Observations of a Correlated Gamma-Ray and Optical Flare for BL Lacertae

Bloom, S. D.; Bertsch, D. L.; Hartman, R. C.; Sreekumar, P.; Thompson, D. J.; Balonek, T. J.; Beckerman, Eli; Davis, Scott; Whitman, K.; Miller, H. R.; Nair, A. D.; Roberts, Lewis C.; Tosti, G.; Massaro, E.; Nesci, R.; Maesano, M.; Montagni, F.; Jang, Miran; Bock, Heike; Dietrich, M.; Herter, M.; Otterbein, K.; Pfeiffer, M.; Seitz, T.; Wagner, S. J.

doi:10.1086/311035

Cited by 81 publications

(57 citation statements)

References 14 publications

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“…In the SED, the synchrotron peak of the low-energy component falls in the near-IR band, and its inverse-Compton reaches a maximum , together with simultaneous optical-to-radio data from the GASP-WEBT collaboration, and contemporaneous γ-ray data from Fermi (from Abdo et al 2010a). The 1997 SED includes EGRET data from Bloom et al (1997), OSSE data from the HEASARC archive, ASCA+RXTE spectra from Tanihata et al (2000), while lowfrequency data are from the WEBT archive. Solid lines represent model fits; we distinguish the low-energy emission component (dotted lines) from the high-energy one (dashed lines); the contribution by an accretion disc of ∼16 000 K and 5 × 10 44 erg s −1 is marked with a dotted-dashed line.…”

Section: Discussionmentioning

confidence: 99%

“…The data from the EGRET instrument onboard CGRO in Fig. 9 were taken from Bloom et al (1997), while those from the OSSE detector were derived from the high energy astrophysics science archive research center 9 (HEASARC). The low-frequency information is from the WEBT archive; the range of optical flux variation in the period is indicated.…”

Section: Modelling the Sedmentioning

confidence: 99%

“…The multiwavelength data available for the Raiteri et al (2009) analysis lacked simultaneous information in the γ-ray band, so that the inverse-Compton spectral region was poorly constrained. But in 2008 the Fermi satellite was able to detect BL Lacertae (Abdo et al 2010a), even if in a low state compared to the past detections by the compton gamma ray observatory 2 (CGRO, Hartman et al 1999;Bloom et al 1997). In the same period, observations in the UV and X-ray bands were performed by Swift, while in the optical, near-IR, mm and cm radio bands the source was monitored by the GLAST-AGILE Support Program (GASP) of the WEBT.…”

Section: Introductionmentioning

confidence: 96%

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Another look at the BL Lacertae flux and spectral variability

Raiteri

Villata

Bruschini

et al. 2010

A&A

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Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with related inverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range. Methods. The GLAST-AGILE Support Program (GASP) of the whole earth blazar telescope (WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability.Results. The GASP-WEBT observations show an optical flare in 2008 February-March, and oscillations of several tenths of mag on a few-day time scale afterwards. The radio flux is only mildly variable. The UV data from both XMM-Newton and Swift seem to confirm a UV excess that is likely caused by thermal emission from the accretion disc. The X-ray data from XMM-Newton indicate a strongly concave spectrum, as well as moderate (∼4-7%) flux variability on an hour time scale. The Swift X-ray data reveal fast (interday) flux changes, not correlated with those observed at lower energies. We compare the spectral energy distribution (SED) corresponding to the 2008 low-brightness state, which was characterised by a synchrotron dominance, to the 1997 outburst state, where the inverse-Compton emission was prevailing. A fit with an inhomogeneous helical jet model suggests that two synchrotron components are at work with their self inverse-Compton emission. Most likely, they represent the radiation from two distinct emitting regions in the jet. We show that the difference between the source SEDs in 2008 and 1997 can be explained in terms of pure geometrical variations. The outburst state occurred when the jet-emitting regions were better aligned with the line of sight, producing an increase of the Doppler beaming factor. Conclusions. Our analysis demonstrates that the jet geometry can play an extremely important role in the BL Lacertae flux and spectral variability. Indeed, the emitting jet is probably a bent and dynamic structure, and hence changes in the emitting regions viewing angles are likely to happen, with strong consequences on the source multiwavelength behaviour.

