Abstract
The black hole binary Cygnus X-1 has a 5.6-d orbital period. We first detected a clear intensity modulation with the orbital period in its high/soft state with six-year MAXI data, as well as ...in its low/hard state. In the low/hard state, the folded light curves showed an intensity drop at the superior conjunction of the black hole by a modulation factor (MF), which is the amplitude divided by the average, of 8 ± 1%, 4 ± 1%, and 3 ± 2% for 2–4, 4–10, and 10–20 keV bands, respectively, showing a spectral hardening at the superior conjunction of the black hole. Spectral analysis in the low/hard state, with a model consisting of a power law and a photoelectric absorption, showed that the hydrogen column density, N
H, increased from (2.9 ± 0.4) × 1021 cm−2 to (4.7 ± 1.1) × 1021 cm−2 around the superior conjunction. The flux of the power-law component decreased by 6 ± 1%. On the other hand, MFs for the folded light curves in the high/soft state, were 4 ± 1% and 4 ± 2% for the 2–4 keV and 4–10 keV bands, respectively. We applied a model consisting of a power law and a disk blackbody with a photoelectric absorption. A modulation of the flux of the power-law component was found to be 7 ± 5% in MF, while the modulation of N
H was less than 1 × 1021 cm−2. These results can be interpreted as follows: the modulation of both states can be mainly explained by scattering of X-rays by an ionized stellar wind, but, only at the superior conjunction in the low/hard state, a large photoelectric absorption appears because of the low ionization state of the wind in the line of sight at phase 0. Such a condition can be established by reasonable parameters of an inhomogeneous wind and the observed luminosities.
Various transient phenomena on a timescale ranging from seconds to days appear at unexpected sky positions in X-rays. MAXI, Monitor of All-sky X-ray Image, on the International Space Station has been ...monitoring about 95% of the sky a day and has detected transient objects since 2009 August. Here, we describe quasi-real-time data processing systems of MAXI and a subsequent nova-alert system to find transient objects, and present the capabilities for the nova-alert system to detect transient events with excess fluxes from ≳80 mCrab in a single scan transit to ≳8 mCrab for 4 d, and to send prompt alert information to the world in less than 30 s after the onboard detection of a burst, making the best use of the International Space Station (ISS) real-time network. We also report on highlights of scientific results obtained with the system until the end of the first extended mission phase, 2015 March. Including 15 X-ray novae solely or independently discovered, we have reported on 177 transient phenomena, such as X-ray bursts, outbursts, and state transitions of X-ray binaries and X-ray flares from active stars and blazars, via the Astronomer's Telegram, and on 63 burst phenomena of other types via the Gamma-ray Coordinates Network. We summarize the results of these transient sources and phenomena focusing on the detections with the nova-alert system, and some new transients yet unpublished or requiring attention.
The archetypical very-high-energy gamma -ray blazar Mrk 421 was monitored for more than three years with the Gas Slit Camera on board Monitor of All Sky X-ray Image (MAXI), and its long-term X-ray ...variability was investigated. The MAXI light curve in the 3-10 keV range was transformed into the periodogram in the frequency range functionof = 1 x 10 super(-8)-2 x 10 super(-6) Hz. The artifacts on the periodogram, resulting from data gaps in the observed light curve, were extensively simulated for variations with a power-law-like power spectrum density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as alpha = 1.60 + or - 0.25. This index is smaller than that obtained in the higher-frequency range (functionof > ~ 1 x 10 super(-5) Hz), namely, alpha = 2.14 + or - 0.06 in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at functionof sub(b) = 5 x 10 super(-6) Hz, consistent with the break of functionof sub(b) = 9.5 x 10 super(-6) Hz suggested from the ASCA data. The low-frequency PSD index of Mrk 421 derived with MAXI falls well within the range of typical values among nearby Seyfert galaxies ( alpha = 1-2). The physical implications from these results are briefly discussed.
We present a catalog of high Galactic-latitude (b > 10degrees) X-ray sources detected in the first 37 months of data of the Monitor of All-sky X-ray Image/Gas Slit Camera (MAXI/GSC). To achieve the ...best sensitivity, we develop a background model of the GSC that well reproduces the data based on the detailed on-board calibration. Source detection is performed through image fits with a Poisson likelihood algorithm. The catalog contains 500 objects detected with significances of s sub(D,4-10 keV) > or =, slanted 7 in the 4-10 keV band. The limiting sensitivity is approximate7.5 x 10 super(-12) erg cm super(-2) s super(-1) (approximate0.6 mCrab) in the 4-10 keV band for 50% of the survey area, which is the highest ever achieved in an all-sky survey mission covering this energy band. We summarize the statistical properties of the catalog and results from cross matching with the Swift/BAT 70 month catalog, the meta-catalog of X-ray detected clusters of galaxies, and the MAXI/GSC 7 month catalog. Our catalog lists the source name (2MAXI), position and its error, detection significances and fluxes in the 4-10 keV and 3-4 keV bands, the hardness ratio, and the basic information of the likely counterpart available for 296 sources.
MAXI observations of gamma-ray bursts Serino, Motoko; Sakamoto, Takanori; Kawai, Nobuyuki ...
