arXiv subjects

H. Negoro

Publications and source records attributed to H. Negoro.

At least 19 recordsLinked to original sources

arXiv2024

Searching for similarities in the accretion flow of Seyfert 1 galaxies and cataclysmic variables based on flare profiles of IRAS 13224-3809, 1H 0707-495, Mrk 766 and MV Lyr

AIMS: We studied fast variability of three selected AGNs, IRAS 13224-3809, 1H 0707-495 and Mrk 766, and the cataclysmic variable MV Lyr observed by XMM-Newton and Kepler spacecrafts, respectively. Our goal is to search for common origin of the variability and to test the so-called sandwich model where a geometrically thick corona is surrounding a geometrically thin disc. METHODS: We study substructures of the averaged flare profiles. The flare profile method identifies individual flares in the light curve, and averages them. Direct fitting of the profile substructures identify individual characteristic frequencies seen in standard power density spectra (PDS) as a break frequency or quasi-periodic oscillation. The credibility of flare profile substructures is demonstrated by comparison with autocorrelation function. RESULTS: We found that the flare profiles of AGNs are similar to that of the cataclysmic variable in the low state. We explain this as a consequence of a truncated inner disc in a sandwich model. The same scenario is also able to explain the presence of characteristic break frequencies in X-ray PDS, but not seen in optical. We also searched for substructures in the flare profile of IRAS 13224-3809. In addition to a permanently present main flare, we found transient side-lobe appearing before the main flare and only seen in a high flux period. This complex flare profile of this AGN suggests that an additional source of X-rays appears during the high flux period. We propose a scenario in which an accretion flow fluctuation enters the sandwich corona and propagates further to some very central part of the accretion disc.

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arXiv2020

Alternation of the flickering morphology between the high and low state in MV Lyr

AIMS: We studied unique data of a nova-like system MV Lyr during transition from the high to low state and vice versa taken by the Kepler space telescope. We were interested in evolution of frequency components found previously by Scaringi et al. in different data also obtained by Kepler. METHODS: We divided the light curve into 10 day segments and investigated the corresponding power density spectra. We searched for individual frequency components by fitting with Lorentzian functions. Additionally, we investigated the variability using averaged shot profiles calculated from the light curve divided into 10 equally spaces subsamples. RESULTS: We found very complex changes of the power density spectra. We focused our study onto three frequency components. Strong activity increase is seen at low frequencies. Contrariwise, the high frequency part of the spectrum strongly decreases in power with specific rise in characteristic frequencies of the individual components. We discuss various scenarios of this phenomenology as reprocessing of X-rays in a receding accretion disc or a radiation from a more active region at the outer disc. Finally, we show that various cataclysmic variables show similar characteristic frequencies in their power density spectra. These are dependent on activity stage, making the situation similar to X-ray binaries.

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arXiv2019

Similar shot profile morphology of fast variability in cataclysmic variable, X-ray binary and blazar; the MV Lyr case

The cataclysmic variable MV Lyr was present in the Kepler field yielding a light curve with the duration of almost 1500 days with 60 second cadence. Such high quality data of this nova-like system with obvious fast optical variability show multicomponent power density spectra. Our goal is to study the light curve from different point of view, and perform a shot profile analysis. We search for characteristics not discovered with standard power density spectrum based methods. The shot profile method identifies individual shots in the light curve, and averages them in order to get all substructures with typical time scales. We also tested the robustness of our analysis using simple shot noise model. We obtained mean profiles with multicomponent features. The shot profile method distinguishes substructures with similar time scales which appear as a single degenerate feature in power density spectra. Furthermore, this method yields the identification of another high frequency component in the power density spectra of Kepler and XMM-Newton data not detected so far. Moreover, we found side-lobes accompanied with the central spike, making the profile very similar to another Kepler data of blazar W2R 1926+42, and Ginga data of Cyg X-1. All three objects show similar time scale ratios of the rising vs. declining part of the central spikes, while the two binaries have also similar rising profiles of the shots described by a power-law function. The similarity of both binary shot profiles suggests that the shots originate from the same origin, e.g. aperiodic mass accretion in the accretion disc. Moreover, the similarity with the blazar may imply that the ejection fluctuations in the blazar jet are connected to accretion fluctuations driving the variability in binaries. This points out to connection between jet and the accretion disc.

