arXiv subjects

Natalie A. Webb

Publications and source records attributed to Natalie A. Webb.

45 records · Page 3Linked to original sources

arXiv2011

Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: a Tidal Disruption Event Candidate

We have discovered an ultrasoft X-ray transient source, 2XMMi J184725.1-631724, which was detected serendipitously in two XMM-Newton observations in the direction of the center of the galaxy IC 4765-f01-1504 at a redshift of 0.0353. These two observations were separated by 211 days, with the 0.2-10 keV absorbed flux increasing by a factor of about 9. Their spectra are best described by a model dominated by a thermal disk or a single-temperature blackbody component (contributing >80% of the flux) plus a weak power-law component. The thermal emission has a temperature of a few tens of eV, and the weak power-law component has a photon index of ~3.5. Similar to the black hole X-ray binaries in the thermal state, our source exhibits an accretion disk whose luminosity appears to follow the $L\propto T^4$ relation. This would indicate that the black hole mass is about 10^5-10^6 M_sun using the best-fitting inner disk radius. Both XMM-Newton observations show variability of about 21% on timescales of hours, which can be explained as due to fast variations in the mass accretion rate. The source was not detected by ROSAT in an observation in 1992, indicating a variability factor of >64 over longer timescales. The source was not detected again in X-rays in a Swift observation in 2011 February, implying a flux decrease by a factor of >12 since the last XMM-Newton observation. The transient nature, in addition to the extreme softness of the X-ray spectra and the inactivity of the galaxy implied by the lack of strong optical emission lines, makes it a candidate tidal disruption event. If this is the case, the first XMM-Newton observation would have been in the rising phase, and the second one in the decay phase.

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arXiv2011

The Cool Accretion Disk in ESO 243-49 HLX-1: Further Evidence of an Intermediate Mass Black Hole

With an inferred bolometric luminosity exceeding 10^42 erg/s, HLX-1 in ESO 243-49 is the most luminous of ultraluminous X-ray sources and provides one of the strongest cases for the existence of intermediate mass black holes. We obtain good fits to disk-dominated observations of the source with BHSPEC, a fully relativistic black hole accretion disk spectral model. Due to degeneracies in the model arising from the lack of independent constraints on inclination and black hole spin, there is a factor of 100 uncertainty in the best-fit black hole mass M. Nevertheless, spectral fitting of XMM-Newton observations provides robust lower and upper limits with 3000 Msun < M < 3 x 10^5 Msun, at 90% confidence, placing HLX-1 firmly in the intermediate-mass regime. The lower bound on M is entirely determined by matching the shape and peak energy of the thermal component in the spectrum. This bound is consistent with (but independent of) arguments based solely on the Eddington limit. Joint spectral modelling of the XMM-Newton data with more luminous Swift and Chandra observations increases the lower bound to 6000 Msun, but this tighter constraint is not independent of the Eddington limit. The upper bound on M is sensitive to the maximum allowed inclination i, and is reduced to M < 10^5 Msun if we limit i < 75 deg.

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arXiv2010

A New 626 s Periodic X-ray Source in the Direction of the Galactic Center

Here we report the detection of a 626 s periodic modulation from the X-ray source 2XMM J174016.0-290337 located in the direction of the Galactic center. We present temporal and spectral analyses of archival XMM-Newton data and photometry of archived near-infrared data in order to investigate the nature of this source. We find that the X-ray light curve shows a strong modulation at 626 +/- 2 s with a confidence level > 99.9% and a pulsed fraction of 54%. Spectral fitting demonstrates that the spectrum is consistent with an absorbed power law. No significant spectral variability was observed over the 626 s period. We have investigated the possibility that the 626 s period is orbital in nature (either that of an ultra-compact X-ray binary or an AM CVn) or related to the spin of a compact object (either an accretion powered pulsar or an intermediate polar). The X-ray properties of the source and the photometry of the candidate near-infrared counterparts are consistent with an accreting neutron star X-ray binary on the near-side of the Galactic bulge, where the 626 s period is most likely indicative of the pulsar spin period. However, we cannot rule out an ultra-compact X-ray binary or an intermediate polar with the data at hand. In the former case, if the 626 s modulation is the orbital period of an X-ray binary, it would be the shortest period system known. In the latter case, the modulation would be the spin period of a magnetic white dwarf. However, we find no evidence for absorption dips over the 626 s period, a low temperature black body spectral component, or Fe Kalpha emission lines. These features are commonly observed in intermediate polars, making 2XMM J174016.0-290337 a rather unusual member of this class if confirmed. We instead suggest that 2XMM J174016.0-290337 could be a new addition to the emerging class of symbiotic X-ray binaries.

