arXiv subjects

N. J. Brassington

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

At least 19 recordsLinked to original sources

arXiv2014

Low-Mass X-ray Binaries and Globular Clusters Streamers and Arcs in NGC4278

We report significant inhomogeneities in the projected two-dimensional (2D) spatial distributions of Low-Mass X-ray Binaries (LMXBs) and Globular Clusters (GCs) of the intermediate mass elliptical galaxy NGC4278. In the inner region of NGC4278, a significant arc-like excess of LMXBs extending south of the center at ~50" in the western side of the galaxy can be associated to a similar over-density of the spatial distribution of red GCs from~Brassington et al. (2009). Using a recent catalog of GCs produced by Usher et al.(2013) and covering the whole field of the NGC4278 galaxy, we have discovered two other significant density structures outside the D25 isophote to the W and E of the center of NGC4278, associated to an over-density and an under-density respectively. We discuss the nature of these structures in the context of the similar spatial inhomogeneities discovered in the LMXBs and GCs populations of NGC4649 and NGC4261, respectively. These features suggest streamers from disrupted and accreted dwarf companions.

astro-ph.GA↗

arXiv2012

The Spectral and Temporal Properties of Transient Sources in Early-Type Galaxies

We report the spectral and temporal variability properties of 18 candidate transient and potential transient (TC and PTC) sources detected in deep multi-epoch Chandra observation of the nearby elliptical galaxies, NGC 3379, NGC 4278 and NGC 4697. Only one source can be identified with a background counterpart, leaving 17 TCs + PTCs in the galaxies. Of these, 14 are in the galaxy field, supporting the theoretical picture that the majority of field X-ray binaries (XRBs) will exhibit transient accretion for >75% of their lifetime. Three sources are coincident with globular clusters (GCs), including two high-luminosity candidate black hole (BH) XRBs, with Lx=5.4E38 erg/s, and Lx=2.8E39 erg/s, respectively. The spectra, luminosities and temporal behavior of these 17 sources suggest that the transient population is heterogeneous, including neutron star (NS) and BH XRBs in both normal and high-rate accretion modes, and super soft sources containing white dwarf binaries. Our TC and PTC detections are noticeably fewer that the number expected from the populations synthesis (PS) models of Fragos et al. (2009), tailored to our new Chandra pointings of NGC 4278. We attribute this discrepancy to the PS assumption that the transient population is composed of NS XRBs, as well as differences between the statistical analysis and error estimates used in the model and our observations.

astro-ph.CO↗

arXiv2012

AGN activity and the misaligned hot ISM in the compact radio elliptical NGC4278

The analysis of a deep (579 ks) Chandra ACIS pointing of the elliptical galaxy NGC4278, which hosts a low luminosity AGN and compact radio emission, allowed us to detect extended emission from hot gas out to a radius of \sim 5 kpc, with a 0.5--8 keV luminosity of 2.4x10^{39} erg/s. The emission is elongated in the NE-SW direction, misaligned with respect to the stellar body, and aligned with the ionized gas, and with the Spitzer IRAC 8\mum non-stellar emission. The nuclear X-ray luminosity decreased by a factor of \sim 18 since the first Chandra observation in 2005, a dimming that enabled the detection of hot gas even at the position of the nucleus. Both in the projected and deprojected profiles, the gas shows a significantly larger temperature (kT=0.75 keV) in the inner \sim 300 pc than in the surrounding region, where it stays at \sim 0.3 keV, a value lower than expected from standard gas heating assumptions. The nuclear X-ray emission is consistent with that of a low radiative efficiency accretion flow, accreting mass at a rate close to the Bondi one; estimates of the power of the nuclear jets require that the accretion rate is not largely reduced with respect to the Bondi rate. Among possibile origins for the central large hot gas temperature, such as gravitational heating from the central massive black hole and a recent AGN outburst, the interaction with the nuclear jets seems more likely, especially if the latter remain confined, and heat the nuclear region frequently. The unusual hot gas distribution on the galactic scale could be due to the accreting cold gas triggering the cooling of the hot phase, a process also contributing to the observed line emission from ionize gas, and to the hot gas temperature being lower than expected; alternatively, the latter could be due to an efficiency of the type Ia supernova energy mixing lower than usually adopted.

