When a Standard Candle Flickers

Wilson-Hodge, C.; Cherry, M. L.; Case, G.; Baumgartner, W. H.; Beklen, E.; Bhat, P. N.; Briggs, M. S.; Camero-Arranz, A.; Chaplin, V.; Connaughton, V.; Finger, Mark H.; Gehrels, N.; Greiner, J.; Jahoda, K.; Jenke, P.; Kippen, R. M.; Kouveliotou, C.; Krimm, H. A.; Kuulkers, E.; Lund, N.; Meegan, C.; Natalucci, L.; Pačiesas, W. S.; Preece, R.; Rodi, J.; Shaposhnikov, Nikolai; Skinner, G.; Swartz, Doug; Kienlin, A. von; Diehl, R.; Zhang, Xiaoling

doi:10.1088/2041-8205/727/2/l40

Cited by 156 publications

(129 citation statements)

References 40 publications

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“…Fitting the Crab orbital light curve with a constant, a σ sys,A = 0.06 × F systematic error needs to be added to obtain a reduced χ 2 equal to 1. A further source of uncertainty is expected following the finding of longterm variability in the Crab light curve (up to ∼7% on a year time scale; Wilson-Hodge et al 2011); therefore, the value of the sys A term is likely to be overestimated here. Owing to the difficulty of disentangling the systematic component from the intrinsic Crab variability, we prefer to choose a conservative approach and to adopt the sys A = 0.06 value.…”

Section: Light Curve Systematic Errorsmentioning

confidence: 96%

Long-term variability of AGN at hard X-rays

Soldi¹,

Beckmann²,

Baumgartner³

et al. 2014

A&A

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Aims. Variability at all observed wavelengths is a distinctive property of active galactic nuclei (AGN). Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by absorption along the line of sight. Characterizing the intrinsic hard X-ray variability of a large AGN sample and comparing it to the results obtained at lower X-ray energies can significantly contribute to our understanding of the mechanisms underlying the high-energy radiation. Methods. Swift/BAT provides us with the unique opportunity to follow, on time scales of days to years and with regular sampling, the 14-195 keV emission of the largest AGN sample available up to date for this kind of investigation. As a continuation of an early work using the first 9 months of BAT data, we study the amplitude of the variations and their dependence on subclass and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. Results. About 80% of the AGN in the sample are found to exhibit significant variability on month-to-year time scales. In particular, radio loud sources are the most variable, and Seyfert 1.5−2 galaxies are slightly more variable than Seyfert 1, while absorbed and unabsorbed objects show similar timing properties. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes in the absorption column density. In general, the variations in the 14-24 and 35-100 keV bands are correlated well, suggesting a common origin to the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14-24 keV than at 35-100 keV, and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on a time scale of a month. In addition, sources with harder spectra are found to be more variable than softer ones, unlike what it is observed below 10 keV. These properties are generally consistent with a variable, in flux and shape, power law continuum, pivoting at energies > ∼ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).

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Section: Light Curve Systematic Errorsmentioning

confidence: 96%

Long-term variability of AGN at hard X-rays

Soldi¹,

Beckmann²,

Baumgartner³

et al. 2014

A&A

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“…Consequently, the lack of information about the site of the flare in the PWN has led to a variety of models being suggested (see § 3). Given the lack of contemporaneous variability at lower frequencies, it might be possible that some of the energy released during the process associated with the γ-ray flares will manifest itself as more gradual flux changes at lower frequencies 4 occurring on much longer timescales (e.g., hard X-ray variability reported by Kouzu et al 2013;Wilson-Hodge et al 2011).…”

Section: Multiwavelength Properties Of the Crabmentioning

confidence: 99%

Pulsar-Wind Nebulae

et al. 2015

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“…Meyer et al 2010), its emission shows substantial variability at high energies (HE; E > 100 MeV) (see e.g. Tavani et al 2011;Abdo et al 2011;Striani et al 2011Striani et al , 2013bBuehler et al 2012), as well as at X-ray energies (Wilson-Hodge et al 2011), albeit with a smaller relative amplitude of flux changes (≈5%) and on longer time scales of a few months. The most recent example is the flare detected with Fermi-LAT (Ojha et al 2013;Mayer et al 2013) and AGILE (Striani et al 2013a;Verrecchia et al 2013) in March 2013, when the peak photon flux of the synchrotron component above 100 MeV was (103.4 ± 0.8) × 10 −7 cm −2 s −1 compared to (6.1 ± 0.1) × 10 −7 cm −2 s −1 in its quiescent state, and variability was measured on time scales of a few hours.…”

Section: Introductionmentioning

confidence: 99%

H.E.S.S. observations of the Crab during its March 2013 GeV gamma-ray flare

Abramowski¹,

Aharonian²,

Benkhali³

et al. 2014

A&A

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Context. On March 4, 2013 the Fermi-LAT and AGILE reported a flare from the direction of the Crab nebula in which the high-energy (HE; E > 100 MeV) flux was six times above its quiescent level. Simultaneous observations in other energy bands give us hints about the emission processes during the flare episode and the physics of pulsar wind nebulae in general. Aims. We search for variability in the emission of the Crab nebula at very-high energies (VHE; E > 100 GeV), using contemporaneous data taken with the H.E.S.S. array of Cherenkov telescopes. Methods. Observational data taken with the H.E.S.S. instrument on five consecutive days during the flare were analysed for the flux and spectral shape of the emission from the Crab nebula. Night-wise light curves are presented with energy thresholds of 1 TeV and 5 TeV.Results. The observations conducted with H.E.S.S. on March 6 to March 10, 2013 show no significant changes in the flux. They limit the variation in the integral flux above 1 TeV to less than 63% and the integral flux above 5 TeV to less than 78% at a 95% confidence level.

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When a Standard Candle Flickers

Cited by 156 publications

References 40 publications

Long-term variability of AGN at hard X-rays

Long-term variability of AGN at hard X-rays

Pulsar-Wind Nebulae

H.E.S.S. observations of the Crab during its March 2013 GeV gamma-ray flare

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