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ABSTRACT: We present results based on XMM-Newton observations of the nearby spiral galaxy M51 (NGC 5194 and NGC 5195). We confirm the presence of the seven known ultraluminous X-ray sources (ULXs) with luminosities exceeding the Eddington luminosity for a 10 M☉ black hole, a low-luminosity active galactic nucleus (LLAGN) with 2-10 keV luminosity of 1.6 × 1039 ergs s-1, and soft thermal extended emission from NGC 5194 detected with Chandra. In addition, we also detected a new ULX with luminosity of ~1039 ergs s-1. We have studied the spectral and temporal properties of the LLAGN and eight ULXs in NGC 5194 and an ULX in NGC 5195. Two ULXs in NGC 5194 show evidence for short-term variability, and all but two ULXs vary on long timescales (over a baseline of ~2.5 yr), providing strong evidence that these are accreting sources. One ULX in NGC 5194, source 69, shows possible periodic behavior in its X-ray flux. We derive a period of 5925 ± 200 s at a confidence level of 95% on the basis of three cycles. This period is lower than the period of 7620 ± 500 s derived from a Chandra observation in 2000. The higher effective area of XMM-Newton enables us to identify multiple components in the spectra of ULXs. Most ULXs require at least two components, a power law and a soft X-ray excess component that is modeled by an optically thin plasma or a multicolor disk blackbody (MCD). However, the soft excess emissions inferred from all ULXs except source 69 are unlikely to be physically associated with the ULXs, as their strengths are comparable to that of the surrounding diffuse emission. The soft excess emission of source 69 is well described either by a two-temperature MEKAL plasma or a single-temperature MEKAL plasma (kT ~ 690 eV) and an MCD (kT ~ 170 eV). The MCD component suggests a cooler accretion disk compared to those in Galactic X-ray binaries, consistent with those expected for intermediate-mass black holes (IMBHs). An iron Kα line (EW ~ 700 eV) or K absorption edge at ~7.1 keV is present in the EPIC pn spectrum of source 26. The spectrum of the ULX in NGC 5195, source 12, is consistent with a simple power law. The LLAGN in NGC 5194 shows an extremely flat hard X-ray power law (Γ ~ 0.7), a narrow iron Kα line at 6.4 keV (EW ~ 3 keV), and strong soft X-ray excess emission. The full-band spectrum is well described by a two-component MEKAL plasma and reflection from cold material such as a putative torus.
The Astrophysical Journal 12/2008; 635(1):198. · 6.02 Impact Factor
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ABSTRACT: We present three XMM-Newton observations of the ultraluminous compact X-ray source Holmberg II X-1 in its historical brightest and faintest states. The source was in its brightest state in 2002 April with an isotropic X-ray luminosity of ~2 × 1040 ergs s-1 but changed to a peculiar low/soft state in 2002 September in which the X-ray flux dropped by a factor of ~4 and the spectrum softened. In all cases, a soft excess component, which can be described by a simple or multicolor disk blackbody (kT ~ 120-170 eV), is statistically required in addition to a power-law continuum (Γ ~ 2.4-2.9). Both spectral components became weaker and softer in the low/soft state; however, the dramatic variability is seen in the power-law component. This spectral transition is opposite to the "canonical" high/soft-low/hard transitions seen in many Galactic black hole binaries. There is a possible contribution from an optically thin thermal plasma. When this component is taken into account, the spectral transition appears to be normal—a drop of the power-law flux and a slightly softer blackbody component in the low state.