Publications of the Astronomical Society of Japan,
10/2014, Letnik:
66, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Abstract
The Monitor of All-sky X-ray Image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs), including bursts with soft spectra, such as X-ray flashes (XRFs). MAXI/GSC is sensitive to ...the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which are capable of detecting GRBs. Since the beginning of the MAXI operation on 2009 August 15, GSC observed 35 GRBs up to the middle of 2013. One third of them were also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N–log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range as that of MAXI/GSC, we find the possibility that many of the MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N–log S distribution suggests that the MAXI XRFs are distributed over a closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss the possibility of confusion of such galactic transients with the MAXI GRB samples.
The black hole binary Cygnus X-1 has a 5.6 day orbital period. We first detected a clear intensity modulation with the orbital period in its high/soft state with 6 year MAXI data, as well as in its ...low/hard state. In the low/hard state, the folded light curves showed an intensity drop at the superior conjunction of the black hole by a modulation factor (MF), which is the amplitude divided by the average, with 8+-1%, 4+-1% and 3+-2% for 2-4 keV, 4-10 and 10-20 keV bands, showing a spectral hardening at the superior conjunction of the black hole. Spectral analysis with a model consisting of a power law and a photoelectric absorption, showed that the hydrogen column density increased from (2.9+-0.4)E+21 to (4.7+-1.1)E+21 cm^-2 around the superior conjunction, although more complex spectral variation, such as a partial absorption, was suggested, and the flux of the power law component decreased with 6+-1%. On the other hand, the MFs of the folded light curves in the high/soft state, were 4+-1% and 4+-2% for 2-4 keV and 4-10 keV bands, respectively. We applied a model consisting of a power law and a diskblackbody with a photoelectric absorption and found a modulation of the flux of the power law component with 7+-5% in MF, while the modulation of the hydrogen column density was less than 1E+21 cm^-2. These results can be interpreted as follows; the modulation of both states can be mainly explained by scattering of the X-rays by an ionized stellar wind, but only at the superior conjunction in the low/hard state, a large photoelectric absorption appears, because of a low ionization state of the wind in the line of sight at the phase. Such a condition can be established by reasonable parameters of an in-homogeneous wind and the observed luminosities.
Since August 2009, MAXI experiment on the ISS has been performing all-sky X-ray monitoring. With MAXI, we detected flaring activities of some blazers, including Mrk 421, Mrk 501, and 3C 273. ...Recently, new X-ray flaring activities were detected from two blazers, MAXI J1930+093 = 2FGL J1931.1+0938 (Atel#5943) and 2MAXI J0243-582 = BZB J0244-5819 (Atel#6012). The MAXI monitoring also covers black hole binaries, including Cyg X-1 and Cyg X-3 which emit GeV gamma-rays. Their gamma-ray emission was found to coincide with their X-ray state transitions. We present light curves and outstanding events of these sources.
Monitor of All-sky X-ray Image (MAXI) on the International Space Station has been observing the X-ray sky since 2009 August 15. It has accumulated the X-ray data for about four years, so far. X-ray ...objects are usually variable and their variability can be studied by the power spectrum density (PSD) of the X-ray light curves.We applied our method to calculate PSDs of several kinds of objects observed with MAXI. We obtained significant PSDs from 16 Seyfert galaxies.For blackhole binary Cygnus X-1 there was a difference in the shape of PSD between the hard state and the soft state. For high mass X-ray binaries, Cen X-3, SMC X-1, and LMC X-4, there were several peaks in the PSD corresponding to the orbital period and the superorbital period.
The archetypical very-high-energy gamma-ray blazar Mrk 421 was monitored for more than 3 years with the Gas Slit Camera onboard Monitor of All Sky X-ray Image (MAXI), and its longterm X-ray ...variability was investigated. The MAXI lightcurve in the 3 -- 10 keV range was transformed to the periodogram in the frequency range \(f = 1 \times 10^{-8}\) -- \(2 \times 10^{-6}\) Hz. The artifacts on the periodogram, resulting from data gaps in the observed lightcurve, were extensively simulated for variations with a power-law like Power Spectrum Density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as \(\alpha = 1.60 \pm 0.25\). This index is smaller than that obtained in the higher frequency range (\(f > 1 \times 10^{-5}\) Hz), namely, \(\alpha = 2.14 \pm 0.06\) in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at \(f_{\rm b} = 5 \times 10^{-6}\) Hz, consistent with the break of \(f_{\rm b} = 9.5 \times 10^{-6}\) Hz, suggested from the ASCA data. The low frequency PSD index of Mrk 421 derived with MAXI falls well within the range of the typical value among nearby Seyfert galaxies (\(\alpha = 1\) -- \(2\)). The physical implications from these results are briefly discussed.
Monitor of all-sky image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs) including the bursts with soft spectra, such as X-ray flashes (XRFs). MAXI/GSC is sensitive to the energy range ...from 2 to 30 keV. This energy range is lower than other currently operating instruments which is capable of detecting GRBs. Since the beginning of the MAXI operation on August 15, 2009, GSC observed 35 GRBs up to the middle of 2013. One third of them are also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N - log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range to that of MAXI/GSC, we find a possibility that many of MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N - log S distribution suggests that the MAXI XRFs distribute in closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss a possibility of a confusion of those galactic transients with the MAXI GRB samples.
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