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Register a new userCyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku
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Y. Terada
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The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory, on 2005 September 14 for a net exposure of 22 ksec. The source was in a declining phase of a minor outburst, exhibiting 3--50 keV luminosity of about $3.7 times 10^{35}$ ergs s$^{-1}$ at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mCrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as those measured when the source is almost two orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X 0331+53.
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The transient X-ray binary pulsar A0535+262 was observed with Suzaku on 2005 September 14 when the source was in the declining phase of the August-September minor outburst. The ~103 s X-ray pulse profile was strongly energy dependent, a double peaked profile at soft X-ray energy band (<3 keV) and a single peaked smooth profile at hard X-rays. The width of the primary dip is found to be increasing with energy. The broad-band energy spectrum of the pulsar is well described with a Negative and Positive power-law with EXponential (NPEX) continuum model along with a blackbody component for soft excess. A weak iron K_alpha emission line with an equivalent width ~25 eV was detected in the source spectrum. The blackbody component is found to be pulsating over the pulse phase implying the accretion column and/or the inner edge of the accretion disk may be the possible emission site of the soft excess in A0535+262. The higher value of the column density is believed to be the cause of the secondary dip at the soft X-ray energy band. The iron line equivalent width is found to be constant (within errors) over the pulse phase. However, a sinusoidal type of flux variation of iron emission line, in phase with the hard X-ray flux suggests that the inner accretion disk is the possible emission region of the iron fluorescence line.
We present results obtained from an extensive near-infrared spectroscopic and photometric observations of the Be/X-ray binary A0535+262/HDE 245770 at different phases of its ~111 day orbital period. This observation campaign is a part of the monitoring programme of selective Be/X-ray binary systems aimed at understanding the X-ray and near-IR properties at different orbital phases, especially during the periastron passage of the neutron star. The near-IR observations were carried out using the 1.2 m telescope at Mt. Abu IR observatory. Though the source was relatively faint for spectroscopic observations with 1.2 m telescope, we monitored the source during the 2011 February--March giant outburst to primarily investigate whether any drastic changes in the near-IR JHK spectra take place at the periastron passage. Changes of such a striking nature were expected to be detectable in our spectra. Photometric observations of the Be star show a gradual and systematic fading in the JHK light curves since the onset of the X-ray outburst that could suggest a mild evacuation/truncation of the circumstellar disc of the Be companion. Near-IR spectroscopy of the object shows that the JHK spectra are dominated by the emission lines of hydrogen Brackett and Paschen series and HeI lines at 1.0830, 1.7002 and 2.0585 micron. The presence of all hydrogen emission lines in the JHK spectra, along with the absence of any significant change in the continuum of the Be companion during X-ray quiescent and X-ray outburst phases suggest that the near-IR line emitting regions of the disc are not significantly affected during the X-ray outburst.
The present paper reports on the RXTE observations of the binary X-ray pulsar 4U0115+63, covering an outburst in 1999 March-April with 44 pointings. The 3-30 keV PCA spectra and the 15-50 keV HEXTE spectra were analyzed jointly for the cyclotron resonance features. When the 3-50 keV luminosity at an assumed distance of 7 kpc was in the range (5-13)x10^{37} erg s^{-1}, harmonic double cyclotron features were observed in absorption at ~11 and ~22 keV, as was measured previously during typical outbursts. As the luminosity decreased below ~5x10^{37} erg s^{-1}, the second resonance disappeared, and the fundamental resonance energy gradually increased, up to $sim$16 keV at 0.16x10^{37} erg s^{-1}. These results reconfirm the report by Mihara et al. (2004) using Ginga, who observed a single absorption at ~16 keV in a minor (~10^{37} erg s^{-1}) outburst of this object. The luminosity-dependent cyclotron resonance energy may be understood as a result of a decrease in the accretion column height, in response to a decrease in the mass accretion rate.
X-ray behavior of the dwarf novae (DNe) outside the quiescent state has not been fully understood. We thus assembled 21 data sets of the 15 DNe observed by the Suzaku satellite by the end of 2013, which include spectra taken during not only the quiescence, but also the transitional, outburst, and super-outburst states. Starting with the traditional cooling flow model to explain the X-ray emission from the boundary layer, we made several modifications to account for the observed spectra. As a result, we found that the best-fit spectral model depends strongly on the state of the DNe with only a few exceptions. Spectra in the quiescent state are explained by the cooling flow model plus a Fe fluorescent line emission attenuated by an interstellar extinction. Spectra in the transitional state require an additional partial covering extinction. Spectra in the outburst and super-outburst state require additional low-temperature thin-thermal plasma component(s). Spectra in the super-outburst state further require a high value of the minimum temperature of the boundary layer. We present an interpretation on the required modifications to the cooling flow model for each state.
We report the diffuse X-ray emissions from the Sgr A and B regions observed with Suzaku. From the Sgr A region, we found many K-shell transition lines of iron and nickel. The brightest are K alpha lines from FeI, FeXXV and FeXXVI at 6.4 keV, 6.7 keV and 6.9 keV. In addition, K alpha lines of NiI and NiXXVII, K beta of FeI, FeXXV and FeXXVI, and K gamma of FeXXV and FeXXVI are detected for the first time. The center energy of K alpha of FeXXV favors collisional excitation as the origin for this line emission. The ionization temperature determined from the flux ratio of K alpha of FeXXV and FeXXVI is similar to the electron temperature determined from the flux ratio of K alpha and K beta of FeXXV, which are in the range of 5-7 keV. Consequently, the Galactic Center diffuse X-rays (GCDX) are consistent with emission from a plasma nearly in ionization equilibrium. The radio complex Sgr B region also exhibits K alpha lines of FeI, FeXXV and FeXXVI. The 6.7 keV line (FeXXV) map exhibits a local excess at (l,b) = (0.612, 0.01), and could be a new young SNR. The 6.4 keV image is clumpy with local excesses near Sgr B2 and at (l,b) = (0.74, -0.09). Like Sgr B2, this latter excess may be another X-ray reflection Nebulae (XRN).
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