X-Ray Fluctuations from Locally Unstable Advection-dominated Disks

Manmoto, T.; Takeuchi, Mitsuru; Mineshige, Shin; Matsumoto, Ryoji; Negoro, H.

doi:10.1086/310097

Cited by 72 publications

(74 citation statements)

References 19 publications

(24 reference statements)

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“…In the context of X-ray fluctuations in a RIAF scenario, Manmoto et al (1996) and Takeuchi & Mineshige (1997) were able to explain the Xray shots seen in Cygnus X-1 light curves (Negoro et al 1994) as due to a disturbance in an extended RIAF. However, contrarily to Cygnus X-1 that switches from a soft state to a hard state just after the peak, NGC 4278 remains in a soft state during the whole observation (including the variation).…”

Section: Variabilitymentioning

confidence: 94%

X-ray and multiwavelength view of NGC 4278

Younes

Porquet

Sabra

et al. 2010

A&A

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Context. The emission mechanism responsible for the bulk of energy from radio to X-rays in low ionization emission line regions (LINERs) and low luminosity active galactic nuclei (LLAGN) has been long debated. Based on UV to X-ray and radio to UV flux ratios, some argue that LINERs/LLAGN are a scaled-down version of their more luminous predecessors Seyfert galaxies. Others, based on the lack of X-ray short (hours) time-scale variability, the non detection of an iron line at 6.4 keV, and the faint UV emission compared to typical AGNs, suggest the truncation of the classical thin accretion disk in the inner regions of the AGN where a radiatively inefficient accretion flow (RIAF) structure forms. Aims. We investigate the LINER-Seyfert connection by studying the unabsorbed LINER galaxy NGC 4278 that accretes at a low rate (L bol/Edd ∼ 7 × 10 −6 ) but exhibits a broad Hα line, and a point-like nucleus in radio, optical, UV and X-rays. Methods. We analyzed one XMM-Newton and seven Chandra X-ray observations of NGC 4278 spread over a three year period, allowing the study of the X-ray variability at different time-scales (hours, months, years). We also examined the radio to X-ray spectral energy distribution to constrain the accretion mode in the nucleus of NGC 4278. Results. Long time-scale (months) variability is observed where the flux increased by a factor of ∼3 on a time-scale of a few months and by a factor of 5 between the faintest and the brightest observation separated by ∼3 years. During the XMM-Newton observation, where the highest flux level is detected, we found a 10% flux increase on a short time-scale of a few hours, while the light curves for the different Chandra observations do not show short time-scale (minutes to hours) variability. A combination of an absorbed power law (N H ≈ 10 20 cm −2 , Γ = 2.2 +0.1 −0.2 ) plus a thermal component (kT ≈ 0.6 keV) were able to fit the Chandra spectra. The XMM-Newton spectra, where the highest X-ray flux is detected, are well fitted with an absorbed power-law with no need for a thermal component as the emission from the power-law component is dominant. The power-law photon index is ∼2.1 and the hydrogen column density is of the order of 10 20 cm −2 . Neither a narrow nor a broad Fe Kα emission line at 6.4 keV are detected with a 22 eV and 118 eV upper limits derived on their equivalent widths. We derive optical fluxes from archival HST ACS observations and detected optical variability on time-scales of years. For the first time for this source, thanks to the optical/UV monitor on board XMM-Newton, we obtained simultaneous UV and X-ray flux measurements. We constructed SEDs based on simultaneous or quasi simultaneous observations and compared them to LINER, radio-loud, and radio-quiet quasar SEDs. We find that at a low X-ray flux the NGC 4278 SED resembles that of typical LINER sources where the radio to X-ray emission can be considered as originating from a jet and/or RIAF, whereas at a high X-ray flux, NGC 4278 SED is more like a low luminosity Seyfert SED...