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arXiv2019

The 2018 X-ray and Radio Outburst of Magnetar XTE J1810-197

We present the earliest X-ray observations of the 2018 outburst of XTE J1810-197, the first outburst since its 2003 discovery as the prototypical transient and radio-emitting anomalous X-ray pulsar (AXP). The Monitor of All-sky X-ray Image (MAXI) detected XTE J1810-197 immediately after a November 20-26 visibility gap, contemporaneous with its reactivation as a radio pulsar, first observed on December 8. On December 13 the Nuclear Spectroscopic Telescope Array (NUSTAR) detected X-ray emission up to at least 30 keV, with a spectrum well-characterized by a blackbody plus power-law model with temperature kT = 0.74+/-0.02 keV and photon index Gamma = 4.4+/-0.2 or by a two-blackbody model with kT = 0.59+/-0.04 keV and kT = 1.0+/-0.1 keV, both including an additional power-law component to account for emission above 10 keV, with Gamma_h = -0.2+/-1.5 and Gamma_h = 1.5+/-0.5, respectively. The latter index is consistent with hard X-ray flux reported for the non-transient magnetars. In the 2-10 keV bandpass, the absorbed flux is 2E-10 erg/s/cm^2, a factor of 2 greater than the maximum flux extrapolated for the 2003 outburst. The peak of the sinusoidal X-ray pulse lags the radio pulse by approx. 0.13 cycles, consistent with their phase relationship during the 2003 outburst. This suggests a stable geometry in which radio emission originates on magnetic field lines containing currents that heat a spot on the neutron star surface. However, a measured energy-dependent phase shift of the pulsed X-rays suggests that all X-ray emitting regions are not precisely co-aligned.

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arXiv2015

Shot Analysis of Kepler Blazar W2R 1926+42

Blazars show rapid and violent variabilities, which timescale are often less than a day. We studied intraday variations by applying a shot analysis technique to Kepler monitoring of blazar W2R 1926+42 in Quarter 14. We obtained a mean profile calculated from 195 rapid variations. The mean profile shows three components; one is a sharp structure distributed within $\pm$0.1 day of the peak, and two slow-varying components. This spiky-peak component reflects features of rapid variations directly. The profile of peak component shows an exponential rise and decay of which timescales are different, 0.0416 and 0.0588 day respectively. This component is too sharp to represent a standard function which is often used to express blazar variations. This asymmetric profile at the peak is difficult to be explained by a simple variation of the Doppler factor by changing a geometry of the emitting region. This result indicates that intraday variations arise from a production of high-energy accelerated particles in the jet.

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arXiv2013

Extraordinary luminous soft X-ray transient MAXI J0158-744 as an ignition of a nova on a very massive O-Ne white dwarf

We present the observation of an extraordinary luminous soft X-ray transient, MAXI J0158-744, by the Monitor of All-sky X-ray Image (MAXI) on 2011 November 11. This transient is characterized by a soft X-ray spectrum, a short duration (1.3 x 10^3 s < ΔT_d < 1.10 x 10^4 s), a very rapid rise (< 5.5 x 10^3 s), and a huge peak luminosity of 2 x 10^40 erg s^-1 in 0.7-7.0 keV band. With Swift observations and optical spectroscopy from the Small and Moderate Aperture Research Telescope System (SMARTS), we confirmed that the transient is a nova explosion, on a white dwarf in a binary with a Be star, located near the Small Magellanic Cloud. An extremely early turn-on of the super-soft X-ray source (SSS) phase (< 0.44 d), the short SSS phase duration of about one month, and a 0.92 keV neon emission line found in the third MAXI scan, 1296 s after the first detection, suggest that the explosion involves a small amount of ejecta and is produced on an unusually massive O-Ne white dwarf close to, or possibly over, the Chandrasekhar limit. We propose that the huge luminosity detected with MAXI was due to the fireball phase, a direct manifestation of the ignition of the thermonuclear runaway process in a nova explosion.

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arXiv2013

Propeller-Effect Interpretation of MAXI/GSC Light Curves of 4U 1608-52 and AqlX-1 and application to XTE J1701-462

We present the luminosity dwell-time distributions during the hard states of low-mass X-ray binaries containing a neutron star, 4U 1608-52 and AqlX-1, observed with MAXI/GSC. The luminosity distributions show a steep cut-off in the low-luminosity side at $\sim1.0 \times 10^{36}$ erg s$^{-1}$ in both the two sources. The cut-off implies a rapid luminosity decrease in their outburst decay phases, and the feature can be interpreted as due the propeller effect. We estimated the surface magnetic field of the neutron star to be (0.5--1.6) $\times 10^8$ G in 4U 1608-52 and (0.6--1.9) $\times 10^8$ G in AqlX-1 from the cut-off luminosity. We applied the same propeller mechanism to the similar rapid luminosity decrease observed in the transient Z-source, XTE J1701-462, with RXTE/ASM. Assuming that spin period of the neutron star is in the order of milliseconds, the observed cut-off luminosity deduces surface magnetic field in the order of $10^9$ G.

