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Title: The X-Ray Binary Population of the Nearby Dwarf Starburst Galaxy IC 10: Variable and Transient X-Ray Sources

Abstract

We have monitored the Cassiopeia dwarf galaxy (IC 10) in a series of 10 Chandra ACIS-S observations to capture its variable and transient X-ray source population, which is expected to be dominated by High Mass X-ray Binaries (HMXBs). We present a sample of 21 X-ray sources that are variable between observations at the 3 σ level, from a catalog of 110 unique point sources. We find four transients (flux variability ratio greater than 10) and a further eight objects with ratios >5. The observations span the years 2003–2010 and reach a limiting luminosity of >10{sup 35} erg s{sup −1}, providing sensitivity to X-ray binaries in IC 10 as well as flare stars in the foreground Milky Way. The nature of the variable sources is investigated from light curves, X-ray spectra, energy quantiles, and optical counterparts. The purpose of this study is to discover the composition of the X-ray binary population in a young starburst environment. IC 10 provides a sharp contrast in stellar population age (<10 My) when compared to the Magellanic Clouds (40–200 My) where most of the known HMXBs reside. We find 10 strong HMXB candidates, 2 probable background Active Galactic Nuclei, 4 foreground flare-stars or active binaries,more » and 5 not yet classifiable sources. Complete classification of the sample requires optical spectroscopy for radial velocity analysis and deeper X-ray observations to obtain higher S/N spectra and search for pulsations. A catalog and supporting data set are provided.« less

Authors:
;  [1]; ;  [2];  [3];  [4]
  1. Department of Physics and Applied Physics, University of Massachusetts Lowell, MA, 01854 (United States)
  2. University of Washington, Department of Astronomy, Box 351580, Seattle, WA 98195 (United States)
  3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138 (United States)
  4. Department of Mathematical Sciences, University of Massachusetts Lowell, MA, 01854 (United States)
Publication Date:
OSTI Identifier:
22663841
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 836; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; CATALOGS; CLASSIFICATION; COMPARATIVE EVALUATIONS; GALAXY NUCLEI; LUMINOSITY; MAGELLANIC CLOUDS; MASS; MILKY WAY; PULSATIONS; RADIAL VELOCITY; SENSITIVITY; SPECTROSCOPY; STARS; VISIBLE RADIATION; X RADIATION; X-RAY SOURCES; X-RAY SPECTRA

