K. A. Pounds

The Signature of Supernova Ejecta Measured in the X-ray Afterglow of Gamma-Ray Burst 011211

Now that γ-ray bursts (GRBs) have been determined to lie at cosmological distances, their isotropic burst energies are estimated to be as high as 1054 erg (ref. 2), making them the most energetic phenomena in the Universe. The nature of the progenitors responsible for the bursts remains, however, elusive. The favoured models range from the merger of two neutron stars in a binary system to the collapse of a massive star. Spectroscopic studies of the afterglow emission could reveal details of the environment of the burst, by indicating the elements present, the speed of the outflow and an estimate of the temperature. Here we report an X-ray spectrum of the afterglow of GRB011211, which shows emission lines of magnesium, silicon, sulphur, argon, calcium and possibly nickel, arising in metal-enriched material with an outflow velocity of the order of one-tenth the speed of light. These observations strongly favour models where a supernova explosion from a massive stellar progenitor precedes the burst event and is responsible for the outflowing matter.

Correlated hard X-ray and ultraviolet variability in NGC 5548

Contemporaneous X-ray (from the Ginga satellite) and ultraviolet (IUE) observations of NGC 5548 in 1989-1990 show that the 1350 A and 2-10 keV flux both varied by a factor of 2.6 in a highly correlated fashion. Adding an additional simultaneous IUE-Exosat observation from 1984 significantly weakens the correlation, since the UV flux was 3.5 times larger then than its average 1990 value, while the X-ray flux was only 70 percent higher. In 1990 the UV and X-ray flux variations appear to be simultaneous to within no more than six days. These results, together with the simultaneity of the variations at 5000 and 1350 A, are incompatible with the standard geometrically thin accretion model. They may be explained, however, if part of the UV and optical radiation originates from reprocessing of X-rays emitted above the disk. Thus essentially all the UV flux in 1990 would be due to reprocessing. But the large UV outburst in 1984 probably represents a genuine increase in the disk throughput due to instability in the disk itself. The similar relationship between UV and X-ray emission in NGC 4151 suggests that this scheme may be applicable to Seyfert galaxies as a class.

The unusual X-ray and optical properties of the ultrasoft active galactic nucleus Zwicky 159.034 (RE J1237+264)

Zwicky 159.034, one of the Seyfert galaxies identified with EUV sources detected during the ROSAT Wide Field Camera (WFC) all-sky survey, has unusual properties. The ROSAT Position Sensitive Proportional Counter (PSPC) 0.1--2.5 keV X-ray spectrum, obtained simultaneously with the WFC survey, appears extremely steep. Subsequent deeper pointed observations with ROSAT revealed that its 0.1--2.5 keV count rate had decreased by an extremely large amount (a factor of about 70). This is comparable to the variability amplitude seen in another ultrasoft Seyfert, E1615+061. There appears to be about 10 times as much flux in the soft component as in any expected hard tail suggesting that, in the absence of partial covering of the hard flux, the soft flux cannot arise via reprocessing of the hard tail. Its peculiar optical spectrum has permitted lines with widths about 1200--1500 km s^{-1}, and forbidden lines are weak or absent. Its optical spectrum shows evidence for variability, and might be used to study the role reprocessed X-rays play in the formation of optical lines.

Reddening, Emission-Line, and Intrinsic Absorption Properties in the Narrow-Line Seyfert 1 Galaxy Arakelian 564*

We use Hubble Space Telescope UV and optical spectra of the narrow-line Seyfert 1 (NLS1) galaxy Ark 564 to investigate its internal reddening and properties of its emission-line and intrinsic UV absorption gas. We find that the extinction curve of Ark 564, derived from a comparison of its UV/optical continuum to that of an unreddened NLS1, lacks a 2200 ? bump and turns up toward the UV at a longer wavelength (4000 ?) than the standard Galactic, LMC, and SMC curves. However, it does not show the extremely steep rise to 1200 ? that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The emission lines and continuum experience the same amount of reddening, indicating the presence of a dust screen that is external to the narrow-line region. Echelle spectra from the Space Telescope Imaging Spectrograph show intrinsic UV absorption lines due to Ly?, N V, C IV, Si IV, and Si III, centered at a radial velocity of -190 km s-1 (relative to the host galaxy). Photoionization models of the UV absorber indicate that it has a sufficient column (NH = 1.6 ? 1021 cm-2) and is at a sufficient distance from the nucleus (D > 95 pc) to be the source of the dust screen. Thus, Ark 564 contains a dusty lukewarm absorber similar to that seen in NGC 3227.

X-ray observations of the warm absorber in NGC 3783

We present a high signal-to-noise ROSAT position sensitive proportional counter (PSPC) observation of the Seyfert 1 galaxy NGC 3783. The 0.1-2.5 keV spectrum shows strong evidence for an absorption edge most likely associated with O VII-VIII. Such a feature, which has now been detected in a number of PSPC spectra (Nandra et al.; Nandra & Pounds; Turner et al.), is the signature of highly ionized material, the so-called warm absorber, in the line of sight to the X-ray source. The result is strongly supported by analysis of previously unpublished Ginga data and reanalysis of a 1985 EXOSAT observation of NGC 3783

