arXiv subjects

J. N. Reeves

Publications and source records attributed to J. N. Reeves.

At least 127 records · Page 7Linked to original sources

arXiv2005

Discovery of an Afterglow Extension of the Prompt Phase of Two Gamma Ray Bursts Observed by Swift

BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, show clearly a prompt component joining the onset of the afterglow emission. By fitting a power law form to the gamma-ray spectrum, we extrapolate the time dependent fluxes measured by the BAT, in the energy band 15-350 keV, into the spectral regime observed by the XRT 0.2-10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to ~300 s, followed by a flatter decay. The duration of the steep decay, ~100 s in the source frame after correcting for cosmological time dilation, agrees roughly with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where one e-folding decay time t_e(BAT) ~ 24+/-2 s, and t_e(XRT) ~ 35+/-2 s. For GRB 050319, a power law decay -(d ln f/d ln t) = n, where n is about 3, provides a reasonable fit. The early time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission, and provide our first quantitative measure of the decay of the prompt gamma-ray emission over a large dynamic range in flux. The initial steep decay is expected due to the delayed high latitude photons from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs remains observable for hundreds of seconds longer than previously thought.

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arXiv2005

New insights into ultraluminous X-ray sources from deep XMM-Newton observations

The controversy over whether ultraluminous X-ray sources (ULXs) contain a new intermediate-mass class of black holes (IMBHs) remains unresolved. We present new analyses of the deepest XMM-Newton observations of ULXs that address their underlying nature. We examine both empirical and physical modelling of the X-ray spectra of a sample of thirteen of the highest quality ULX datasets, and find that there are anomalies in modelling ULXs as accreting IMBHs with properties simply scaled-up from Galactic black holes. Most notably, spectral curvature above 2 keV in several sources implies the presence of an optically-thick, cool corona. We also present a new analysis of a 100 ks observation of Holmberg II X-1, in which a rigorous analysis of the temporal data limits the mass of its black hole to no more than 100 solar masses. We argue that a combination of these results points towards many (though not necessarily all) ULXs containing black holes that are at most a few 10s of solar mass in size.

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arXiv2005

A deep XMM-Newton observation of the ultraluminous X-ray source HoII X-1 : the case against a 1000 solar mass black hole

We present results from a 112 ks long look by XMM-Newton at the ultraluminous X-ray source (ULX) Holmberg II X-1, long thought to be the one of best candidates for the missing class of intermediate mass black holes (IMBHs). Our data comprises the first high quality XMM-Newton/RGS spectrum of an ULX, and an XMM-Newton/EPIC spectrum with unprecedented signal-to-noise. A detailed timing analysis shows that any variability on time-scales of minutes to hours is very weak (< few per cent fractional rms), though larger amplitude variations on much shorter time-scales could be hidden by photon counting statistics. This result suggests that if Ho II X-1 harbours an IMBH, then we are observing this source in a highly unusual and atypical state when compared with the known variability behaviour of other accreting systems of large mass. Moreover unlike Galactic X-ray binaries, our spectral analysis indicates the possible presence of an optically-thick low temperature corona. Taken together our timing and spectral analysis suggests that the compact companion is most-likely a high luminosity analogue of black hole binary systems similar to GRS 1915+105, the Galactic microquasar, harbouring a compact object of mass no greater than 100 solar masses.

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arXiv2005

The soft X-ray blast in the apparently sub-luminous GRB 031203

GRB031203 was a very low apparent luminosity gamma-ray burst (GRB). It was also the first GRB with a dust-scattered X-ray halo. The observation of the halo allowed us to infer the presence of a large soft X-ray fluence in the total burst output. It has, however, also been claimed that GRB031203 was intrinsically sub-energetic, representative of a class of spectrally hard, low-energy bursts quite different from other GRBs. Reanalysis of the available data confirms our original finding that GRB031203 had a very large soft X-ray component, the time of which can be constrained to within a few minutes after the burst, implying that while GRB031203 did indeed have a very low apparent luminosity, it was also very soft. Notions propagated in the literature regarding the uncertainties in the determination of the soft X-ray fluence from the halo data and on the available constraints from the hard X-ray data are addressed: the properties of the scattering dust along the line of sight (grain sizes, precise location and the geometry) are determined directly from the high quality X-ray data so that there is little uncertainty about the scatterer; constraints on the X-ray lightcurve from the Integral spacecraft at the time of the soft X-ray blast are not complete because of a slew in the spacecraft pointing shortly after the burst. Claims that GRB031203 was intrinsically under-energetic and that it represents a deviation from the luminosity-peak energy relation do not appear to be substantiated by the data, regardless of whether the soft X-ray component is declared part of the prompt emission or the afterglow. We conclude that the difference between the soft and hard X-ray spectra from XMM-Newton and Integral indicate that a second soft pulse probably occurred in this burst as has been observed in other GRBs, notably GRB050502B.

