EPIC-pn Spectrum Research Articles

The GRB 011211 X-Ray Lines: Toward Resolving the Significance Debate

We demonstrate for the GRB 011211 X-ray afterglow lines that the significance estimation methods used by Reeves et al. and Rutledge & Sako are consistent and that the conflicting estimates found by those authors arise from different assumptions in the continuum modeling. If the column density is fixed at the Galactic value as in the Reeves et al. analysis, then both methods agree that the lines are significant at the ~3 σ level, in agreement with the original claim of Reeves et al. If we allow the column density to vary, values in excess of the Galactic column density are preferred by the data, and the lines are found from both methods to be ~1.5 σ significant, in agreement with the claim of Rutledge & Sako. We argue that the column density parameter should be fixed at the Galactic value when analyzing the portion (first 5 ks) of the XMM-Newton EPIC-PN spectrum containing the potential emission lines, because the full 25 ks observation is well modeled assuming Galactic absorption. The presence of cold gas local to the gamma-ray burst, which would be required to produce a time-variable NH, is unlikely. We also discuss the case for emission lines for GRB 030227, for which the value of NH is not critical.

TheXMM-Newtonview of Mrk 3 and IXO 30

We present the analysis of the XMM-Newton EPIC pn spectrum of the Seyfert 2 galaxy, Mrk 3. We confirm that the source is dominated by a pure Compton reflection component and an iron Kα line, both produced as reflection from a Compton-thick torus, likely responsible also for the large column density (1.36 +0.03 0.04 × 10 24 cm 2 ) which is pierced by the primary powerlaw only at high energies. A low inclination angle and an iron underabundance of a factor ≃ 0.82, suggested by the amount of reflection and the depth of the iron edge, are consistent with the iron Kα line EW with respect to the Compton reflection component, being 610 +30 50 eV. Moreover, the iron line width, σ = 32 +13 14 eV, if interpreted in terms of Doppler broadening due to the Keplerian rotation of the torus, puts an estimate to the inner radius of the latter, r = 0.6 +1.3 0.3 sin 2 i pc. Finally, two different photoionised reflectors are needed to take into account a large number of soft X-ray emission lines from N, O, Ne, Mg, Si, Fe L and the Fe xxv emission line at 6.71 +0.03

Integral field spectroscopy of the ultraluminous X-ray source Holmberg II X-1

We present optical integral field observations of the H II region containing the ultraluminous X-ray source Holmberg II X-1. We confirm the existence of an X-ray ionized nebula as the counterpart of the source owing to the detec- tion of an extended He II λ4686 region (21 × 47 pc) at the Chandra ACIS-S position. An extended blue object with a size of 11 × 14 pc is coincident with the X-ray/He II λ4686 region, which could indicate that it is either a young stellar complex or a cluster. We have derived an X-ray to optical luminosity ratio of LX/LB ≥ 170, and presumable it is LX/LB ∼ 300−400 using the recent HST ACS data. We find a complex velocity dispersion at the position of the ULX. In addition, there is a radial velocity variation in the X-ray ionized region found in the He II emission of ±50 km s −1 on spatial scales of 2-3 �� . We believe that the pu- tative black hole not only ionizes the surrounding HII gas, but also perturbs it dynamically (via jets or the accretion disk wind). The spatial analysis of the public Chandra ACIS-S data reveals a point-like X-ray source and gives marginal indication of an extended component (� 15% of the total flux). The XMM-Newton EPIC-PN spectrum of HoII X-1 is best fitted with an absorbed power law in addition to either a thermal thick plasma or a thermal thin plasma or a multi-colour disk black body (MCD). In all cases, the thermal component shows a relatively low temperature (kT ∼ 0.14−0.22 keV). Finally we discuss the optical/X-ray properties of HoII X-1 with regards to the possible nature of the source. The existence of an X-ray ionized nebula coincident with the ULX and the soft X-ray component with a cool accretion disk favours the interpretation as an intermediate-mass black hole (IMBH). However, the complex velocity behaviour at the position of the ULX indicates a dynamical influence of the black hole on the local HII gas.

