arXiv subjects

Jimmy A. Irwin

Publications and source records attributed to Jimmy A. Irwin.

At least 55 records · Page 3Linked to original sources

arXiv2013

Discovery of a Highly Variable Dipping Ultraluminous X-ray source in M94

We report the discovery of a new ultraluminous X-ray source (ULX) 2XMM J125048.6+410743 within the spiral galaxy M94. The source has been observed by ROSAT, Chandra, and XMM-Newton on several occasions, exhibiting as a highly variable persistent source or a recurrent transient with a flux variation factor of >100, a high duty cycle (at least ~70%), and a peak luminosity of Lx ~ 2X10^{39} erg/s (0.2-10 keV, absorbed). In the brightest observation, the source is similar to typical low-luminosity ULXs, with the spectrum showing a high-energy cutoff but harder than that from a standard accretion disk. There are also sporadical short dips, accompanied by spectral softening. In a fainter observation with Lx ~ 3.6X10^{38} erg/s, the source appears softer and is probably in the thermal state seen in Galactic black-hole X-ray binaries (BHBs). In an even fainter observation (Lx ~ 9X10^{37} erg/s), the spectrum is harder again, and the source might be in the steep-powerlaw state or the hard state of BHBs. In this observation, the light curve might exhibit ~7 hr (quasi-)periodic large modulations over two cycles. The source also has a possible point-like optical counterpart from HST images. In terms of the colors and the luminosity, the counterpart is probably a G8 supergiant or a compact red globular cluster containing ~2X10^5 K dwarfs, with some possible weak UV excess that might be ascribed to accretion activity. Thus our source is a candidate stellar-mass BHB with a supergiant companion or with a dwarf companion residing in a globular cluster. Our study supports that some low-luminosity ULXs are supercritically accreting stellar-mass BHBs.

astro-ph.HE↗

arXiv2013

The Megasecond Chandra XVP Observation of NGC 3115: Witnessing the Flow of Hot Gas within the Bondi Radius

Observational confirmation of hot accretion model predictions has been hindered by the challenge to resolve spatially the Bondi radii of black holes with X-ray telescopes. Here, we use the Megasecond Chandra X-ray Visionary Project (XVP) observation of the NGC~3115 supermassive black hole to place the first direct observational constraints on the spatially and spectroscopically resolved structures of the X-ray emitting gas inside the Bondi radius of a black hole. We measured temperature and density profiles of the hot gas from a fraction out to tens of the Bondi radius (R_B = 2.4-4.8 arcsec = 112-224 pc). The projected temperature jumps significantly from ~0.3 keV beyond 5 arcsec to ~0.7 keV within ~4-5 arcsec, but then abruptly drops back to ~0.3 keV within ~3 arcsec. This is contrary to the expectation that the temperature should rise toward the center for a radiatively inefficient accretion flow. A hotter thermal component of ~1 keV inside 3 arcsec (~150 pc) is revealed using a two component thermal model, with the cooler ~0.3 keV thermal component dominating the spectra. We argue that the softer emission comes from diffuse gas physically located within $\sim 150$~pc from the black hole. The density profile is broadly consistent with rho ~ r^{-1} within the Bondi radius for either the single temperature or the two-temperature model. The X-ray data alone with physical reasoning argue against the absence of a black hole, supporting that we are witnessing the onset of the gravitational influence of the supermassive black hole.