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arXiv2005

Evidence for Gravitational Infall of Matter onto the Super-Massive Black Hole in the quasar PG 1211+143?

We report the detection of redshifted iron K-alpha absorption lines in the Chandra LETG spectrum of the narrow-line quasar, PG 1211+143. The absorption lines are observed at 4.22 keV and 4.93 keV in the quasar spectrum, corresponding to 4.56 keV and 5.33 keV in the rest frame of PG 1211+143. From Monte Carlo simulations, the chance probability of both lines being false detections is low at 1.36e-04. Highly redshifted ionized iron K-alpha (Fe XXV or Fe XXVI) is the most plausible identification for the lines at their observed energies. If identified with H-like iron K-alpha at 6.97 keV, then the relativistic velocity shifts required are 0.40c and 0.26c. The extreme velocities can be explained by pure gravitational redshift if the matter exists in a stable orbit within 6 gravitational radii of the black hole. This would require a Kerr metric for the black hole. Alternatively the absorption may be the result of matter infalling directly onto the black hole, with a maximum observed velocity of 0.38c at 6Rg in the Schwarzschild metric. This matter may originate in a failed outflow or jet, which does not escape the gravitational potential of the black hole.

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arXiv2005

XMM-Newton spectroscopy of high-redshift QSOs

XMM-Newton observations of 29 high redshift (z>2) quasars, including seven radio-quiet, 16 radio-loud and six Broad Absorption Line (BAL) objects, are presented; due to the high redshifts, the rest-frame energy bands extend up to \~30-70 keV. Over 2-10 keV, the quasars can be well fitted in each case by a simple power-law, with no strong evidence for iron emission lines. The lack of iron lines is in agreement both with dilution by the radio jet emission (for the radio-loud quasars) and the X-ray Baldwin effect. No Compton reflection humps at higher energies (i.e., above 10 keV in the rest frame) are detected either. Over the broad-band (0.3-10 keV), approximately half (nine out of 16) of the radio-loud quasars are intrinsically absorbed, with the values of N_H generally being 1-2 x 10^22 cm^-2 in the rest frames of the objects. None of the seven radio-quiet objects shows excess absorption, while four of the six BAL quasars are absorbed. The radio-loud quasars have flatter continuum slopes than their radio-quiet counterparts (Gamma_RL ~ 1.55; Gamma_RQ ~ 1.98 over 2-10 keV), while, after modelling the absorption, the underlying photon index for the six BAL quasars is formally consistent with the non-BAL radio-quiet objects.

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arXiv2005

Evidence for Orbital Motion of Material Close to the Central Black Hole of Mrk 766

Time-resolved X-ray spectroscopy has been obtained for the narrow line Seyfert galaxy Mrk766 from XMM-Newton observations. We present analysis in the energy-time plane of EPIC pn data in the 4-8 keV band with energy resolution R~50. A component of Fe Ka emission detected in the maps shows a variation of photon energy with time that appears both to be statistically significant and to be consistent with sinusoidal variation. We investigate the interpretation that there exists a component of line emission from matter in a Keplerian orbit around a supermassive black hole. The orbit has a period ~165 ks and a line-of-sight velocity ~13,500 km/s. This yields a lower limit for the central mass of M > 4.9x10^5 solar masses within a radius of 3.6 x 10^13 cm (2.4 A.U.). The orbit parameters are consistent with higher black hole masses, but the lack of any substantial gravitational redshift of the orbit implies an upper limit to the black hole mass of 4.5x10^7 solar masses.