The Astrophysical Journal 12/2008; 608(1):L57. · 6.02 Impact Factor
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Peter J. Brown,
Luc Dessart,
Stephen T. Holland,
Stefan Immler,
Wayne Landsman,
Stéphane Blondin,
Alexander J. Blustin,
Alice Breeveld, Gulab C. Dewangan,
Neil Gehrels,
Robert B. Hutchins,
Robert P. Kirshner,
Keith O. Mason,
Paolo A. Mazzali,
Peter Milne,
Maryam Modjaz,
and Peter W. A. Roming
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ABSTRACT: We report early photospheric-phase observations of the Type IIP supernova (SN) 2005cs obtained by the Swift ultraviolet-optical and X-ray telescopes. Observations started within 2 days of discovery and continued on a regular basis for 3 weeks. During this time the V-band magnitude remained essentially constant, while the UV was initially bright, but steadily faded until below the brightness of an underlying UV-bright H II region. This UV decay is similar to SNe II observed by the International Ultraviolet Explorer. UV grism spectra show the P Cygni absorption of Mg II 2798 Å, indicating a photospheric origin of the UV flux. Based on non-LTE model atmosphere calculations with the CMFGEN code, we associate the rapid evolution of the UV flux with the cooling of the ejecta, the peak of the spectral energy distribution (SED) shifting from ~700 Å on June 30 to ~1200 Å on July 5. Furthermore, the corresponding recombination of the ejecta, e.g., the transition from Fe III to Fe II, induces a considerable strengthening of metal line-blanketing at and above the photosphere, blocking more and more effectively this fading UV flux. SN 2005cs was not detected in the X-ray band, and the upper limit to the X-ray luminosity yields a limit to the mass-loss rate of the progenitor of 1 × 10-5 yr-1 (vw/10 km s-1). Overall, Swift represents a unique opportunity to capture the early and fast evolution of Type II SNe in the UV, providing additional constraints on the reddening, the SED shortward of 4000 Å, and the ionization state and temperature of the photon-decoupling regions.
The Astrophysical Journal 12/2008; 659(2):1488. · 6.02 Impact Factor
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ABSTRACT: This paper reports the discovery of a bright X-ray transient source, Suzaku J1305-4913, in the south-west arm of the nearby Seyfert II galaxy NGC 4945. It was detected at a 0.5 -- 10 keV flux of $2.2 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ during the Suzaku observation conducted on 2006 January 15 -- 17, but was undetectable in a shorter observation on 2005 August 22 --23, with an upper limit of $1.7 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (90% confidence level). At a distance of 3.7 Mpc, the bolometric luminosity of the source becomes $L_{\rm bol} = 4.4 \times 10^{39} \alpha$ erg s$^{-1}$, where $\alpha = (\cos 60^\circ / \cos i)$ and $i$ is the disk inclination. Therefore, the source is classified into so-called ultraluminous X-ray sources (ULXs). The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with the innermost accretion disk temperature of $T_{\rm in} = 1.69_{-0.05}^{+0.06}$ keV. During the 2006 January observation, it varied by a factor of 2 in intensity, following a clear correlation of $L_{\rm bol} \propto T_{\rm in}^4$. It is inferred that the innermost disk radius $R_{\rm in}$ stayed constant at $R_{\rm in} = 79_{-3.9}^{+4.0} \alpha^{1/2}$ km, suggesting the presence of a standard accretion disk. Relating $R_{\rm in}$ with the last stable orbit around a non-rotating black hole yields a rather low black hole mass, $\sim 9 \alpha^{1/2}$ solar masses, which would imply that the source is shining at a luminosity of $\sim3 \alpha^{1/2} $ times the Eddington limit. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with a mass of 20 -- 130 solar masses.