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arXiv2010

A Redshift for the Intermediate Mass Black Hole Candidate HLX-1: Confirmation of its Association with the Galaxy ESO 243-49

In this Letter we report a spectroscopic confirmation of the association of HLX-1, the brightest ultra-luminous X-ray source, with the galaxy ESO 243-49. At the host galaxy distance of 95 Mpc, the maximum observed 0.2 - 10 keV luminosity is 1.2E42 erg/s. This luminosity is ~400 times above the Eddington limit for a 20 Msun black hole, and has been interpreted as implying an accreting intermediate mass black hole with a mass in excess of 500 Msun (assuming the luminosity is a factor of 10 above the Eddington value). However, a number of other ultra-luminous X-ray sources have been later identified as background active galaxies or foreground sources. It has recently been claimed that HLX-1 could be a quiescent neutron star X-ray binary at a Galactic distance of only 2.5 kpc, so a definitive association with the host galaxy is crucial in order to confirm the nature of the object. Here we report the detection of the Halpha emission line for the recently identified optical counterpart at a redshift consistent with that of ESO 243-49. This finding definitively places HLX-1 inside ESO 243-49, confirming the extreme maximum luminosity and strengthening the case for it containing an accreting intermediate mass black hole of more than 500 Msun.

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arXiv2008

The Magellanic system X-ray sources

Using archival X-ray data from the second XMM-Newton serendipitous source catalogue, we present comparative analysis of the overall population of X-ray sources in the Large and Small Magellanic Clouds. We see a difference between the characteristics of the brighter sources in the two populations in the X-ray band. Utilising flux measurements in different energy bands we are able to sort the X-ray sources based on similarities to other previously identified and classified objects. In this manner we are able to identify the probable nature of some of the unknown objects, identifying a number of possible X-ray binaries and Super Soft Sources.

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arXiv2004

Faint X-ray sources in globular clusters in the XMM-Newton and Chandra era

X-ray imaging and spectroscopy can be used to probe the binary content of globular clusters. Binaries are thought to play a key role in the dynamical evolution of the clusters by serving as an internal source of energy which counters the tendency of their cores to collapse. Various kinds of binaries are found in clusters in X-rays, including BY Dra or RS CVn systems, Cataclysmic Variables and low mass X-ray binaries as well as millisecond pulsars. In this review, we will present the most recent X-ray observations of globular clusters as performed with XMM-Newton and Chandra. We will illustrate the power of X-ray observations to separate the different X-ray source populations, and emphasize how the results are important in understanding the binary formation processes.

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arXiv2003

Discovery of a quiescent neutron star binary in the globular cluster M13

We have discovered with XMM-Newton an X-ray source in the core of the globular cluster M13, whose X-ray spectral properties suggest that it is a quiescent neutron star X-ray binary. The spectrum can be well fitted with a pure hydrogen atmosphere model, with T=76 +/- 3 eV, R=12.8 +/- 0.4 km and an X-ray luminosity of 7.3 +/- 0.6 x 10^{32} erg/s. In the light of this result, we have discovered a strong correlation between the stellar encounter rate and the number of quiescent neutron stars found in the ten globular clusters observed so far by either XMM-Newton or Chandra. This result lends strong support to the idea that these systems are primarily produced by stellar encounters in the core of globular clusters.

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arXiv2002

An XMM-Newton observation of the globular cluster Omega Centauri

We report on a deep XMM-Newton EPIC observation of the globular cluster Omega Cen performed on August 13th, 2001. We have detected 11 and 27 faint X-ray sources in the core and half mass radii, searching down to a luminosity of 1.3 x 10^{31} erg s^{-1} in the 0.5-5 keV range. Most sources have bolometric X-ray luminosities between ~ 10^{31}-10^{32} erg s^{-1}. We present the color-color and hardness-intensity diagrams of the source sample, as well as high-quality EPIC spectra of the brightest objects of the field; including the two candidate Cataclysmic Variables (CVs) in the core and the quiescent neutron star low-mass X-ray binary candidate. The spectra of the latter objects fully support their previous classification. We show that the bulk of sources are hard and spectrally similar to CVs. The lack of soft faint sources might be related to the absence of millisecond pulsars in the cluster. The XMM-Newton observations reveal the presence of an excess of sources well outside the core of the cluster where several RS CVn binaries have already been found. We have also analyzed a publicly available Chandra ACIS-I observation performed on January 24-25th, 2000, to improve the XMM-Newton source positions and to search for source intensity variations between the two data sets. 63 XMM-Newton sources have a Chandra counterpart, and 15 sources within the half-mass radius have shown time variability. Overall, the general properties of the faint X-ray sources in Omega Cen suggest that they are predominantly CVs and active binaries (RS CVn or BY Dra).

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arXiv2001

First XMM-Newton Observations of the Globular Cluster M22

We have examined preliminary data of the globular cluster, M22, from the EPIC MOS detectors on board XMM-Newton. We have detected 27 X-ray sources within the centre of the field of view, 24 of which are new detections. Three sources were found within the core of the cluster. From spectral analysis of the X-ray sources, it is possible that the object at the centre of the core is a quiescent X-ray transient and those lying further out are maybe cataclysmic variables.

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