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Register a new userThe Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115 (II): properties of point sources
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We have carried out an in-depth study of low-mass X-ray binaries (LMXBs) detected in the nearby lenticular galaxy NGC 3115, using the Megasecond Chandra X-Ray Visionary Project observation (total exposure time 1.1 Ms). In total we found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above 2sigma detection, with 0.3--8 keV luminosity L_X ~10^36-10^39 erg/s. Other than 13 transient candidates, the sources overall have less long-term variability at higher luminosity, at least at L_X > 2x10^37 erg/s. In order to identify the nature and spectral state of our sources, we compared their collective spectral properties based on single-component models (a simple power law or a multicolor disk) with the spectral evolution seen in representative Galactic LMXBs. We found that in the L_X versus photon index Gamma_PL and L_X versus disk temperature kT_MCD plots, most of our sources fall on a narrow track in which the spectral shape hardens with increasing luminosity below L_X~7x10^37 erg/s but is relatively constant (Gamma_PL~1.5 or kT_MCD~1.5 keV) above this luminosity, similar to the spectral evolution of Galactic neutron star (NS) LMXBs in the soft state in the Chandra bandpass. Therefore we identified the track as the NS LMXB soft-state track and suggested sources with L_X<7x10^37 erg/s as atolls in the soft state and those with L_X>7x10^37 erg/s as Z sources. Ten other sources (five are transients) displayed significantly softer spectra and are probably black hole X-ray binaries in the thermal state. One of them (persistent) is in a metal-poor GC.
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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.
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.
We present a detailed, broadband X-ray spectral analysis of the ULX pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the $XMM$-$Newton$, $NuSTAR$ and $Chandra$ observatories. The broadband $XMM$-$Newton+NuSTAR$ spectrum of P13 is qualitatively similar to the rest of the ULX sample with broadband coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures $sim$0.5 and $sim$1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be similar, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron stars magnetic field soon after its onset, implying a limit of $B lesssim 6 times 10^{12}$ G for the dipolar component of the central neutron stars magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual off states. These data require both a hard powerlaw component, suggesting residual accretion onto the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.
A very sensitive X-ray investigation of the giant HII region N11 in the LMC was performed using the Chandra X-ray Observatory. The 300ks observation reveals X-ray sources with luminosities down to 10^32 erg/s, increasing by more than a factor of 5 the number of known point sources in the field. Amongst these detections are 13 massive stars (3 compact groups of massive stars, 9 O-stars and one early B-star) with log(Lx/Lbol)~-6.5 to -7, which may suggest that they are highly magnetic or colliding wind systems. On the other hand, the stacked signal for regions corresponding to undetected O-stars yields log(Lx/Lbol)~-7.3, i.e., an emission level comparable to similar Galactic stars despite the lower metallicity. Other point sources coincide with 11 foreground stars, 6 late-B/A stars in N11, and many background objects. This observation also uncovers the extent and detailed spatial properties of the soft, diffuse emission regions but the presence of some hotter plasma in their spectra suggests contamination by the unresolved stellar population.
As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of optical sources in the GBS footprint. This consists of two regions centered at Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning (l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of observations for each line of sight in r, i and H{alpha} filters. It is complete down to r = 20.2 and i = 19.2 mag; the mean 5{sigma} depth is r = 22.5 and i = 21.1 mag. The mean root-mean-square residuals of the astrometric solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the 1640 unique X-ray sources detected in Chandra observations of the GBS area, and find candidate optical counterparts to 1480 X-ray sources. We use a false alarm probability analysis to estimate the contamination by interlopers, and expect ~ 10 per cent of optical counterparts to be chance alignments. To determine the most likely counterpart for each X-ray source, we compute the likelihood ratio for all optical sources within the 4{sigma} X-ray error circle. This analysis yields 1480 potential counterparts (~ 90 per cent of the sample). 584 counterparts have saturated photometry (r<17, i<16), indicating these objects are likely foreground sources and the real counterparts. 171 candidate counterparts are detected only in the i-band. These sources are good qLMXB and CV candidates as they are X-ray bright and likely located in the Bulge.
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