astro-ph.HE↗

arXiv2012

Hot Diffuse Emission in the Nuclear Starburst Region of NGC 2903

(Abridged) We present a deep Chandra observation of the late-type barred spiral galaxy NGC 2903. The Chandra data reveal soft (kT_e ~ 0.2-0.5keV) diffuse emission in the nuclear starburst region and extending ~5kpc to the north and west of the nucleus. Much of this soft hot gas is likely to be from local active star-forming regions; however, besides the nuclear region, the morphology of hot gas does not strongly correlate with sites of active star formation. The central ~650 pc radius starburst zone exhibits much higher surface brightness diffuse emission than the surrounding regions and a harder spectral component in addition to its soft component. We interpret the hard component as being of thermal origin with kT_e~3.6keV and to be directly associated with a wind fluid produced by supernovae and massive star winds. The inferred terminal velocity for this hard component, ~1100 km/s, exceeds the local galaxy escape velocity suggesting a potential outflow. The softer extended emission does not display an obvious outflow geometry. However, the column density through which the X-rays are transmitted is lower to the west of the nucleus compared to the east and the surface brightness is higher there suggesting some soft hot gas originates from above the disk; viewed directly from the western zone but through the intervening galaxy disk from the eastern zone. There are several point-like sources in the nuclear region with X-ray spectra typical of compact binaries. None of these are coincident with the mass center of the galaxy and we place an upper limit luminosity from any point-like nuclear source to be < 2x10^38 ergs/s in the 0.5-8.0keV band which indicates that NGC 2903 lacks an active galactic nucleus. Heating from the nuclear starburst and a galactic wind may be responsible for preventing cold gas from accreting onto the galactic center.

astro-ph.CO↗

arXiv2012

Luminous [O III] and [N II] from Tidally Disrupted Horizontal Branch Stars

We model the emission lines generated in the photoionised debris of a tidally disrupted horizontal branch star. We find that at late times, the brightest optical emission lines are [N II] λλ6548,6583 and [O III] λλ4959,5007. Models of a red clump horizontal branch star undergoing mild disruption by a massive (50 -- 100 M_\sun) black hole yield an emission line spectrum that is in good agreement with that observed in the NGC 1399 globular cluster hosting the ultraluminous X-ray source CXOJ033831.8 - 352604. We make predictions for the UV emission line spectrum that can verify the tidal disruption scenario and constrain the mass of the BH.

astro-ph.HE↗

arXiv2011

Resolving the Bondi Accretion Flow toward the Supermassive Black Hole of NGC 3115 with Chandra

Gas undergoing Bondi accretion onto a supermassive black hole (SMBH) becomes hotter toward smaller radii. We searched for this signature with a Chandra observation of the hot gas in NGC 3115, which optical observations show has a very massive SMBH. Our analysis suggests that we are resolving, for the first time, the accretion flow within the Bondi radius of an SMBH. We show that the temperature is rising toward the galaxy center as expected in all accretion models in which the black hole is gravitationally capturing the ambient gas. There is no hard central point source that could cause such an apparent rise in temperature. The data support that the Bondi radius is at about 4 arcsec-5 arcsec (188-235 pc), suggesting an SMBH of 2 x 10^9 M_sun that is consistent with the upper end of the optical results. The density profile within the Bondi radius has a power-law index of 1.03^{+0.23}_{-0.21} which is consistent with gas in transition from the ambient medium and the accretion flow. The accretion rate at the Bondi radius is determined to be {\dot M}_B = 2.2 x 10^{-2} M_sun yr^{-1}. Thus, the accretion luminosity with 10% radiative efficiency at the Bondi radius (10^{44} erg s^{-1}) is about six orders of magnitude higher than the upper limit of the X-ray luminosity of the nucleus.

astro-ph.HE↗

arXiv2010

Evidence for a Stellar Disruption by an IMBH in an Extragalactic Globular Cluster

We report [O III] 5007 Angstrom and [N II] 6583 Angstrom emission from a globular cluster harboring the ultraluminous X-ray source (ULX) CXOJ033831.8-352604 in the Fornax elliptical galaxy NGC1399. No accompanying Balmer emission lines are present in the spectrum. One possibility is that the forbidden lines emanate from X-ray illuminated debris of a star that has been tidally-disrupted by an intermediate-mass black hole (IMBH), with this debris also feeding the black hole leading to the observed X-ray emission. The line strengths indicate that the minimum size of the emitting region is ~10^15 cm, and if the 70 km/s half-widths of the emission lines represent rotation around the black hole, a minimum black hole mass of 1000 solar mass is implied. The non-detection of H-alpha and H-beta emission lines suggests a white dwarf star was disrupted, although the presence of strong nitrogen emission is somewhat of a mystery.