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arXiv2005

Chandra High-Resolution X-ray Spectroscopy of the Fe K Line in the Seyfert 1 Galaxy NGC 3783

We report on the results of detailed X-ray spectroscopy of the Fe K region in the Seyfert 1 galaxy NGC 3783 from five ~170 ks observations with the Chandra high energy gratings. Monitoring was conducted over an interval of ~125 days in 2001. The combined data constitute the highest signal-to-noise Fe K spectrum having the best velocity resolution in the Fe K band to date (FWHM ~1860 km/s). The data show a resolved Fe K line core with a center energy of 6.397 +/- 0.003 keV, consistent with an origin in neutral or lowly ionized Fe, located between the BLR and NLR, as found by Kaspi et al. (2002). We also find that excess flux around the base of the Fe K line core can be modeled with either a Compton scattering ``shoulder'' or an emission line from a relativistic accretion disk, having an inclination angle of 11 degrees or less. This disk line model is as good as a Compton-shoulder model for the base of the Fe K line core. In the latter model, the column density is 7.5 [+2.7,-0.6] x 10^{23} cm^{-2}, which corresponds to a Thomson optical depth of ~0.60. An intrinsic width of 1500 [+460,-340] km/s FWHM is still required in this model. Moreover, more complicated scenarios involving both a Compton-shoulder and a disk line cannot be ruled out. We confirm an absorption feature due to He-like Fe (FWHM = 6405 [+5020,-2670] km/s), found in previous studies.

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arXiv2005

A changing inner radius in the accretion disc of Q0056-363?

Q0056-363 is the most powerful X-ray quasar known to exhibit a broad, likely relativistic iron line (Porquet & Reeves 2003). It has been observed twice by XMM-$Newton$, three and half years apart (July 2000 and December 2003). In the second observation, the UV and soft X-ray fluxes were fainter, the hard X-ray power law flatter, and the iron line equivalent width (EW) smaller than in the 2000 observation. These variations can all be explained, at least qualitatively, if the disc is truncated in the second observation. We report also on the possible detection of a transient, redshifted iron absorption line during the 2003 observation.

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arXiv2004

Complex X-ray Absorption and the Fe Kalpha Profile in NGC 3516

We present data from simultaneous Chandra, XMM-Newton and BeppoSAX observations of the Seyfert 1 galaxy NGC 3516, taken during 2001 April and Nov. We have investigated the nature of the very flat observed X-ray spectrum. Chandra grating data show the presence of X-ray absorption lines, revealing two distinct components of the absorbing gas, one which is consistent with our previous model of the UV/X-ray absorber while the other, which is outflowing at a velocity of ~1100 km/s has a larger column density and is much more highly ionized. The broad-band spectral characteristics of the X-ray continuum observed with XMM during 2001 April, reveal the presence of a third layer of absorption consisting of a very large column (~2.5 x 10E23 cm^-2) of highly ionized gas with a covering fraction ~50%. This low covering fraction suggests that the absorber lies within a few lt-days of the X-ray source and/or is filamentary in structure. Interestingly, these absorbers are not in thermal equilibrium with one another. The two new components are too highly ionized to be radiatively accelerated, which we suggest is evidence for a hydromagnetic origin for the outflow. Applying our model to the Nov dataset, we can account for the spectral variability primarily by a drop in the ionization states of the absorbers, as expected by the change in the continuum flux. When this complex absorption is accounted for we find the underlying continuum to be typical of Seyfert 1 galaxies. The spectral curvature attributed to the high column absorber, in turn, reduces estimates of the flux and extent of any broad Fe emission line from the accretion disk.

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arXiv2004

An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?

We report on a short XMM-Newton observation of the radio-quiet Narrow Line Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong excess of counts between 6-9 keV in the rest frame. This feature can be modeled by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV appears blue-shifted with respect to the iron Kalpha emission band between 6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar rest frame. The line profile can be fitted by reflection from the inner accretion disk, but an inclination angle of >60 deg is required to model the extreme blue-wing of the line. Furthermore the extreme strength of the line requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2 keV is dominated by the pure-reflection component from the disk, while little or none of the direct hard power-law is observed. Alternatively the spectrum above 2 keV may instead be explained by an ionized absorber, if the column density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV. This absorber could originate in a large column density, high velocity outflow, perhaps similar to those which appear to be observed in several other high accretion rate AGN. Further observations, especially at higher spectral resolution, are required to distinguish between the accretion disk reflection or outflow scenarios.