astro-ph.HE↗

arXiv2012

Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291

We present Chandra studies of the X-ray binary (XRB) populations in the bulge and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available Chandra observations totaling an effective exposure of 179 ks. We report photometric properties of these sources in a point-source catalog. There are ~40% of the bulge sources and ~25% of the ring sources showing >3σlong-term variability in their X-ray count rate. The X-ray colors suggest that a significant fraction of the bulge (~75%) and ring (~65%) sources are likely low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate that it is a low-luminosity AGN with moderate obscuration; spectral variability is observed between individual observations. We construct 0.3-8.0 keV X-ray luminosity functions (XLFs) for the bulge and ring XRB populations, taking into account the detection incompleteness and background AGN contamination. We reach 90% completeness limits of ~1.5\times10^{37} and ~2.2\times10^{37} erg/s for the bulge and ring populations, respectively. Both XLFs can be fit with a broken power-law model, and the shapes are consistent with those expected for populations dominated by LMXBs. We perform detailed population synthesis modeling of the XRB populations in NGC 1291, which suggests that the observed combined XLF is dominated by an old LMXB population. We compare the bulge and ring XRB populations, and argue that the ring XRBs are associated with a younger stellar population than the bulge sources, based on the relative overdensity of X-ray sources in the ring, the generally harder X-ray color of the ring sources, the overabundance of luminous sources in the combined XLF, and the flatter shape of the ring XLF.

astro-ph.CO↗

arXiv2010

Field and Globular Cluster LMXBs in NGC 4278

We report a detailed spectral analysis of the population of low-mass X-ray binaries (LMXBs) detected in the elliptical galaxy NGC~4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs), and three in the stellar field. The range of (0.3-8 keV) $L_X$ for these sources suggests that they may be black hole binaries (BHBs). Comparison of our results with simulations allows us to discriminate between disk and power-law dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the $L_X \sim T^4_{in}$ relation of Galactic BHBs are in the 5-15 $M_{\odot}$ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges suggests that while the high luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger $N_H$ in the GC spectrum; we speculate that this may point to either a metallicity effect, or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncover a previously unreported $L_X - R_G$ correlation (where $R_G$ is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC $L_X - R_G$ correlation is the by-product of a luminosity effect, and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation.

astro-ph.HE↗

arXiv2010

The X-ray Spectra of the Luminous LMXBs in NGC 3379: Field and Globular Cluster Sources

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX>1.2E38 erg/s). We also present a set of spectral simulations, produced to aid the interpretation of low-count single-component spectral modeling. These simulations demonstrate that it is possible to infer the spectral states of X-ray binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters, and one is a confirmed field source. Due to the nature of the luminosity cut all sources are either neutron star binaries emitting at or above the Eddington luminosity or black hole binaries. The spectra from these sources are well described by single-component models, with parameters consistent with Galactic LMXB observations, where hard-state sources have a range in photon index of 1.5-1.9 and thermally dominated sources have inner disc temperatures between ~0.7-1.55 keV. The large variability observed in the brightest globular cluster source (LX>4E38 erg/s) suggests the presence of a black hole binary. At its most luminous this source is observed in a thermally dominated state with kT=1.5 keV, consistent with a black hole mass of ~4 Msol. This observation provides further evidence that globular clusters are able to retain such massive binaries. We also observed a source transitioning from a bright state (LX~1E39 erg/s), with prominent thermal and non-thermal components, to a less luminous hard state (LX=3.8E38 erg/s, Gamma=1.85). In its high flux emission this source exhibits a cool-disc component of ~0.14 keV, similar to spectra observed in some ultraluminous X-ray sources. Such a similarity indicates a possible link between `normal' stellar mass black holes in a high accretion state and ULXs.

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arXiv2009

Long Term Monitoring of the Dynamics and Particle Acceleration of Knots in the Jet of Centaurus A

We present new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years at the VLA and with Chandra, with which we measure the X-ray and radio spectral indices of jet knots, flux density variations in the jet knots, polarization variations, and proper motions. We compare the observed properties with current knot formation models and particle acceleration mechanisms. We rule out impulsive particle acceleration as a formation mechanism for all of the knots as we detect the same population of knots in all of the observations and we find no evidence of extreme variability in the X-ray knots. We find the most likely mechanism for all the stationary knots is a collision resulting in a local shock followed by a steady state of prolonged, stable particle acceleration and X-ray synchrotron emission. In this scenario, the X-ray-only knots have radio counterparts that are too faint to be detected, while the radio-only knots are due to weak shocks where no particles are accelerated to X-ray emitting energies. Although the base knots are prime candidates for reconfinement shocks, the presence of a moving knot in this vicinity and the fact that there are two base knots are hard to explain in this model. We detect apparent motion in three knots; however, their velocities and locations provide no conclusive evidence for or against a faster moving `spine' within the jet. The radio-only knots, both stationary and moving, may be due to compression of the fluid.

astro-ph.CO↗

arXiv2009

Comparing GC and Field LMXBs in Elliptical Galaxies with deep Chandra and Hubble data