astro-ph.HE↗

arXiv2009

X-Ray Searches for Emission from the WHIM in the Galactic Halo and the Intergalactic Medium

At least 50% of the baryons in the local universe are undetected and predicted to be in a hot dilute phase (1E5-1E7 K) in low and moderate overdensity environments. We searched for the predicted diffuse faint emission through shadowing observations whereby cool foreground gas absorbs more distant diffuse emission. Observations were obtained with Chandra and XMM-Newton. Using the cold gas in two galaxies, NGC 891 and NGC 5907, shadows were not detected and a newer observation of NGC 891 fails to confirm a previously reported X-ray shadow. Our upper limits lie above model predictions. For Local Group studies, we used a cloud in the Magellanic Stream and a compact high velocity cloud to search for a shadow. Instead of a shadow, the X-ray emission was brighter towards the Magellanic Stream cloud and there is a less significant brightness enhancement toward the other cloud also. The brightness enhancement toward the Magellanic Stream cloud is probably due to an interaction with a hot ambient medium that surrounds the Milky Way. We suggest that this interaction drives a shock into the cloud, heating the gas to X-ray emitting temperatures.

astro-ph.CO↗

arXiv2009

Elemental Abundances in the X-Ray Gas of Early-Type Galaxies with XMM and Chandra Observations

The source of hot gas in elliptical galaxies is thought to be due to stellar mass loss, with contributions from supernova events and possibly from infall from a surrounding environment. This picture predicts supersolar values for the metallicity of the gas toward the inner part of the galaxy, which can be tested by measuring the gas phase abundances. We use high-quality data for 10 nearby early-type galaxy from XMM-Newton, featuring both the EPIC and the Reflection Grating Spectrometer, where the strongest emission lines are detected with little blending; some Chandra data are also used. We find excellent consistency in the elemental abundances between the different XMM instruments and good consistency with Chandra. Differences in abundances with aperture size and model complexity are examined, but large differences rarely occur. For a two-temperature thermal model plus a point source contribution, the median Fe and O abundances are 0.86 and 0.44 of the Solar value, while Si and Mg abundances are similar to that for Fe. This is similar to stellar abundances for these galaxies but supernovae were expected to enhance the gas phase abundances considerably, which is not observed.

astro-ph.HE↗

arXiv2008

Deep Chandra X-ray Observations of Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697

Chandra X-ray observations routinely resolve tens to hundreds of low-mass X-ray binaries (LMXBs) per galaxy in nearby massive early-type galaxies. These studies have raised important issues regarding the behavior of this population of remnants of the once massive stars in early-type galaxies, namely the connection between LMXBs and globular clusters (GCs) and the nature of the LMXB luminosity function (LF). In this paper, we combine five epochs of Chandra observations and one central field Hubble Space Telescope Advance Camera for Surveys observation of NGC 4697, one of the nearest, optically luminous elliptical (E6) galaxies, to probe the GC-LMXB connection and LMXB-LF down to a detection/completeness limit of (0.6/1.4) x 10^{37} ergs/s. We detect 158 sources, present their luminosities and hardness ratios, and associate 34 LMXBs with GCs. We confirm that GCs with higher encounter rates (Γ_h) and redder colors (higher metallicity Z) are more likely to contain GCs, and find that the expected number of LMXBs per GC is proportional to Γ_h^{0.79+0.18/-0.15} Z^{0.50+0.20/-0.18}, consistent with fainter X-ray sources in Galactic GCs and LMXBs in Virgo early-type galaxies. Approximately 11+/-2% / 8 +/-2% of GCs in NGC 4697 contain an LMXB at the detection/completeness limit. We propose that the larger proportion of metal-rich GCs in NGC 4697 compared to the Milky Way explains why these fractions are much higher than those of the Milky Way at similar luminosities. We confirm that a broken power-law is the best fit to the LMXB-LF, although we cannot rule out a cutoff power-law, and argue that this raises the possibility that there is no universal form for the LMXB-LF in early-type galaxies. We find marginal evidence for different LFs of LMXBs in GCs and the field and different spectra of GC-LMXBs and Field-LMXBs.