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arXiv2004

A high velocity ionised outflow and XUV photosphere in the narrow emission line quasar PG1211+143

We report on the analysis of a ~60 ksec XMM observation of the bright, narrow emission line quasar PG 1211+143. Absorption lines are seen in both EPIC and RGS spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionised outflow velocity of ~24000 km s^-1. The highest energy lines require a column density of N_H ~ 5 x 10^23 cm^-2, at an ionisation parameter of log(xi) ~ 3.4. If the origin of this high velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius ~130 Schwarzschild radii, providing a natural explanation for the Big Blue Bump (and strong soft X-ray) emission in PG 1211+143.

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arXiv2004

XMM-Newton observation of the Seyfert 1.8 ESO 113-G010: discovery of a highly redshifted iron line at 5.4 keV

We present a spectral analysis of the Seyfert 1.8 ESO 113-G010 observed with XMM-Newton for 4 ks. The spectrum shows a soft excess below 0.7 keV and more interestingly a narrow emission Gaussian line at 5.4 keV (in its rest-frame), most probably originating from a redshifted iron Kalpha line. No significant line at or above 6.4 keV is found contrary to other objects showing redshifted lines, ruling out a strong blue-wing to the line profile. The line is detected at 99% confidence, from performing Monte Carlo simulations which fully account for the range of energies where a narrow iron line is likely to occur. The energy of the line could indicate emission from relativistic (0.17 - 0.23 c) ejected matter moving away from the observer, as proposed for Mrk 766 by Turner et al. (2004). Alternatively, the emission from a narrow annulus at the surface of the accretion disk is unlikely due to the very small inclination angle (i.e. less than 10 deg) required to explain the narrow, redshifted line in this intermediate Seyfert galaxy. However emission from a small, localized hot-spot on the disk, occurring within a fraction of a complete disk orbit, could also explain the redshifted line. This scenario would be directly testable in a longer observation, as one would see significant variations in the energy and intensity of the line within an orbital timescale.

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arXiv2004

Resolving the large scale spectral variability of the luminous Seyfert 1 galaxy 1H 0419-577: Evidence for a new emission component and absorption by cold dense matter

An XMM-Newton observation of the luminous Seyfert 1 galaxy 1H 0419-577 in September 2002, when the source was in an extreme low-flux state, found a very hard X-ray spectrum at 1-10 keV with a strong soft excess below ~1 keV. Comparison with an earlier XMM-Newton observation when 1H 0419-577 was `X-ray bright' indicated the dominant spectral variability was due to a steep power law or cool Comptonised thermal emission. Four further XMM-Newton observations, with 1H 0419-577 in intermediate flux states, now support that conclusion, while we also find the variable emission component in intermediate state difference spectra to be strongly modified by absorption in low ionisation matter. The variable `soft excess' then appears to be an artefact of absorption of the underlying continuum while the `core' soft emission can be attributed to recombination in an extended region of more highly ionised gas. We note the wider implications of finding substantial cold dense matter overlying (or embedded in) the X-ray continuum source in a luminous Seyfert 1 galaxy.

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arXiv2004

XMM-Newton observations of high-luminosity radio-quiet QSOs

XMM Newton observations of five high-luminosity radio-quiet QSOs (Q 0144-3938, UM 269, PG 1634+706, SBS 0909+532 and PG 1247+267) are presented. Spectral energy distributions were calculated from the XMM-Newton EPIC (European Photon Imaging Camera) and OM (Optical Monitor) data, with bolometric luminosities estimated in the range from 7 x 10^45 to 2 x 10^48 erg s^-1 for the sample, peaking in the UV. At least four of the QSOs show a similar soft excess, which can be well modelled by either one or two blackbody components, in addition to the hard X-ray power-law. The temperatures of these blackbodies (~100-500 eV) are too high to be direct thermal emission from the accretion disc, so Comptonization is suggested. Two populations of Comptonizing electrons, with different temperatures, are needed to model the broad-band spectrum. The hotter of these produces what is seen as the hard X-ray power-law, while the cooler (~0.25-0.5 keV) population models the spectral curvature at low energies. Only one of the QSOs shows evidence for an absorption component, while three of the five show neutral iron emission. Of these, PG 1247+267 seems to have a broad line (EW ~ 250 eV), with a strong, associated reflection component (R ~ 2), measured out to 30 keV in the rest frame of the QSO. Finally, it is concluded that the X-ray continuum shape of AGN remains essentially constant over a wide range of black hole mass and luminosity.