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Section: Variabilitymentioning

confidence: 94%

X-ray and multiwavelength view of NGC 4278

Younes

Porquet

Sabra

et al. 2010

A&A

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“…A wide class of models for the variability is based on the idea of inwards propagating mass accretion fluctuations that originate at certain radii in the disk and vary on the local viscous timescales (e.g., Manmoto et al 1996;Lyubarskii 1997;Nowak et al 1999b;Kotov et al 2001;Arévalo & Uttley 2006). This class of models can be traced back to the original work of Lightman & Eardley (1974) and Shakura & Sunyaev (1976) on disk instabilities as the source for black hole variability.…”

Section: Models For X-ray Variabilitymentioning

confidence: 99%

Long term variability of Cygnus X-1

Grinberg

Pottschmidt

Böck

et al. 2014

A&A

116

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We present the most extensive analysis of Fourier-based X-ray timing properties of the black hole binary Cygnus X-1 to date, based on 12 years of bi-weekly monitoring with RXTE from 1999 to 2011. Our aim is a comprehensive study of timing behavior across all spectral states, including the elusive transitions and extreme hard and soft states. We discuss the dependence of the timing properties on spectral shape and photon energy, and study correlations between Fourier-frequency dependent coherence and time lags with features in the power spectra. Our main results follow. (a) The fractional rms in the 0.125-256 Hz range in different spectral states shows complex behavior that depends on the energy range considered. It reaches its maximum not in the hard state, but in the soft state in the Comptonized tail above 10 keV. (b) The shape of power spectra in hard and intermediate states and the normalization in the soft state are strongly energy-dependent in the 2.1-15 keV range. This emphasizes the need for an energy-dependent treatment of power spectra and a careful consideration of energy-and mass-scaling when comparing the variability of different source types, e.g., black hole binaries and AGN. PSDs during extremely hard and extremely soft states can be easily confused for energies above ∼5 keV in the 0.125-256 Hz range. (c) The coherence between energy bands drops during transitions from the intermediate into the soft state but recovers in the soft state. (d) The time lag spectra in soft and intermediate states show distinct features at frequencies related to the frequencies of the main variability components seen in the power spectra and show the same shift to higher frequencies as the source softens. Our results constitute a template for other sources and for physical models for the origin of the X-ray variability. In particular, we discuss how the timing properties of Cyg X-1 can be used to assess the evolution of variability with spectral shape in other black hole binaries. Our results suggest that none of the available theoretical models can explain the full complexity of X-ray timing behavior of Cyg X-1, although several ansatzes with different physical assumptions are promising.

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“…They all depend on the black hole mass M BH and Schwarzschild radius R S . We review them below, from the fastest to the slowest, following Edelson & Nandra (1999) and Manmoto et al (1996).…”

Section: Event Duration and Black-hole Physical Time Scalesmentioning

confidence: 99%

AGN variability time scales and the discrete-event model

Favre¹,

Courvoisier²,

Paltani³

2005

A&A

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We analyse the ultraviolet variability time scales in a sample of 15 type 1 Active Galactic Nuclei (AGN) observed by IUE. Using a structure function analysis, we demonstrate the existence in most objects of a maximum variability time scale of the order of 0.02-1.00 year. We do not find any significant dependence of these maximum variability time scales on the wavelength, but we observe a weak correlation with the average luminosity of the objects. We also observe in several objects the existence of long-term variability, which seems decoupled from the short-term one. We interpret the existence of a maximum variability time scale as a possible evidence that the light curves of type 1 AGN are the result of the superimposition of independent events. In the framework of the so-called discrete-event model, we study the event energy and event rate as a function of the object properties. We confront our results to predictions from existing models based on discrete events. We show that models based on a fixed event energy, like supernova explosions, can be ruled out. In their present form, models based on magnetic blobs are also unable to account for the observed relations. Stellar collision models, while not completely satisfactory, cannot be excluded.

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X-Ray Fluctuations from Locally Unstable Advection-dominated Disks

Cited by 72 publications

References 19 publications

X-ray and multiwavelength view of NGC 4278

X-ray and multiwavelength view of NGC 4278

Long term variability of Cygnus X-1

AGN variability time scales and the discrete-event model

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