(abridged) We present a statistical study of the low-mass X-ray binary (LMXB) populations of three nearby, old elliptical galaxies: NGC 3379, NGC 4278, and NGC 4697. With a cumulative ~1 Ms Chandra ACIS observing time, we detect 90-170 LMXBs within the D25 ellipse of each galaxy. Cross-correlating Chandra X-ray sources and HST optical sources, we identify 75 globular cluster (GC) LMXBs and 112 field LMXBs. In the low luminosity range allowed by our deeper data (LX < 5 x 1037 erg s-1), we find a significant relative lack of GC-LMXBs, when compared with field sources. Using the co-added sample from the three galaxies, we find that the incompleteness-corrected X-ray luminosity functions (XLFs) of GC and field LMXBs differ at ~4# significance at LX < 5 x 1037 erg s-1. As previously reported, these XLFs are consistent at higher luminosities. Our observations may indicate a potential predominance of GC-LMXBs with donors evolved beyond the main sequence, when compared to current models, but their efficient formation requires relatively high initial binary fractions in clusters. The field LMXB XLF can be fitted with either a single power-law model plus a localized excess at a luminosity of 5-6 x 1037 erg s-1, or a broken power-law with a similar low-luminosity break. This XLF may be explained with NS-red-giant LMXBs, contributing to ~15% of total LMXBs population at ~5x1037 erg s-1. The difference in the GC and field XLFs is consistent with different origins and/or evolutionary paths between the two LMXB populations, although a fraction of the field sources are likely to have originated in GCs.

astro-ph.CO↗

arXiv2009

Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field

We study the X-ray luminosity function (XLF) of low mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low luminosity limit and comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of 8e35 erg/s, about 2-3 times deeper than previous investigations. We confirm the presence of the low luminosity break in the overall LMXB XLF at log(L_X)=37.2-37.6 below which the luminosity distribution follows a constant dN/d(ln L). Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest that there is an enhanced fraction of helium accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the 4 times higher ionization temperature of He, such systems are subject to accretion disk instabilities at approximately 20 times higher mass accretion rate, and therefore are not observed as persistent sources at low luminosities.

astro-ph.HE↗

arXiv2009

Deep Chandra Monitoring Observations of NGC 4278: Catalog of Source Properties

We present the properties of the discrete X-ray sources detected in our monitoring program of the globular cluster (GC) rich elliptical galaxy, NGC 4278, observed with Chandra ACIS-S in six separate pointings, resulting in a co-added exposure of 458-ks. From this deep observation, 236 sources have been detected within the region overlapped by all observations, 180 of which lie within the D25 ellipse of the galaxy. These 236 sources range in Lx from 3.5E36 erg/s (with 3sigma upper limit <1E37 erg/s) to ~2E40 erg/s, including the central nuclear source which has been classified as a LINER. From optical data, 39 X-ray sources have been determined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, with ten of these sources exhibiting Lx>1E38 erg/s. From X-ray source photometry, it has been determined that the majority of the 236 point sources that have well constrained colors, have values that are consistent with typical LMXB spectra, with 29 of these sources expected to be background objects from the logN-logS relation. There are 103 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. 3 of these sources have been identified as transient candidates, with a further 3 possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.

astro-ph.CO↗

arXiv2009

Transient Low-Mass X-Ray Binary Populations in Elliptical Galaxies NGC 3379 and NGC 4278

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star (NS) low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration and recurrence time. We apply this prescription to the population synthesis (PS) models of field LMXBs presented by Fragos et al. (2008), and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources (Brassington et al., 2008, 2009). We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Our comparison suggests that LMXBs formed through evolution of primordial field binaries are dominant in globular cluster (GC) poor elliptical galaxies, while they still have a significant contribution in GC rich ones.

astro-ph.CO↗

arXiv2009

High-energy particle acceleration at the radio-lobe shock of Centaurus A

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations. The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths.

astro-ph.HE↗

arXiv2008

Discovery of Hot Gas in Outflow in NGC3379

We report the discovery of a faint (L_x ~ 4 10^37 erg/s, 0.5-2 keV), out-flowing gaseous hot interstellar medium (ISM) in NGC 3379. This represents the lowest X-ray luminosity ever measured from a hot phase of the ISM in a nearby early type galaxy. The discovery of the hot ISM in a very deep Chandra observation was possible thanks to its unique spectral and spatial signatures, which distinguish it from the integrated stellar X-ray emission, responsible for most of the unresolved emission in the Chandra data. This hot component is found in a region of about 800 pc in radius at the center of the galaxy and has a total mass M~ 3 10^5 solar masses. Independent theoretical prediction of the characteristics of an ISM in this galaxy, based on the intrinsic properti es of NGC 3379, reproduce well the observed luminosity, temperature, and radial distribution and mass of the hot gas, and indicate that the gas is in an outflowing phase, predicted by models but not observed in any system so far.