Discovery of absorption features of the accretion disc corona and systematic acceleration of the X-ray burst rate in XB 1323-619

We present results from analysis of the observation of the dipping, quasi-periodic bursting low mass X-ray binary XB 1323-619 made with XMM-Newton in 2003 January. In spectral analysis of the non-dip, non-burst EPIC PN spectrum, a number of absorption lines were discovered, notably at 6.70 and 6.98 keV, which we identify with scattering by high ionization state ions Fe xxv and Fe xxvi. Such features have been seen in other dipping sources, but their origin was not understood. Curve of growth analysis provided a consistent solution in which the line ratio was reproduced assuming collisional ionization with kT = 31 keV, close to the electron temperature we previously determined for the accretion disc corona (ADC) in this source. We thus propose that the absorption lines in the dipping low mass X-ray binaries (LMXB) are produced in the ADC. Spectral evolution in dipping was well described by the progressive covering model, which we have previously shown to give very good explanations of many dipping sources. We discuss the proposal of Boirin et al. (2004b), based on analysis of the present observation, that spectral evolution in all of the dipping LMXB may be explained by subjecting the continuum to a highly ionized absorber. This would require a decrease in X-ray intensity by a factor of ∼3 in dipping at energies where photoelectric absorption is not effective (40-100 keV), which previous BeppoSAX analysis of several sources firmly rules out. In XB 1323-619, any decrease was less than 10 ± 10 per cent and so the ionized absorber proposal can be ruled out. We find a remarkable linear increase in the rate of X-ray bursts with time over the 14-yr period since 1989 and a systematic non-linear increase in source luminosity (L). The linear variation of burst rate with L shows that the burst rate is proportional to mass accretion rate and if continued implies that the gap between bursts will become zero on 2008 January 11. In reality, we expect the source to undergo a transition from X-ray bursting to X-ray flaring, which would confirm the suggestion we previously made that flaring is unstable nuclear burning effectively consisting of a superposition of X-ray bursts.

Revealing a Cool Accretion Disk in the Ultraluminous X‐Ray Source M81 X‐9 (Holmberg IX X‐1): Evidence for an Intermediate‐Mass Black Hole

We report the results of an analysis of two XMM-Newton EPIC-pn spectra of the bright ultraluminous X-ray source M81 X-9 (Holmberg IX X-1) obtained in snapshot observations. Soft thermal emission is clearly revealed in spectra dominated by hard power-law components. Depending on the model used, M81 X-9 was observed at a luminosity of LX = × 1040 ergs s-1 (0.3-10.0 keV). The variability previously observed in this source signals that it is an accreting source that likely harbors a black hole. Remarkably, accretion disk models for the soft thermal emission yield very low inner disk temperatures (kT = 0.17-0.29 keV, including 90% confidence errors and variations between observations and disk models) and improve the fit statistic over any single-component continuum model at the 6 σ level of confidence. This represents much stronger evidence for a cool disk than prior evidence that combined spectra from different observatories, and the strongest evidence of a cool disk in an ultraluminous X-ray source apart from that for NGC 1313 X-1. In common with NGC 1313 X-1, scaling the temperatures measured in M81 X-9 to those commonly seen in stellar-mass Galactic black holes at their highest observed fluxes (kT 1 keV) may imply that M81 X-9 harbors a black hole with a mass on the order of 103 M☉; the measured disk component normalization and broadband luminosity imply black hole masses on the order of 102 M☉. It is therefore possible that these sources harbor 103 M☉ black holes accreting at LX 0.1 × LEdd. While these results do not represent proof that M81 X-9 harbors an intermediate-mass black hole, radio and optical observations suggest that beaming and anisotropic emission from a stellar-mass black hole are unlikely to account for the implied luminosity. We further argue that the strength of the hard emission in these sources and well-established phenomena frequently observed in stellar-mass black holes near to the Eddington limit suggest that optically thick photospheres are unlikely to be the origin of the cool thermal emission in bright ultraluminous X-ray sources. For comparison to M81 X-9, we have also analyzed the previously unpublished EPIC-pn spectrum of NGC 1313 X-1; cool disk emission is again observed, and refined spectral fit parameters and mass estimates are reported.