09/2007;
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Takeshi Itoh,
Chris Done,
Kazuo Makishima,
Grzegorz Madejski,
Hisamitsu Awaki,
Naoki Isobe,
Poshak Gandhi, Gulab C. Dewangan,
Richard E. Griffiths,
Naohisa Anabuki,
Takashi Okajima,
James Reeves,
Tadayuki Takahashi,
Yoshihiro Ueda,
Satohi Eguchi,
Tahir Yaqoob
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ABSTRACT: Suzaku observed a nearby Seyfert 2 galaxy NGC4945, which hosts one of the brightest active galactic nuclei above 20 keV. Combining data from the X-ray CCD camera (XIS) and the Hard X-ray Detector (HXD), the AGN intrinsic nuclear emission and its reprocessed signals were observed simultaneously. The intrinsic emission is highly obscured with an absorbing column of $\sim 5 \times 10^{24}$ cm$^{-2}$, and was detectable only above $\sim 10$ keV. The spectrum below 10 keV is dominated by reflection continuum and emission lines from neutral/ionized material. Along with a neutral iron K$\alpha$ line, a neutral iron K$\beta$ and a neutral nickel K$\alpha$ line were detected for the first time from this source. The neutral lines and the cold reflection continuum are consistent with both originating in the same location. The Compton down-scattered shoulder in the neutral Fe-K$\alpha$ line is $\sim 10%$ in flux of the narrow core, which confirms that the line originates from reflection rather than transmission. The weakness of the Compton shoulder also indicates that the reflector is probably seen nearly edge-on. Flux of the intrinsic emission varied by a factor of $\sim 2$ within $\sim 20$ ks, which requires the obscuring material to be geometrically thin. Broadband spectral modeling showed that the solid angle of the neutral reflector is less than a few $\times 10^{-2} \times 2\pi$. All this evidence regarding the reprocessed signals suggests that a disk-like absorber/reflector is viewed from a near edge-on angle.
09/2007;
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James N. Reeves,
Hisamitsu Awaki, Gulab C. Dewangan,
Andy C. Fabian,
Yasushi Fukazawa,
Luigi Gallo,
Richard Griffiths,
Hajime Inoue,
Hideyo Kunieda,
Alex Markowitz, [......],
Richard Mushotzky,
Takashi Okajima,
Andy Ptak,
Tadayuki Takahashi,
Yuichi Terashima,
Masayoshi Ushio,
Shin Watanabe,
Tomonori Yamasaki,
Makoto Yamauchi,
Tahir Yaqoob
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ABSTRACT: We report on a 100 ks Suzaku observation of the bright, nearby (z=0.008486) Seyfert 1.9 galaxy MCG -5-23-16. The broad-band (0.4-100 keV) X-ray spectrum allows us to determine the nature of the high energy emission with little ambiguity. The X-ray continuum consists of a cutoff power-law of photon index $\Gamma=1.9$, absorbed through Compton-thin matter of column density $N_{\rm H}=1.6\times10^{22}$ cm$^{-2}$. A soft excess is observed below 1 keV and is likely a combination of emission from scattered continuum photons and distant photoionized gas. The iron K line profile is complex, showing narrow neutral iron K$\alpha$ and K$\beta$ emission, as well as a broad line which can be modeled by a moderately inclined accretion disk. The line profile shows either the disk is truncated at a few tens of gravitational radii, or the disk emissivity profile is relatively flat. A strong Compton reflection component is detected above 10 keV, which is best modeled by a combination of reflection off distant matter and the accretion disk. The reflection component does not appear to vary. The overall picture is that this Seyfert 1.9 galaxy is viewed at moderate (50 degrees) inclination through Compton-thin matter at the edge of a Compton-thick torus covering $2\pi$ steradians, consistent with unified models.
11/2006;
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ABSTRACT: We report the first clear evidence for the simultaneous presence of a low frequency break and a QPO in the fluctuation power spectrum of a well known ultraluminous X-ray source (ULX) in M82 using long XMM-Newton observations. The break occurs at a frequency of 34.2_{-3}^{+6}mHz. The QPO has a centroid at 114.3\pm1.5mHz, a coherence Q~3.5 and an amplitude (rms) of 19% in the 2-10keV band. The power spectrum is approximately flat below the break frequency and then falls off above the break frequency as a power law with the QPO superimposed. This form of the power spectrum is characteristic of the Galactic X-ray binaries (XRBs) in their high or intermediate states. M82 X-1 was likely in an intermediate state during the observation. The EPIC PN spectrum is well described by a model comprising an absorbed power-law (Gamma~2) and an iron line at ~6.6keV with a width sigma~0.2keV and an equivalent width of ~180eV. Using the well established correlations between the power and energy spectral parameters for XRBs, we estimate a black hole mass for M82 X-1 in the range of 25-520Msun including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation. Comment: Final version, accepted for publication in ApJ Letters
09/2005;