astro-ph↗

arXiv2008

Measurements of Variability of Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697 from Multi-Epoch Chandra X-ray Observations

Multi-epoch Chandra X-ray observations of nearby massive early-type galaxies open up the study of an important regime of low-mass X-ray binary (LMXB) behavior -- long term variability. In a companion paper, we report on the detection of 158 X-ray sources down to a detection/completeness limit of 0.6/1.4 x 10^{37} ergs/s using five Chandra observations of NGC 4697, one of the nearest (11.3 Mpc), optically luminous (M_B < -20), elliptical (E6) galaxy. In this paper, we report on the variability of LMXB candidates measured on timescales from seconds to years. At timescales of seconds to hours, we detect five sources with significant variability. Approximately 7% of sources show variability between any two observations, and 16+/-4% of sources do not have a constant luminosity over all five observations. Among variable sources, we identify eleven transient candidates, with which we estimate that if all LMXBs in NGC 4697 are long-term transients then they are on for ~ 100 yr and have a 7% duty cycle. These numbers are consistent with those found for brighter LMXBs in M87 and NGC 1399, which suggests that there does not appear to be a measurable difference between the outburst durations of long-term transient neutron star LMXBs and black hole LMXBs. We discuss in detail a transient supersoft source, whose properties are not easily explained by standard explanations for supersoft sources.

astro-ph↗

arXiv2007

Variable Low-Mass X-ray Binaries in Early-Type Galaxies

As the Chandra X-ray Observatory mission matures, increasing numbers of nearby galaxies are being observed multiple times, sampling the variability of extragalactic X-ray binaries on timescales extending from seconds to years. We present results on luminous low-mass X-ray binaries from several early-type galaxies. We show that instantaneous LMXB luminosity functions of early-type galaxies do not significantly change between observations; a relatively low fraction of sources are strongly variable on <~ 5 yr timescales. We discuss the implications that a relatively small number of transient LMXBs are being discovered in early-type galaxies.

astro-ph↗

arXiv2007

The Search for Million Degree Gas Through The NVII Hyperfine Line

Gas in the million degree range occurs in a variety of astronomical environments, and it may be the main component of the elusive missing baryons at low redshift. The NVII ion is found in this material and it has a hyperfine spin-flip transition with a rest frequency of 53.042 GHz, which can be observed for z > 0.1, when it is shifted into a suitably transparent radio band. We used the 42-48 GHz spectrometer on the Green Bank Telescope to search for both emission and absorption from this NVII transmission. For absorption studies, 3C273, 3C 279, 3C 345, and 4C+39.25 were observed but no feature were seen above the 5 sigma level. For emission line studies, we observed Abell 1835, Abell 2390 and the star-forming galaxy PKS 1345+12, but no features were seen exceeding 5 sigma. We examine whether the strongest emission feature, in Abell 2390 (3.7 sigma), and the strongest absorption feature, toward 4C+39.25 (3.8 sigma), might be expected from theoretical models. The emission feature would require ~1E10 Msolar of 1E6 K gas, which is inconsistent with X-ray limits for the O VII Kalpha line, so it is unlikely to be real. The NVII absorption feature requires a NVII column of 6E16 cm^-2, higher than model predictions by at least an order of magnitude, which makes it inconsistent with model expectations. The individual observations were less than 1 hr in length, so for lengthy observations, we show that NVII absorption line observations can begin to be useful in in the search for hot intergalactic gas.

astro-ph↗

arXiv2006

The Remarkable Stability of Probable Black Hole Low-Mass X-ray Binaries in Nearby Galaxies