XMM-Newton Observations of Red AGNs

XMM-Newton spectra of five red 2MASS active galactic nuclei (AGNs), selected from a sample observed by Chandra to be relatively X-ray bright and to cover a range of hardness ratios, confirm the presence of substantial absorbing material in three sources with optical classifications ranging from type 1 to type 2. A flat (hard) power-law continuum is observed in the other two. The combination of X-ray absorption and broad optical emission lines suggests either a small (nuclear) absorber or a favored viewing angle so as to cover the X-ray source but not the broad emission-line region (BELR). A soft excess is detected in all three type 1 sources. We speculate that this may arise in an extended region of ionized gas, perhaps linked to the polarized (scattered) optical light present in these sources. The spectral complexity revealed by XMM-Newton emphasizes the limitations of the low-S/N Chandra data. The new results strengthen our earlier conclusions that the observed X-ray continua of red AGNs are unusually hard at energies 2 keV. Their observed spectra are consistent with contributing significantly to the missing hard or absorbed population of the cosmic X-ray background (CXRB), although their intrinsic power-law slopes are typical of broad-line (type 1) AGNs (Γ ~ 1.7-1.9). This suggests that the missing X-ray-absorbed CXRB population may include type 1 AGNs or QSOs in addition to the type 2 AGNs generally assumed.

Soft X-ray emission lines in the afterglow spectrum of GRB 011211: A detailed XMM-Newton analysis

We report on an XMM-Newtonobservation of the X-ray afterglow of the Gamma Ray Burst GRB 011211, originally detected by Beppo-SAX on 11th December 2001. The early afterglow spectrum obtained by XMM-Newton, observed 11 hours after the initial burst, appeared to reveal decaying H-like K emission lines of Mg, Si, S, Ar and Ca, arising in enriched material with an outflow velocity of order 0.1c (Reeves et al. 2002). This was attributed to matter ejected from a massive stellar progenitor occurring shortly before the burst itself. Here, we present a detailed re-analysis of the XMM-NewtonEPIC observations of GRB 011211. In particular, we show that the detection of the soft X-ray line emission appears robust, regardless of detector background, calibration, spectral binning, or the spectral model that is assumed. We demonstrate that thermal emission, from an optically thin plasma, is the most plausible model that can account for the soft X-ray emission, which appears to be the case for at least two burst afterglow spectra observed by XMM-Newton. The X-ray spectrum of GRB 011211 appears to evolve with time after the first 10 ks of the XMM-Newtonobservation as the Si and S emission lines are only detected during the first 10 ks of observation. The observations suggest that thermal emission is present during the early afterglow spectrum, whilst a power-law component dominates the latter stages. Finally we estimate the mass of the ejected material in GRB 011211 to be of the order 4-20 solar masses.

The variable XMM-Newton spectrum of Markarian 766

The narrow-line Seyfert 1 galaxy Mrk 766 was observed for GO ks with the XMM-Newton observatory. The source shows a complex X-ray spectrum. The 2-10 keV spectrum can be adequately represented by a power law and broad Fe Ka emission. Between 0.7 and 2 keV the spectrum is harder and exhibits a flux deficit with respect to the extrapolated medium energy slope. Below 0.7 keV, however, there is a strong excess of emission. The RGS spectrum shows an edge-like feature at 0.7 keV; the energy of this feature is inconsistent with that expected for an OVII edge from a warm absorber, Mrk 766 varies by a factor of similar to2 in overall count rate in the EPIC and RGS instruments on a timescale of a few thousand seconds, while no significant flux changes are observed in the ultraviolet with the Oh I. The X-ray variability is spectrally dependent with the largest amplitude variability occurring ill the 0.4-2 keV band. The spectral variability can be explained by a change in flux and slope of the medium energy continuum emission, superimposed on a less variable (or constant) low energy emission component.

The X-ray spectrum of A0620—00

A REMARKABLE change in the X-ray spectrum of the transient source A0620—00 (ref. 1) was observed by the Ariel V Sky Survey Experiment (SSE) during the rise to maximum brightness, The SSE detector being used has an effective area (behind its field collimator) of 290 cm2 and covers the energy band 2–18 keV in 8 channels. The channel boundaries are marked in Fig. 1, which shows sample pulse height spectra taken as the source intensified. These raw data have not been corrected for the detector efficiency and at this stage include no input spectral assumptions. Two points emerge clearly: when first detected the X-ray spectrum of A0620—00 was hard and remained so as the overall intensity rose to the precursor peak and, secondly, the subsequent rise to maximum intensity was entirely due to a strong enhancement of X rays below 10 keV, giving a marked softening in the overall spectrum. In fact, the observed flux above 10 keV actually fell during this period.

Comparison of high- and low-state X-ray spectra in the type 1.5 quasi-stellar object 2MASS 0918+2117

When observed by XMM-Newton in 2003 the type 1.5 QSO 2MASS 0918+2117 was found to be in a low state, with an X-ray flux ∼4-5 times fainter than during an earlier Chandra observation. The 2-6 keV spectrum was unusually hard (photon index Γ∼1.25), with evidence for a reflection-dominated continuum, while a soft excess visible below ∼1 keV prevented confirmation of the anticipated low energy absorber (Wilkes et al. 2005). In a second XMM-Newton observation in 2005 the X-ray flux is found to have recovered, with a 2-10 keV continuum spectrum now typical of a broadline active galaxy (Γ∼2) and a deficit of flux below ∼1 keV indicative of continuum absorption in a column NH∼4 × 10 21 cm. We find the preferred ionisation state of the absorbing gas to be low, which then leaves a residual soft excess of similar spectral form and flux to that found in the 2003 XMM-Newton observation. Although observed at different epochs we note that dust in the absorbing column could also explain the red nucleus and strong optical polarisation of 2MASS 0918+2117.