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Section: Discussionmentioning

confidence: 99%

Section: Modelling the Sedmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

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Another look at the BL Lacertae flux and spectral variability

Raiteri

Villata

Bruschini

et al. 2010

A&A

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“…A K-S test gives p = 0.004 for the distributions to come from the same population, which indicates that our TeV and non-TeV sources have different magnitude distributions, which is not surprising considering that the non-TeV sources reside at higher cosmological distances. Because the TeV sources are much brighter in the optical than the non-TeV sources, they may also be brighter in the gamma-ray bands as the fluxes of BL Lac objects in these two regimes are correlated (e.g., Bloom et al 1997;Hovatta et al 2014;Wierzcholska et al 2015). This may in part also explain why some objects in our non-TeV sample are not detected at TeV energies with the current instruments.…”

Section: Host Galaxy Contributionmentioning

confidence: 91%

Optical polarization of high-energy BL Lacertae objects

Hovatta

Lindfors

Blinov

et al. 2016

A&A

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Context. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. Aims. We wish to understand if there are differences in the BL Lac objects that are detected with the current-generation TeV instruments compared to those that have not yet been detected. Methods. We use a maximum likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculate the time derivative of the electric vector position angle (EVPA) change. We also study the spread in the Stokes Q/I − U/I-plane and rotations in the polarization plane. Results. The mean polarization fraction of the TeV-detected BL Lacs is 5% while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q/I − U/I-plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seems to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and nonTeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.

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“…BL Lac was first detected at energies >100 MeV by EGRET in 1995 (Catanese et al 1997). In July 1997 during an optical flare of BL Lac, it was again significantly detected by EGRET (Bloom et al 1997;Böttcher & Bloom 2000) and by X-ray telescopes (Madejski et al 1999;Grove & Johnson 1997;Ravasio et al 2003). During this flare the spectrum of BL Lac was significantly harder than the time-averaged spectrum of 3EG J2202+4217 (Hartman et al 1999).…”

Section: A4 Bl Lacertaementioning

confidence: 99%

Gamma-ray absorption method (GRAM) application to the identification of EGRET unidentified sources

Iyudin

Burwitz

Greiner

et al. 2007

A&A

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Recently we reported the very first detection of γ-ray resonant absorption along the line of sight toward γ-ray bright quasars (QSOs) like 3C 273, 3C 279, PKS 0528+0134, and BL Lacertae. These detections resulted from the analysis of absorption troughs in SEDs derived on the base of mostly EGRET data that were collected during monitoring campaigns of the Virgo and galactic anticenter regions by the Compton Gamma Ray Observatory (CGRO), as well as during ToO observations of QSOs flares. Among three resonant absorption mechanisms that affect the γ-ray spectrum of point-like sources, we now pinpoint the ∆-isobar resonance that has a very stable peak energy of the absorption cross-section for all elements (nuclei) and for individual nucleons. From two absorbers that are usually detected on the sight lines towards γ-ray bright QSOs, we concentrate here on the only one that is at the QSO rest frame. We discuss the advantages and drawbacks of this method distinguishing between a galactic and extragalactic origin of the EGRET unidentified sources (EUIDs). This is compared to the multiwavelength identification approach that was succesfully used in a few cases to identify galactic and extragalactic objects among EUIDs. We applied the GRAM to identifying two of the EGRET unidentified sources as blazars, and predict their respective redshifts.

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Observations of a Correlated Gamma-Ray and Optical Flare for BL Lacertae

Cited by 81 publications

References 14 publications

Another look at the BL Lacertae flux and spectral variability

Another look at the BL Lacertae flux and spectral variability

Optical polarization of high-energy BL Lacertae objects

Gamma-ray absorption method (GRAM) application to the identification of EGRET unidentified sources

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