object PG 1553+11 (1ES 1553+113)

PG 1553+11 was the target of a coordinated three week multiwavelength campaign during 2003 April and May. A significant X-ray flare was observed during the second half of this campaign. Although no optical flare was recorded during the X-ray campaign, optical observations obtained immediately prior to the campaign displayed a higher flux than that recorded during the campaign. An optical flare was observed a few days after the end of the X-ray campaign and may be related to the X-ray flare. Radio observations were made at three frequencies, with no significant changes in flux detected near the times of the optical and X-ray flares. The spectral energy distributions and flux ratios in different wavebands observed for this object are compared to other X-ray selected blazars to demonstrate how PG 1553+11 is an extreme member of this group.

astro-ph↗

arXiv2005

Intrinsic Curvature in the X-ray Spectra of BL Lacertae Objects

We report results from {\it XMM-Newton} observations of thirteen X-ray bright BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The survey was designed to look for evidence of departures of the X-ray spectra from a simple power law shape (i.e., curvature and/or line features), and to find objects worthy of deeper study. Our data are generally well fit by power-law models, with three cases having hard ($Γ<2; dN/dE \propto E^{-Γ}$) spectra that indicate synchrotron peaks at $E \gsim 5$ keV. Previous data had suggested a presence of absorption features in the X-ray spectra of some BL Lacs. In contrast, none of these spectra show convincing examples of line features, either in absorption or emission, suggesting that such features are rare amongst BL Lacs, or, more likely, artifacts caused by instrumental effects. We find significant evidence for intrinsic curvature (steepening by $dΓ/ d({\rm log} E) = 0.4 \pm 0.15$) in fourteen of the seventeen X-ray spectra. This cannot be explained satisfactorily via excess absorption, since the curvature is essentially constant from $0.5-6$ keV, an observation which is inconsistent with the modest amounts of absorption that would be required. We use the {\it XMM-Newton} Optical Monitor data with concurrent radio monitoring to derive broadband spectral energy distributions and peak frequency estimates. From these we examine models of synchrotron emission and model the spectral curvature we see as the result of episodic particle acceleration.

astro-ph↗

arXiv2004

BL Lac X-ray Spectra: simpler than we thought

We report results from {\it XMM-Newton} observations of thirteen X-ray bright BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The spectra are generally well fit by power-law models, with four objects having hard ($α<1; F_ν\propto ν^{-α}$) spectra that indicates synchrotron peaks at $>5$ keV. None of our spectra show line features, indicating that soft X-ray absorption ``notches'' must be rare amongst BL Lacs, rather than common or ubiquitous as had previously been asserted. We find significant curvature in most of the spectra. This curvature is almost certainly intrinsic, as it appears nearly constant from 0.5 to 6 keV, an observation which is inconsistent with the small columns seen in these sources.

astro-ph↗