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arXiv2013

Rapid Spectral Changes of Cygnus X-1 in the Low/Hard State with Suzaku

Rapid spectral changes in the hard X-ray on a time scale down to ~0.1 s are studied by applying "shot analysis" technique to the Suzaku observations of the black hole binary Cygnus X-1, performed on 2008 April 18 during the low/hard state. We successfully obtained the shot profiles covering 10--200 keV with the Suzaku HXD-PIN and HXD-GSO detector. It is notable that the 100-200 keV shot profile is acquired for the first time owing to the HXD-GSO detector. The intensity changes in a time-symmetric way, though the hardness does in a time-asymmetric way. When the shot-phase-resolved spectra are quantified with the Compton model, the Compton y-parameter and the electron temperature are found to decrease gradually through the rising phase of the shot, while the optical depth appears to increase. All the parameters return to their time-averaged values immediately within 0.1 s past the shot peak. We have not only confirmed this feature previously found in energies below ~60 keV, but also found that the spectral change is more prominent in energies above ~100 keV, implying the existence of some instant mechanism for direct entropy production. We discuss possible interpretations on the rapid spectral changes in the hard X-ray band.

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arXiv2011

A contribution of stellar flares to the GRXE -- based on MAXI observations --

Using unbiased observations of MAXI/GSC the potential contribution of stellar flares and CVs to GRXE luminosity is estimated in the energy range of 2 - 10 keV. Currently, a reasonable luminosity has been obtained extrapolating the number of stellar flares and that of CVs toward the Galactic ridge from those of the observed flares and CVs near the solar system. The ionized emission lines of Si to Fe are also simulated making the composite thermal spectrum which is based on MAXI observations of stellar flares and CVs. The present estimated result strongly supports a picture that the cumulative stellar flares contribute primarily to the GRXE in terms of the composite thermal spectrum with emission lines and secondary contribution is due to the thermal spectrum with high temperature from CVs.

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arXiv2011

Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole

Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in the massive black hole at the centre of our Milky Way galaxy could strongly ionize the upper atmosphere of the Earth, if beamed towards us.

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arXiv2010

Measuring spin of a supermassive black hole at the Galactic centre -- Implications for a unique spin

We determine the spin of a supermassive black hole in the context of discseismology by comparing newly detected quasi-periodic oscillations (QPOs) of radio emission in the Galactic centre, Sagittarius A* (Sgr A*), as well as infrared and X-ray emissions with those of the Galactic black holes. We find that the spin parameters of black holes in Sgr A* and in Galactic X-ray sources have a unique value of $\approx 0.44$ which is smaller than the generally accepted value for supermassive black holes, suggesting evidence for the angular momentum extraction of black holes during the growth of supermassive black holes. Our results demonstrate that the spin parameter approaches the equilibrium value where spin-up via accretion is balanced by spin-down via the Blandford-Znajek mechanism regardless of its initial spin. We anticipate that measuring the spin of black holes by using QPOs will open a new window for exploring the evolution of black holes in the Universe.

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arXiv2002

Energy Spectra and Normalized Power Spectral Densities of X-Ray Nova GS 2000+25

The X-ray energy spectra and Normalized Power Spectral Densities (NPSDs) of an X-ray nova, GS 2000+25, were investigated. The X-ray energy spectra of the source consist of two components: a hard component, which can be represented by a power-law, and an ultra-soft component, represented by radiation from an optically-thick accretion disk (the disk component). In a model in which the power-law component is the Compton-scattered radiation, it is found that the temperature of the incident blackbody radiation to the Compton cloud decrease from 0.8 keV to 0.2 keV according to the decay of the intensity, which coincides with that of the inner accretion disk. When the source changed from the high-state to the low-state, both the photon index of the power-law component (or Compton y-parameter) and the NPSD of the hard component dramatically changed as did GS 1124-683. That is, the photon index changed from 2.2--2.6 to 1.7--1.8 and the absolute values of the NPSDs at 0.3 Hz of the hard component in the low-state became about 10-times larger than those of the hard component in the high-state. These X-ray properties were similar to those of other black-hole candidates, such as Cyg X-1, GX 339-4, and LMC X-3.

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arXiv2002

Log-Normal Distributions in Cygnus X-1: Possible Physical Link with Gamma-Ray Bursts and Blazars

Prompted by recent discoveries of log-normal distributions in gamma-ray/X-ray temporal variabilities of gamma-ray bursts (GRBs) and blazars, we re-examine the X-ray variability of Cygnus X-1 in the hard/low state using Ginga data in 1990. It was previously reported that the distributions of the time intervals between X-ray shots (flares) deviated from the Poisson distributions at short time intervals: the occurrence of shots tended to be suppressed for several seconds before and/or after a shot event. Detailed analyses show that this deviation is larger for shots with larger peaks, and that the time-interval distributions for large shots approach log-normal distributions with a peak at the interval of 7--8 s. Furthermore, we also show that the peak-intensity distribution for the shots is consistent with the log-normal distribution, though no typical peak intensity can be seen. This might indicate the presence of a physical link connecting the physics of black hole accretion flow and that of jet/GRB formation.