The most luminous X-ray sources in nearby elliptical galaxies are likely black hole low-mass X-ray binaries (BHLMXBs). In the Milky Way, such systems are always transient, and with the exception of GRS1915+105 have burst durations on the order of weeks or months. However, the low duty cycle of short-duration outburst BHLMXBs makes it improbable that any one source would be caught in an outburst during a single snapshot observation. Long-duration outburst BHLMXBs, although much rarer, would be detectable in a series of snapshot observations separated by several years. Our analysis of multi-epoch Chandra observations of the giant elliptical galaxies NGC1399 and M87 separated by 3.3 and 5.3 yr, respectively, finds that all 37 luminous (>8 x 10^38 ergs/s) X-ray sources that were present in the first epoch observations were still in outburst in all of the following observations. Many of these probable long-duration outburst BHLMXBs reside within globular clusters of the galaxies. Conversely, no definitive short-duration outburst BHLMXBs were detected in any of the observations. This places an upper limit on the ratio of short--to--long-duration outbursters that is slightly lower, but consistent with what is seen in the Milky Way. The fact that none of the luminous sources turned off between the first and last epochs places a 95 per cent lower limit of 50 yr on the mean burst duration of the long-duration outburst sources. The most likely scenario for the origin of these sources is that they are long-period (>30 d) black hole binaries with a red giant donor, much like GRS1915+105. However, unlike GRS1915+105, most of the sources show only modest variability from epoch to epoch.

astro-ph↗

arXiv2006

The Size of the Cooling Region of Hot Gas in Two Elliptical Galaxies

Some early-type galaxies show OVI emission, a tracer of gas at 10^5.5 K, and a predicted product of gas cooling from the X-ray emitting temperatures. We studied the spatial extent and velocity structure of this cooling gas by obtaining spectra of the OVI doublet in NGC 4636 and NGC 5846 with the Far Ultraviolet Spectroscopic Explorer. For NGC 4636, the central LWRS pointing shows that the OVI lines are double-peaked and symmetrical about the systemic velocity of the galaxy, with a separation of 210 km/s. A LWRS observation 30" from the center failed to show additional OVI emission. For NGC 5846, three spectra were obtained with 4" x 20" apertures (MDRS) at the center and 4" to the east and west of the center. The OVI line flux seen in the previous LWRS is contained in the sum of the smaller apertures, with most of the flux in a single non-central MDRS aperture, suggesting a size for the emission < 0.5 kpc; the emission consists of a blue and red peak. For both galaxies, the OVI velocity structure is similar to that of the optical [NII] emission and is consistent with rotation. The compactness and velocity structure of the OVI emission rules out cooling flow models with broadly distributed mass drop-out but is consistent with cooling flow models where the cooling occurs primarily in the central region. The 1E4 K gas may be the end state of the OVI gas.

astro-ph↗

arXiv2006

OVI Observations of Galaxy Clusters: Evidence for Modest Cooling Flows

A prediction of the galaxy cluster cooling flow model is that as gas cools from the ambient cluster temperature, emission lines are produced in gas at subsequently decreasing temperatures. Gas passing through 10^5.5 K emits in the lines of OVI 1032,1035, and here we report a FUSE study of these lines in three cooling flow clusters, Abell 426, Abell 1795, and AWM 7. No emission was detected from AWM 7, but OVI is detected from the centers of Abell 426 and Abell 1795, and possibly to the south of the center in Abell 1795, where X-ray and optical emission line filaments lie. In Abell 426, these line luminosities imply a cooling rate of 32+/-6 Msolar/yr within the central r = 6.2 kpc region, while for Abell 1795, the central cooling rate is 26+/-7 Msolar/yr (within r = 22 kpc), and about 42+/-9 Msolar/yr including the southern pointing. Including other studies, three of six clusters have OVI emission, and they also have star formation as well as emission lines from 1E4 K gas. These observations are generally consistent with the cooling flow model but at a rate closer to 30 Msolar/yr than originally suggested values of 100-1000 Msolar/yr.