astro-ph↗

arXiv2008

Deep Chandra Monitoring Observations of NGC 3379: Catalog of Source Properties

We present the properties of the discrete X-ray sources detected in our monitoring program of the `typical' elliptical galaxy, NGC 3379, observed with Chandra ACIS-S in five separate pointings, resulting in a co-added exposure of 324-ks. From this deep observation, 132 sources have been detected within the region overlapped by all observations, 98 of which lie within the D25 ellipse of the galaxy. These 132 sources range in Lx from 6E35 erg/s (with 3sigma upper limit <4E36 erg/s) to ~2E39 erg/s, including one source with Lx>1E39 erg/s, which has been classified as a ULX. From optical data, 10 X-ray sources have been determined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, with three of these sources exhibiting Lx> 1E38 erg/s. From X-ray source photometry, it has been determined that the majority of the 132 sources that have well constrained colors, have values that are consistent with typical LMXB spectra. Additionally to this, a sub-population of 10 sources has been found to exhibit very hard spectra and it is expected that most of these sources are absorbed background AGN. There are 64 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Five of these sources have been identified as transient candidates, with a further 3 possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.

astro-ph↗

arXiv2008

Evidence for Non-Hydrostatic Gas Motions in the Hot ISM of Centaurus A

We present preliminary results from a deep (600 ks) {\em Chandra} observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A (Cen A). We find a surface brightness discontinuity in the gas $\sim$3.5 kpc from the nucleus spanning a 120$^\circ$ arc. The temperature of the gas is 0.60$\pm$0.05 and 0.68$\pm$0.10 keV, interior and exterior to the discontinuity, respectively. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3$\pm$0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470$\pm$100 km s$^{-1}$, or Mach 1.0$\pm$0.2 (1.2$\pm$0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of $\sim$7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of non-hydrostatic motion of the gas core (i.e. sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kpc-scale jet. The jet could have decollimated crossing the discontinuity and thereby forming the northeast radio lobe.

astro-ph↗

arXiv2008

Models for Low-Mass X-Ray Binaries in the Elliptical Galaxies NGC3379 and NGC4278: Comparison with Observations

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXB) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code (Belczynski et al., 2006), assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested and calibrated using detailed binary star calculations and incorporates all the important physical processes of binary evolution. The simulations are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLF) of point sources in these galaxies. For the first time we explore the population XLF down to luminosities of 3X10^36 erg/s, as probed by the most recent observational results (Kim et al., 2006). We consider models for the formation and evolution of LMXBs in galactic fields with different CE efficiencies, stellar wind prescriptions, magnetic braking laws and initial mass functions. We identify models that produce an XLF in excellent agreement with the observations both in shape and absolute normalization. We also find that the treatment of the outburst luminosity of transient systems remains a crucial factor for the determination of the XLF since the modeled populations are dominated by transient X-ray systems.

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arXiv2007

The LMXB Population of NGC 3379

Presented here are the highlights from the deep Chandra observation of the elliptical galaxy NGC 3379. From the multi-epoch observation of this galaxy, 132 discrete X-ray sources have been detected within the region overlapped by all observations, 98 of which lie within the D25 ellipse of the galaxy. Of these 132 sources, 71 exhibit long-term variability, indicating that they are accreting compact objects. 11 of these sources have been identified as transient candidates, with a further 7 possible transients. In addition to this, from the joint Hubble/Chandra field of view, nine globular clusters (GCs) and 53 field low mass X-ray binaries (LMXBs) have been detected in the galaxy. Comparisons of these two populations reveals that, at higher luminosities the field LMXBs and GC-LMXBs are similar. However, a significant lack of GC-LMXBs has been found at lower luminosities, indicating that not all LMXBs can form in GCs.

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arXiv2007

Where Centaurus A gets its X-ray knottiness

We report an X-ray spectral study of the transverse structure of the Centaurus A jet using new data from the Chandra Cen A Very Large Project. We find that the spectrum steepens with increasing distance from the jet axis, and that this steepening can be attributed to a change in the average spectrum of the knotty emission. Such a trend is unexpected if the knots are predominantly a surface feature residing in a shear layer between faster and slower flows. We suggest that the spectral steepening of the knot emission as a function of distance from the jet axis is due to knot migration, implying a component of transverse motion of knots within the flow.

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