Citation Formats

Laycock, Silas, Cappallo, Rigel, Williams, Benjamin F., Binder, Breanna, Prestwich, Andrea, and Christodoulou, Dimitris M. The X-Ray Binary Population of the Nearby Dwarf Starburst Galaxy IC 10: Variable and Transient X-Ray Sources. United States: N. p., 2017. Web. doi:10.3847/1538-4357/836/1/50.
Laycock, Silas, Cappallo, Rigel, Williams, Benjamin F., Binder, Breanna, Prestwich, Andrea, & Christodoulou, Dimitris M. The X-Ray Binary Population of the Nearby Dwarf Starburst Galaxy IC 10: Variable and Transient X-Ray Sources. United States. doi:10.3847/1538-4357/836/1/50.
Laycock, Silas, Cappallo, Rigel, Williams, Benjamin F., Binder, Breanna, Prestwich, Andrea, and Christodoulou, Dimitris M. Fri . "The X-Ray Binary Population of the Nearby Dwarf Starburst Galaxy IC 10: Variable and Transient X-Ray Sources". United States. doi:10.3847/1538-4357/836/1/50.
@article{osti_22663841,
title = {The X-Ray Binary Population of the Nearby Dwarf Starburst Galaxy IC 10: Variable and Transient X-Ray Sources},
author = {Laycock, Silas and Cappallo, Rigel and Williams, Benjamin F. and Binder, Breanna and Prestwich, Andrea and Christodoulou, Dimitris M.},
abstractNote = {We have monitored the Cassiopeia dwarf galaxy (IC 10) in a series of 10 Chandra ACIS-S observations to capture its variable and transient X-ray source population, which is expected to be dominated by High Mass X-ray Binaries (HMXBs). We present a sample of 21 X-ray sources that are variable between observations at the 3 σ level, from a catalog of 110 unique point sources. We find four transients (flux variability ratio greater than 10) and a further eight objects with ratios >5. The observations span the years 2003–2010 and reach a limiting luminosity of >10{sup 35} erg s{sup −1}, providing sensitivity to X-ray binaries in IC 10 as well as flare stars in the foreground Milky Way. The nature of the variable sources is investigated from light curves, X-ray spectra, energy quantiles, and optical counterparts. The purpose of this study is to discover the composition of the X-ray binary population in a young starburst environment. IC 10 provides a sharp contrast in stellar population age (<10 My) when compared to the Magellanic Clouds (40–200 My) where most of the known HMXBs reside. We find 10 strong HMXB candidates, 2 probable background Active Galactic Nuclei, 4 foreground flare-stars or active binaries, and 5 not yet classifiable sources. Complete classification of the sample requires optical spectroscopy for radial velocity analysis and deeper X-ray observations to obtain higher S/N spectra and search for pulsations. A catalog and supporting data set are provided.},
doi = {10.3847/1538-4357/836/1/50},
journal = {Astrophysical Journal},
number = 1,
volume = 836,
place = {United States},
year = {Fri Feb 10 00:00:00 EST 2017},
month = {Fri Feb 10 00:00:00 EST 2017}
}
  • We report the discovery of a large amplitude (factor of ∼100) X-ray transient (IC 10 X-2, CXOU J002020.99+591758.6) in the nearby dwarf starburst galaxy IC 10 during our Chandra monitoring project. Based on the X-ray timing and spectral properties, and an optical counterpart observed with Gemini, the system is a high-mass X-ray binary consisting of a luminous blue supergiant and a neutron star. The highest measured luminosity of the source was 1.8 × 10{sup 37} erg s{sup –1}during an outburst in 2003. Observations before, during, and after a second outburst in 2010 constrain the outburst duration to be less thanmore » 3 months (with no lower limit). The X-ray spectrum is a hard power law (Γ = 0.3) with fitted column density (N{sub H} = 6.3 × 10{sup 21} atom cm{sup –2}), consistent with the established absorption to sources in IC 10. The optical spectrum shows hydrogen Balmer lines strongly in emission at the correct blueshift (-340 km s{sup –1}) for IC 10. The N III triplet emission feature is seen, accompanied by He II [4686] weakly in emission. Together these features classify the star as a luminous blue supergiant of the OBN subclass, characterized by enhanced nitrogen abundance. Emission lines of He I are seen, at similar strength to Hβ. A complex of Fe II permitted and forbidden emission lines are seen, as in B[e] stars. The system closely resembles galactic supergiant fast X-ray transients, in terms of its hard spectrum, variability amplitude, and blue supergiant primary.« less
  • We present deep Hubble Space Telescope WFPC2 optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury as well as early release Wide Field Camera 3 ultraviolet and infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our data provide a detailed example of how covering such a broad range in wavelength provides a powerful tool for constraining the physical properties of stellar populations. The deepest data reach the ancient red clump at M{sub F814W} {approx} - 0.2. All of the optical data reach the main-sequence turnoff for stars younger than {approx}300 Myr and the blue He-burningmore » sequence for stars younger than 500 Myr. The full color-magnitude diagram (CMD) fitting analysis shows that all three fields in our data set are consistent with {approx}75% of the stellar mass being older than 8 Gyr, in spite of showing a wide range in star formation rates at present. Thus, our results suggest that the scale length of NGC 4214 has remained relatively constant for many gigayears. As previously noted by others, we also find the galaxy has recently ramped up production consistent with its bright UV luminosity and its population of UV-bright massive stars. In the central field we find UV point sources with F336W magnitudes as bright as -9.9. These are as bright as stars with masses of at least 52-56 M{sub sun} and ages near 4 Myr in stellar evolution models. Assuming a standard initial mass function, our CMD is well fitted by an increase in star formation rate beginning 100 Myr ago. The stellar populations of this late-type dwarf are compared with those of NGC 404, an early-type dwarf that is also the most massive galaxy in its local environment. The late-type dwarf appears to have a similar high fraction of ancient stars, suggesting that these dominant galaxies may form at early epochs even if they have low total mass and very different present-day morphologies.« less
