Young Accreting Compact Objects in M31: The Combined Power of NuSTAR, Chandra, and Hubble

Abstract

We present 15 high-mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope. The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard-state black holes, pulsars, and non-magnetized neutron stars associated with optical point-source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point-source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding seven likely high-mass stars and one possible red helium-burning star. The remaining seven HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates-X-ray sources with UV-bright point-source optical counterpart candidates-are found in regions with star formation bursts less than 50 Myr ago, and three are associated with young stellar ages (<10Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403.

Keywords: X-rays: binaries; X-rays: galaxies; galaxies: individual (M31); pulsars: general; stars: black holes; stars: neutron.

Figure 1.

Near-UV image of M31 from the Galaxy Evolution Explorer (GALEX; left; Gil de Paz et al. 2009) and three-color X-ray image of Chandra Field A (right), see Section 2.2 for more information on Field A data reduction. Magenta regions outline the area observed by PHAT. Green outlines the NuSTAR observed region, and cyan outlines the area observed by Chandra, with solid lines indicating ChandraPHAT observations and dashed lines indicating Field A observations. In the UV image, the 64 sources observed by NuSTAR and Chandra that fall within the PHAT footprint are marked with white crosses, and a 5′ circle outlines the densest inner bulge region of M31. In the X-ray image, all sources detected by wavdetect within Field A are marked with white ellipses (including sources that do not match NuSTAR sources or are outside the PHAT footprint, and thus are not presented in our sample in Table 1).

Figure 2.

Results of ACIS extract iteration to improve Chandra source positions in Field A observations (see Section 2.2.1). The top panel shows the change in source position between the input source list from wavdetect and the output AE positions. The bottom panel shows the change between the input and output source positions for the fourth iteration of AE. Note that most of the sources with an off-axis angle >10′ have more than 100 counts. These belong to the nucleus of M31, which is far off axis in the Field A observations and has a high source density. The source with an off-axis angle of ~17′.5 in the bottom panel is not shown in the top panel because it has a Δ position >2″ between the input position from wavdetect and the output position from AE.

Figure 3.

Comparison of NuSTAR and Chandra measured fluxes for 60 hard X-ray sources observed by both telescopes. Sources were matched positionally to within 10″.

Figure 4.

Color–magnitude diagrams (top) and finding charts (bottom) from the PHAT data set for the 10″ × 10″ region surrounding source 004335.91+411433.4. The top two panels show the UV and optical CMDs. The bottom two panels show a UV image in the F336W filter and an RGB optical image with the F160W filter as red, F814W as green, and F336W as blue. The plotted ellipses represent the 1 and 3σ Chandra positional errors. The optical counterpart is identified in the UV finder (lower left) with a cyan circle and on the CMDs with a cyan star. The black points in the background represent other stars in the PHAT photometry catalog within 5″ of the X-ray source position.

Figure 5.

Hardness-intensity diagram and hardness-ratio plots used to classify NuSTAR sources. These plots show sources classified as HMXB candidates as black diamonds. Circles outlined in orange are the two background galaxies with NuSTAR classifications that were identified using HST imaging, described further in Section 5.1. Squares outlined in magenta are globular clusters, which occupy the non-magnetized neutron star region of both diagrams (see Section 5.5 for a further discussion of X-ray sources in clusters). The bands are defined as follows: soft (S = 4–6 keV), medium (M = 6–12 keV), and hard (H = 12–25 keV). Sources are labeled by NuSTAR ID. Background colored sources represent modeled evolutionary tracks of Galactic X-ray binaries with known compact object types, adjusted for the distance of M31 (see A. Zezas et al., 2018 in preparation for information on the Galactic XRB diagnostic regions). NuSTAR data and source classifications for M31 sources from Wik et al. (2018, in preparation).

Figure 6.

BEAST SED fits for the optical counterparts of two HMXB candidates, 004321.48+411556.9 (left) and 004448.13+412247.9 (right). The counterpart in the left panel has a robust fit and is most likely a B-type star. The counterpart in the right panel returns a poor fit. Black points show measured photometry for the optical counterparts from the PHAT data set, listed in Table 2. The colored lines show the median fit ±33% errors of the three different models. Yellow shows the stellar-only model, red shows a stellar+dust model, and blue shows a stellar+dust+bias model. The observational bias is determined using artificial star tests.

Figure 7.

Histogram showing the number of HMXB candidates associated with each time bin based on their spatially resolved star formation histories from Lewis et al. (2015). The time bins are defined by the age resolution of the SFHs. The black line represents the average of 100 random samples of regions in M31 not associated with HMXB candidates, providing a reference for the overall SFH of M31.

Figure 8.

Comparing the NuSTAR X-ray classification and optical counterpart of sources observed with NuSTAR, Chandra, and PHAT. NuSTAR classifications are defined as follows: NS = non-magnetized neutron star, PUL = pulsar, HBH = hard-state black hole, and IBH = intermediate-state black hole. For a further discussion of the NuSTAR classified intermediate-state black hole associated with a cluster, see Section 5.5.

Figure 9.

Images showing background galaxies associated with NuSTAR source 118 (left) and 116 (right). These color images were created with HST imaging from the PHAT survey and use the F160W filter as red, F814W as green, and F336W as blue. Images are 10″ on a side. White circles indicate the 1 and 3σ Chandra positional errors. The 1σ error is not visible on the image on the left as it lies on top of the bright galaxy. The galaxy on the left is bright, extended, and elliptical in shape. The galaxy on the right is much fainter. It is visible as a faint, red, extended source within the 1σ error error circle, indicated with an arrow. For more discussion of background galaxies, see Section 5.1.