astro-ph↗

arXiv2005

OVI in Elliptical Galaxies: Indicators of Cooling Flows

Early-type galaxies often contain a hot X-ray emitting interstellar medium (3-8E6 K) with an apparent radiative cooling time much less than a Hubble time. If unopposed by a heating mechanism, the gas will radiatively cool to temperatures <= 10E4 K at a rate proportional to L_X/T_X, typically 0.03-1 M_solar yr^-1. We can test if gas is cooling through the 3E5 K range by observing the OVI doublet, whose luminosity is proportional to the cooling rate. Here we report on a study of an unbiased sample of 24 galaxies, obtaining Far Ultraviolet Spectroscopic Explorer spectra to complement the X-ray data of ROSAT} and Chandra. The OVI line emission was detected in about 40% of the galaxies and at a luminosity level similar to the prediction from the cooling flow model. There is a correlation between Mdot_OVI and Mdot_X, although there is significant dispersion about the relationship, where the OVI is brighter or dimmer than expected by a factor of three or more. If the cooling flow picture is to be retained, this dispersion requires that cooling flows be time-dependent, as might occur by the activity of an AGN. However, of detected objects, those with the highest or lowest values of Mdot_OVI/Mdot_X are not systematically hot or cool, as one might predict from AGN heating.

astro-ph↗

arXiv2005

XMM-Newton Observation of Diffuse Gas and LMXBs in the Elliptical Galaxy NGC 4649 (M60)

(Abridged) We present an XMM-Newton X-ray observation of the X-ray bright E2 elliptical galaxy NGC 4649. In addition to bright diffuse emission, we resolve 158 discrete sources, ~50 of which are likely to be low-mass X-ray binaries (LMXBs) associated with NGC 4649. We find evidence for variability in three sources between this observation and a previous Chandra observation. Additionally, we detect five sources that were not detected with Chandra despite its better detection limit, suggesting that these sources have since brightened. The total X-ray spectrum of the resolved sources is well-fit by a hard power-law, while the diffuse spectrum requires a hard and a soft component, presumably due to the relatively soft diffuse gas and the harder unresolved sources. A deprojection of the diffuse emission revealed a radial temperature gradient that is hot in the center, drops to a minimum at about 20-50" (1.6-4.1 kpc), and rises again in the outer regions. The diffuse emission appears to require a two-temperature model with heavy element abundance ratios that differ from the solar values. We have verified the existence of faint radial features extending out from the core of NGC 4649 that had previously been seen with Chandra. The fingers are morphologically similar to radial features seen in hydrodynamic simulations of cooling flows in elliptical galaxies. We also find evidence for a longer, previously undetected filament that extends to the northeastern edge of NGC 4649. We conclude that the filament may be the result of a tidal interaction, possibly with NGC 4647, though more work is necessary to verify this conclusion.

astro-ph↗

arXiv2005

The Birthplace of Low-Mass X-ray Binaries: Field Versus Globular Cluster Populations

Recent Chandra studies of low-mass X-ray binaries (LMXBs) within early-type galaxies have found that LMXBs are commonly located within globular clusters of the galaxies. However, whether all LMXBs are formed within globular clusters has remained an open question. If all LMXBs formed within globular clusters, the summed X-ray luminosity of the LMXBs in a galaxy should be directly proportional to the number of globular clusters in the galaxy regardless of where the LMXBs currently reside. We have compared these two quantities over the same angular area for a sample of 12 elliptical and S0 galaxies observed with Chandra and found that the correlation between the two quantities is weaker than expected if all LMXBs formed within globular clusters. This indicates that a significant number of the LMXBs were formed in the field, and naturally accounts for the spread in field-to-cluster fractions of LMXBs from galaxy to galaxy. We also find that the "pollution" of globular cluster LMXBs into the field has been minimal within elliptical galaxies, but there is evidence that roughly half of the LMXBs originally in the globular clusters of S0 galaxies in our sample have escaped into the field. This is due to higher globular cluster disruption rates in S0s resulting from stronger gravitational shocks caused by the passage of globular clusters through the disks of S0 galaxies that are absent in elliptical galaxies.

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