  • Chandra X-ray Observatory imaging spectroscopy of the starburst galaxy Henize 2-10 (He 2-10) reveals a strong nuclear point source and at least two fainter compact sources embedded within a more luminous diffuse thermal component. Spectral fits to the nuclear X-ray source imply an unabsorbed X-ray luminosity L{sub x} >10{sup 40} erg s{sup -1} for reasonable power law or blackbody models, consistent with accretion onto a >50 M{sub sun} black hole behind a foreground absorbing column of N{sub H} >10{sup 23} cm{sup -2}. Two of these point sources have L{sub x} = 2 - 5 x 10{sup 38} erg s{sup -1},more » comparable to luminous X-ray binaries. These compact sources constitute a small fraction ({<=}16%) of the total X-ray flux from He 2-10 in the 0.3-6.0 keV band and just 31% of the X-rays in the hard 1.1-6.0 keV band which is dominated by diffuse emission. Two-temperature solar-composition plasmas (kT {approx_equal} 0.2 keV and kT {approx_equal} 0.7 keV) fit the diffuse X-ray component as well as single-temperature plasmas with enhanced {alpha}/Fe ratios. Since the observed radial gradient of the X-ray surface brightness closely follows that of the H{alpha} emission, the composition of the X-ray plasma likely reflects mixing of the ambient cool/warm interstellar medium (ISM) with an even hotter, low emission measure plasma, thereby explaining the {approx}solar ISM composition. Aperture synthesis 21 cm maps show an extended neutral medium to radii of 60'' so that the warm and hot phases of the ISM, which extend to {approx}30'', are enveloped within the 8 x 10{sup 20} cm{sup -2} contour of the cool neutral medium. This extended neutral halo may serve to inhibit a starburst-driven outflow unless it is predominantly along the line of sight. The high areal density of star formation can also be reconciled with the lack of prominent outflow signatures if He 2-10 is in the very early stages of developing a galactic wind.« less
  • We present results for two ultraluminous X-ray sources (ULXs), IC 342 X-1 and IC 342 X-2, using two epochs of XMM-Newton and NuSTAR observations separated by ∼7 days. We observe little spectral or flux variability above 1 keV between epochs, with unabsorbed 0.3-30 keV luminosities being 1.04{sub −0.06}{sup +0.08}×10{sup 40} erg s{sup –1} for IC 342 X-1 and 7.40 ± 0.20 × 10{sup 39} erg s{sup –1} for IC 342 X-2, so that both were observed in a similar, luminous state. Both sources have a high absorbing column in excess of the Galactic value. Neither source has a spectrum consistent with a black hole binary in low/hard state, and both ULXsmore » exhibit strong curvature in their broadband X-ray spectra. This curvature rules out models that invoke a simple reflection-dominated spectrum with a broadened iron line and no cutoff in the illuminating power-law continuum. X-ray spectrum of IC 342 X-1 can be characterized by a soft disk-like blackbody component at low energies and a cool, optically thick Comptonization continuum at high energies, but unique physical interpretation of the spectral components remains challenging. The broadband spectrum of IC 342 X-2 can be fit by either a hot (3.8 keV) accretion disk or a Comptonized continuum with no indication of a seed photon population. Although the seed photon component may be masked by soft excess emission unlikely to be associated with the binary system, combined with the high absorption column, it is more plausible that the broadband X-ray emission arises from a simple thin blackbody disk component. Secure identification of the origin of the spectral components in these sources will likely require broadband spectral variability studies.« less
  • We estimate the reddening and distance of the nearest starburst galaxy IC 10 using deep near-infrared JHK {sub S} photometry obtained with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru telescope. We estimate the foreground reddening toward IC 10 using the UBV photometry of IC 10 from the Local Group Survey, obtaining E(B - V) = 0.52 +- 0.04 mag. We derive the total reddening including the internal reddening, E(B - V) = 0.98 +- 0.06 mag, using the UBV photometry of early-type stars in IC 10 and comparing the JHK {sub S} photometry of red giant branchmore » stars in IC 10 and the Small Magellanic Cloud. Using the Two Micron All Sky Survey point-source catalog of 20 Galactic globular clusters, we derive a relation between the metallicity [Fe/H]{sub CG97} and the slope of the red giant branch in the K {sub S} - (J - K {sub S}) color-magnitude diagram. The mean metallicity of the red giant branch stars in IC 10 is estimated to be [Fe/H]{sub CG97} = -1.08 +- 0.28. The magnitude of the tip of the red giant branch (TRGB) of IC 10 in the K {sub S} band is measured to be K {sub S,TRGB} = 18.28 +- 0.01. Based on the TRGB method, we estimate the distance modulus of IC 10 to be (m - M){sub 0} = 24.27 +- 0.03(random) +- 0.18(systematic), corresponding to the distance of d = 715 +- 10 +- 60 kpc. This confirms that IC 10 is a member of the Local Group.« less