Variability of Active Galactic Nuclei

Ulrich, M. H.; Maraschi, L.; Urry, C. Megan

doi:10.1146/annurev.astro.35.1.445

Cited by 836 publications

(680 citation statements)

References 297 publications

(304 reference statements)

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“…The X-ray spectra of these sources generally soften as they brighten 15 . In contrast, the extremely low luminosity of Sgr A * precludes the presence of a standard, optically thick accretion disk 5 ; hence, the dominant source of seed photons would be the millimetre-tosubmillimetre synchrotron photons.…”

Section: /55mentioning

confidence: 99%

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

Baganoff

Bautz

Brandt

et al. 2001

Nature

608

705

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Most galactic nuclei are now believed to harbour supermassive black holes 1 . Studies of stellar motions in the central few light-years of our Milky Way Galaxy indicate the presence of a dark object with a mass of ≈ 2.6 × 10 6 solar masses (refs 2, 3). This object is spatially coincident with Sagittarius A * (Sgr A * ), the unique compact radio source located at the dynamical centre of our Galaxy. By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A * is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole 4, 5 . However, Sgr A * is much fainter than expected in all wavebands, especially in X-rays, casting some doubt on this model. Recently, we reported the first strong evidence of X-ray emission from Sgr A * (ref. 6). Here we report the discovery of rapid X-ray flaring from the direction of Sgr A * . These data provide compelling evidence that the X-ray emission is coming from accretion onto a supermassive black hole at the Galactic Centre, and the nature of the variations provides strong constraints on the astrophysical processes near the event horizon of the black hole.Our view of Sgr A * in the optical and ultraviolet wavebands is blocked by the large visual extinction, AV ≈ 30 magnitudes 7 , caused by dust and gas along the line of sight. Sgr A * has not been detected in the infrared due to its faintness and to the bright infrared background from stars and clouds of dust 8 . Detection of X-rays from Sgr A * is therefore essential to constrain the spectrum at energies above the radio-tosubmillimetre band and to test the supermassive-black-hole accretion-flow paradigm 5 .We first observed the Galactic Centre on 21 September 1999 with the imaging array of the Advanced CCD Imaging Spectrometer (ACIS-I) aboard the Chandra X-ray Observatory 9 and discovered an X-ray source coincident within 0. 35 ± 0. 26 (1σ) of the radio source 6 . The luminosity in 1999 was very weak, LX ≈ 2 × 10 33 erg s −1 in the 2-10 keV band, after correction for the inferred neutral hydrogen absorption column NH ≈ 1 × 10 23 cm −2 . This is far fainter than previous X-ray observatories could detect 6 .We observed the Galactic Centre a second time with Chandra/ACIS-I from

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Section: /55mentioning

confidence: 99%

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

Baganoff

Bautz

Brandt

et al. 2001

Nature

608

705

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“…Blazars are known to be variable with the variability correlated across a wide range of energies [29]. Secondary gamma rays, on the other hand, would have this variability washed out [7], and no variability should be seen in the secondary component on the timescale of the VERITAS observations.…”

Section: Pks 1424+240 and Secondary Gamma Raysmentioning

confidence: 99%

Understanding the spectrum of a distant blazar PKS 1424+240 and its implications

Essey

Kusenko

2014

Astroparticle Physics

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We calculate the gamma-ray spectrum of a distant blazar, PKS 1424+240, located at redshift z ≥ 0.6035, where the optical depth to primary gamma rays of the highest observed energies is τ ≥ 5. The spectral shape agrees well with what is expected from secondary gamma rays produced in line-ofsight interactions of cosmic rays with background photons. In particular, we find an acceptable agreement with the source redshifts in the range 0.6 ≤ z ≤ 1.3. We discuss the implications of existing and future data for models of extragalactic background light (EBL) and for the redshift of the source. In the case of a future detection of temporal variability at lower energies, one can set more restrictive limits on both the source redshift and the EBL spectrum.

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“…As such, it is classified as a BL Lacertae (BL Lac) type. Its spectral energy distribution (SED) is well described by a characteristic two-peak shape (for a review, see, e.g., Urry & Padovani 1995;Ulrich et al 1997). In the more general context of blazars, Mrk 421 belongs to a subclass of the so-called high-energypeaked BL Lac (HBL) objects, relatively low luminosity sources with both peaks located at relatively high energies (respectively at ∼1 keV and ∼100 GeV).…”

Section: Introductionmentioning

confidence: 99%

“…The seed photons are most likely the synchrotron photons internal to the jet. Such "synchrotron selfCompton" (SSC) models, developed by many authors (for early examples see, e.g., Jones et al 1974;Ghisellini et al 1985;Marscher & Gear 1985), have been successfully invoked to describe the full SED of HBL objects (e.g., Ulrich et al 1997;Fossati et al 2008;Tavecchio et al 2010).…”

Section: Introductionmentioning

confidence: 99%

MULTIWAVELENGTH STUDY OF QUIESCENT STATES OF Mrk 421 WITH UNPRECEDENTED HARD X-RAY COVERAGE PROVIDED BY NuSTAR IN 2013

Baloković¹,

Paneque²,

Madejski³

et al. 2016

ApJ

116

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We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy(VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep 3 G » power law, with no evidence for an exponential cutoff or additional hard components up to ∼80keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

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Variability of Active Galactic Nuclei

Cited by 836 publications

References 297 publications

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

Understanding the spectrum of a distant blazar PKS 1424+240 and its implications

MULTIWAVELENGTH STUDY OF QUIESCENT STATES OF Mrk 421 WITH UNPRECEDENTED HARD X-RAY COVERAGE PROVIDED BY NuSTAR IN 2013

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