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arXiv2004

XMM-Newton EPIC observations of 21 low-redshift PG quasars

We present an X-ray spectral analysis of 21 low redshift quasars observed with XMM-Newton EPIC. All the sources are Palomar Green quasars with redshifts between 0.05 and 0.4 and have low Galactic absorption along the line-of-sight. A large majority of quasars in the sample (19/21) exhibit a significant soft excess below ~1-1.5keV, whilst two objects (PG1114+445 and IZw1) show a deficit of soft X-ray flux due to the presence of a strong warm absorber. At least half of the objects appear to harbor a warm absorber, as found previously in Seyfert 1 galaxies. We find significant detections of FeKalpha emission lines in at least twelve objects, whilst there is evidence for some broadening of the line profile, compared to the EPIC-pn resolution, in five of these quasars. The determination of the nature of this broadening (e.g., Keplerian motion, a blend of lines, relativistic effects) is not possible with the present data and requires either higher S/N or higher resolution spectra. In seven objects the line is located between 6.7-7keV, corresponding to highly ionized iron, whereas in the other five objects the line energy is consistent with 6.4 keV, i.e. corresponding to near neutral iron. The ionized lines tend to be found in the quasars with the steepest X-ray spectra. We also find a correlation between the continuum power law index and the optical Hbeta width, in both the soft and hard X-ray bands, whereby the steepest X-ray spectra are found in objects with narrow Hbeta widths, which confirms previous ROSAT and ASCA results. The soft and hard band X-ray photon indices are also strongly correlated, i.e. the steepest soft X-ray spectra correspond the steepest hard X-ray spectra.

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arXiv2004

A very low luminosity X-ray flash: XMM-Newton observations of GRB 031203

GRB031203 was observed by XMM-Newton twice, first with an observation beginning 6 hours after the burst, and again after 3 days. The afterglow had average 0.2-10.0keV fluxes for the first and second observations of 4.2+/-0.1x10^-13 and 1.8+/-0.1x10^-13 erg/cm^2/s respectively, decaying very slowly according to a power-law with an index of -0.55+/-0.05. The prompt soft X-ray flux, inferred from a detection of the dust echo of the prompt emission, strongly implies that this burst is very soft and should be classified as an X-ray flash (XRF) and further, implies a steep temporal slope (<~-1.7) between the prompt and afterglow phases or in the early afterglow, very different from the later afterglow decay slope. A power-law (Gamma=1.90+/-0.05) with absorption at a level consistent with the Galactic foreground absorption fits the afterglow spectrum well. A bright, low-redshift (z=0.105) galaxy lies within 0.5 arcsec of the X-ray position and is likely to be the GRB host. At this redshift, GRB031203 is the closest GRB or XRF known after GRB980425. It has a very low equivalent isotropic gamma-ray energy in the burst (~3x10^49 erg) and X-ray luminosity in the afterglow (9x10^42 erg/s at 10 hours), 3-4 orders of magnitude less than typical bursts, though higher than either the faint XRF020903 or GRB980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB980425, indicating that GRB031203 may be representative of low luminosity bursts.

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arXiv2004

XMM-Newton observations of the BL Lac MS0205.7+3509: a dense, low-metallicity absorber

The high-frequency-peaked BL Lac, MS0205.7+3509 was observed twice with XMM-Newton. Both X-ray spectra are synchrotron-dominated, with mean 0.2--10keV fluxes of 2.80+/-0.01 and 3.34+/-0.02 E-12 erg/cm^2/s. The X-ray spectra are well fit by a power-law with absorption above the Galactic value, however no absorption edges are detected, implying a low metallicity absorber (Z_\sun = 0.04(+0.03)(-0.01)) or an absorber with redshift above one (best-fit z=2.1 for an absorber with solar abundances). In either case the absorbing column density must be ~9E21 cm^-2. A new optical spectrum is presented, with a MgII absorption doublet detected at z=0.351, but no other significant features. The optical spectrum shows little reddening, implying a low dust to gas ratio in the absorber. MS0205.7+3509 must therefore be viewed through a high column density, low-metallicity gas cloud, probably at z=0.351 and associated with the galaxy that has been shown to be within ~2" of the BL Lac.

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