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Register a new userThe Effect of Variability on X-Ray Binary Luminosity Functions: Multiple Epoch Observations of NGC 300 with Chandra
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We have obtained three epochs of Chandra ACIS-I observations (totaling $sim$184 ks) of the nearby spiral galaxy NGC~300 to study the logN-logS distributions of its X-ray point source population down to $sim$2$times$10$^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the 0.35-8 keV band (equivalent to $sim$10$^{36}$ erg s$^{-1}$). The individual epoch logN-logS distributions are best described as the sum of a background AGN component, a simple power law, and a broken power law, with the shape of the logN-logS distributions sometimes varying between observations. The simple power law and AGN components produce a good fit for persistent sources (i.e., with fluxes that remain constant within a factor of $sim$2). The differential power law index of $sim$1.2 and high fluxes suggest that the persistent sources intrinsic to NGC~300 are dominated by Roche lobe overflowing low mass X-ray binaries. The variable X-ray sources are described by a broken power law, with a faint-end power law index of $sim$1.7, a bright-end index of $sim$2.8-4.9, and a break flux of $sim$8$times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ ($sim$4$times10^{36}$ erg s$^{-1}$), suggesting they are mostly outbursting, wind-fed high mass X-ray binaries, although the logN-logS distribution of variable sources likely also contains low-mass X-ray binaries. We generate model logN-logS distributions for synthetic X-ray binaries and constrain the distribution of maximum X-ray fluxes attained during outburst. Our observations suggest that the majority of outbursting X-ray binaries occur at sub-Eddington luminosities, where mass transfer likely occurs through direct wind accretion at $sim$1-3% of the Eddington rate.
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The X-ray luminosity functions of galaxies have become a useful tool for population studies of X-ray binaries in them. The availability of long term light-curves of X-ray binaries with the All Sky X-ray Monitors opens up the possibility of constructing X-ray luminosity functions, by also including the intensity variation effects of the galactic X-ray binaries. We have constructed multiple realizations of the X-ray luminosity functions (XLFs) of Milky Way, using the long term light-curves of sources obtained in the 2-10 keV energy band with the RXTE-ASM. The observed spread seen in the value of slope of both HMXB and LMXB XLFs are due to inclusion of variable luminosities of X-ray binaries in construction of these XLFs as well as finite sample effects. XLFs constructed for galactic HMXBs in the luminosity range 10^{36} - 10^{39} erg/sec is described by a power-law model with a mean power-law index of -0.48 and a spread due to variability of HMXBs as 0.19. XLFs constructed for galactic LMXBs in the luminosity range 10^{36} - 10^{39} erg/sec has a shape of cut-off power-law with mean power-law index of -0.31 and a spread due to variability of LMXBs as 0.07.
In 2010 May, an intermediate luminosity optical transient was discovered in the nearby galaxy NGC 300 by a South African amateur astronomer. In the decade since its discovery, multi-wavelength observations of the misnamed ``SN 2010da have continually re-shaped our understanding of this high mass X-ray binary system. In this review, we present an overview of the multi-wavelength observations and attempts to understand the 2010 transient event and, later, the re-classification of this system as NGC~300 ULX-1: a red supergiant + neutron star ultraluminous X-ray source.
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.
We have studied the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXBs) in the nearby lenticular galaxy NGC 3115, using the Megasecond Chandra X-Ray Visionary Project Observation. With a total exposure time of ~1.1 Ms, we constructed the XLF down to a limiting luminosity of ~10^36 erg/s, much deeper than typically reached for other early-type galaxies. We found significant flattening of the overall LMXB XLF from dN/dL propto L^{-2.2pm0.4} above 5.5x10^37 erg/s to dN/dL propto L^{-1.0pm0.1} below it, though we could not rule out a fit with a higher break at ~1.6x10^38 erg/s. We also found evidence that the XLF of LMXBs in globular clusters (GCs) is overall flatter than that of field LMXBs. Thus our results for this galaxy do not support the idea that all LMXBs are formed in GCs. The XLF of field LMXBs seems to show spatial variation, with the XLF in the inner region of the galaxy being flatter than that in the outer region, probably due to contamination of LMXBs from undetected and/or disrupted GCs in the inner region. The XLF in the outer region is probably the XLF of primordial field LMXBs, exhibiting dN/dL propto L^{-1.2pm0.1} up to a break close to the Eddington limit of neutron star LMXBs (~1.7x10^38 erg/s). The break of the GC LMXB XLF is lower, at ~1.1x10^37 erg/s. We also confirm previous findings that the metal-rich/red GCs are more likely to host LMXBs than the metal-poor/blue GCs, which is more significant for more luminous LMXBs, and that more massive GCs are more likely to host LMXBs.
We present results from the major coordinated X-ray observing program on the ULX NGC 1313 X-1 performed in 2017, combining $XMM$-$Newton$, $Chandra$ and $NuSTAR$, focusing on the evolution of the broadband ($sim$0.3-30.0 keV) continuum emission. Clear and unusual spectral variability is observed, but this is markedly suppressed above $sim$10-15 keV, qualitatively similar to the ULX Holmberg IX X-1. We model the multi-epoch data with two-component accretion disc models designed to approximate super-Eddington accretion, allowing for both a black hole and a neutron star accretor. With regards to the hotter disc component, the data trace out two distinct tracks in the luminosity-temperature plane, with larger emitting radii and lower temperatures seen at higher observed fluxes. Despite this apparent anti-correlation, each of these tracks individually shows a positive luminosity-temperature relation. Both are broadly consistent with $Lpropto{T}^{4}$, as expected for blackbody emission with a constant area, and also with $Lpropto{T}^{2}$, as may be expected for an advection-dominated disc around a black hole. We consider a variety of possibilities for this unusual behaviour. Scenarios in which the innermost flow is suddenly blocked from view by outer regions of the super-Eddington disc/wind can explain the luminosity-temperature behaviour, but are difficult to reconcile with the lack of strong variability at higher energies, assuming this emission arises from the most compact regions. Instead, we may be seeing evidence for further radial stratification of the accretion flow than is included in the simple models considered, with a combination of winds and advection resulting in the suppressed high-energy variability.
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