The XMM-Newton view of NGC 6251

We report on the nuclear X-ray properties of the radio galaxy NGC 6251 observed with XMM-Newton . NGC 6251 is a well-known radio galaxy with intermediate FRI/II radio properties. It is optically classified as a Seyfert 2 and hosts a supermassive black hole with mass ~$ 6\times10^8~ M_{\odot}$. The 0.4–10 keV EPIC pn continuum is best fitted by two thermal components ($kT \sim 0.5$ and 1.4 keV, respectively), plus a power law with photon index $\Gamma \sim 1.9$ absorbed by a column density $N_{\rm H} \sim 5 \times 10^{20}$ cm -2 . We confirm the previous ASCA detection of a strong iron line. The line, resolved in the EPIC pn spectrum, is adequately fitted with a broad ($\sigma\sim 0.6$ keV) Gaussian at rest-frame energy 6.4 keV with EW ~ 220 eV. We also detect, for the first time, short-term, low-amplitude variability of the nuclear flux on a timescale of a few ks. The spectral properties argue in favor of the presence of a standard accretion disk, ruling out the base of the jet as the sole origin of the X-rays. The moderate X-ray luminosity and lack of strong intrinsic absorption suggest that NGC 6251 is a “pure” type 2 AGN which lacks a broad-line region.

XMM-Newton observations of MR Vel/RX J0925.7-4758

We report on XMM-Newton observations of the galactic supersoft X-ray source RX J0925.7-4758. The RGS spectrum exhibits a wealth of spectral features from iron and oxygen. XMM-Newton data confirm the finding of previous Chandra HETGS/MEG observations that NLTE models of hot white dwarf atmospheres fail to represent the complex spectrum. There are clear evidences for P Cygni profiles with wind velocities of up to 2000 km/s. Small flux variations with time scales larger than 1000s are present. The strongest power is at ~ 0.21d, a period close to that seen in V band optical light curves. A detailed analysis of the associated changes in the RGS and EPIC pn spectra hint at a mostly grey mechanism suggesting a variation of the visibility of the white dwarf due to occulting material in the accretion disk. Finally, we detect radial velocity changes of 173 +/- 47 km/s between two RGS observations obtained half an orbital cycle apart. The amplitude of the RGS velocity shift is consistent with that of the optical He II 4686 and thus supports the idea that most of the He II optical line emission arises from the accretion disk.

Discovery of pulsations from the Be/X-ray binary RXJ0101.3-7211 in the SMC by XMM-Newton

We report pulsations in the X-ray flux of RX J0101.3 7211 in the Small Magellanic Cloud (SMC) with ap eriod of (455 2) s in XMM-Newton EPIC-PN data. The X-ray spectrum can be described by a power-law with a photon index of 0:6 0:1. Timing analysis of ROSAT PSPC and HRI archival data conrms the pulsations and indicates a period increase of 5 s since 1993. RX J0101.3 7211 varied in brightness during the ROSAT observations with timescales of years with a maximum unabsorbed flux of 6 10 13 erg cm 2 s 1 (0.1{2.4 keV). The flux during the XMM-Newton observation in the ROSAT band was lower than during the faintest ROSAT detection. The unabsorbed luminosity derived from the EPIC-PN spectrum is 2 10 35 erg s 1 (0.2{10.0 keV) assuming a distance of 60 kpc. Optical spectra of the proposed counterpart taken at the 2.3 m telescope of MSSSO in Australia in August 2000 show strong H emission and indicate a Be star. The X-ray and optical data conrm RX J0101.3 7211 as a Be/X-ray binary pulsar in the SMC.