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arXiv2002

On the Nature of X-ray Variability in Ark 564

We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time series, employing techniques not generally applied to AGN light curves, can provide useful information to characterize the engines of these powerful sources.We searched for signs of non-stationarity in the data, but did not find strong evidences for it. We find that the process causing the variability is very likely nonlinear, suggesting that variability models based on many active regions, as the shot noise model, may not be applicable to Ark 564. The complex light curve can be viewed, for a limited range of time scales, as a fractal object with non-trivial fractal dimension and statistical self-similarity. Finally, using a nonlinear statistic based on the scaling index as a tool to discriminate time series, we demonstrate that the high and low count rate states, which are indistinguishable on the basis of their autocorrelation, structure and probability density functions, are intrinsically different, with the high state characterized by higher complexity.

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arXiv2001

Detection of a high frequency break in the X-ray power spectrum of Ark 564

We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of ~ 3.5 decades. We detect a high frequency break at ~ 0.002 Hz. The power spectrum has an rms of ~30% and a slope of ~ -1 and ~ -2 below and above the break frequency. When combined with the results from a long RXTE observation (Pounds et al. 2001), the observed power spectra of Ark 564 and Cyg X-1 (in the low/hard state) are almost identical, showing a similar shape and rms amplitude. However, the ratio of the high frequency breaks is very small (~ 10e{3-4}), implying that these characteristic frequencies are not indicative of the black hole mass. This result supports the idea of a small black hole mass/high accretion rate in Ark 564.

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arXiv2001

SSC of MAXI experiment

Monitor of All-sky X-ray Image (MAXI) on the International Space Station (ISS) has two kinds of X-ray detectors: the Gas Slit Camera (GSC) and the Solid-state Slit Camera (SSC). SSC is an X-ray CCD array, consisting of 16 chips, which has the best energy resolution as an X-ray all-sky monitor in the energy band of 0.5 to 10 keV. Each chip consists of 1024x1024 pixels with a pixel size of 24$μ$m, thus the total area is ~200 cm^2. We have developed an engineering model of SSC, i.e., CCD chips, electronics, the software and so on, and have constructed the calibration system. We here report the current status of the development and the calibration of SSC.

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arXiv2001

Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. III. Optical Observations and the Optical--UV--X-ray Connection

We present the results of a two-year long optical monitoring program of the narrow-line Seyfert 1 galaxy Akn 564. The majority of this monitoring project was also covered by X-ray observations (RXTE) and for a period of ~50 days, we observed the galaxy in UV (HST) and X-rays (RXTE & ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly time-scale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (one day) or long (several months) time-scales. The only significant optical variations can be described as two 2--4 day events with ~10% flux variations. We detect no significant optical line variations and thus cannot infer a reverberation size for the broad-line region. Similarly, the large X-ray variations seem to vanish when the light curve is smoothed over a period of 30 days. The UV continuum follows the X-rays with a lag of ~0.4 days, and the optical band lags the UV band by ~2 days. No significant correlation was found between the entire X-ray dataset and the optical band. Focusing on a 20-day interval around the strongest optical event we detect a significant X-ray--optical correlation with similar events seen in the UV and X-rays. Our data are consistent with reprocessing models on the grounds of the energy emitted in this single event. However, several large X-ray flares produced no corresponding optical emission.

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arXiv2001

A Rapid X-ray Flare in the Radio-loud Narrow Line Quasar PKS 0558-504

We report the detection of a very short time scale flare in the radio-loud narrow line quasars PKS 0558-504 by using ASCA. The X-ray count rates increased and decreased by a factor of two in 33 minutes, and possibly by 40% in as short as two minutes during this flare, confirming that such flare event does occur in this object with imaging detectors. The implied largest rate of change in luminosity in 0.8-10 keV alone, dL/dt=(1.8+/-0.4) 10^{42} ergs s^-2, is several times higher than the limit sets for the isotropic emitting plasma around a Kerr black hole. Either emission from a relativistic boosting jet or a magnetic heated corona may explain such high radiative efficiency. Magnetic field with a strength of at least a few 10^4 Gauss is required in the latter case. The spectrum during this flare is significant harder than the average one. Three radio loud narrow line active galactic nuclei (AGN) possess considerably smaller black holes than in radio loud AGN in bright quasar survey, indicating they are still in a phase of the rapid growth of the black hole by accretion.

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