A Catalog of Transient X-ray Sources in M31

From October 1999 to August 2002, 45 transient X-ray sources were detected in M31 by Chandra and XMM-Newton. We have performed spectral analysis of all XMM-Newton and Chandra ACIS detections of these sources, as well as flux measurements of Chandra HRC detections. The result is absorption-corrected X-ray lightcurves for these sources covering this 2.8 year period, along with spectral parameters for several epochs of the outbursts of most of the transient sources. We supply a catalog of the locations, outburst dates, peak observed luminosities, decay time estimates, and spectral properties of the transient sources, and we discuss similarities with Galactic X-ray novae. Duty cycle estimates are possible for 8 of the transients and range from 40% to 2%; upper limits to the duty cycles are estimated for an additional 15 transients and cover a similar range. We find 5 transients which have rapid decay times and may be ultra-compact X-ray binaries. Spectra of three of the transients suggest they may be faint Galactic foreground sources. If even one is a foreground source, this suggests a surface density of faint transient X-ray sources of >~1 deg$^{-2}$.Comment: 63 pages, 22 figures, 3 tables, accepted for publication in Ap

A Synoptic X-ray Study of M31 with the Chandra-HRC

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of 2.5 years and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term lightcurves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well-fit by a power-law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with L_X > 10^{37} erg/s . We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 solar mass black hole X-ray nova with an orbital period of ~20 days. In the other case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.Comment: 68 pages (27 text), 8 tables, 16 figures, 1 appendix, accepted by ApJ; accepted version contains reorganized text, new tables and figures, and updated result

X-ray/Optical/Radio Observations of a Resolved Supernova Remnant in NGC 6822

The supernova remnant (SNR), Ho 12, in the center of the dwarf irregular galaxy NGC 6822 was previously observed at X-ray, optical, and radio wavelengths. By using archival Chandra and ground-based optical data, we found that the SNR is spatially resolved in X-rays and optical. In addition, we obtained a ~5" resolution radio image of the SNR. These observations provide the highest spatial resolution imaging of an X-ray/optical/radio SNR in that galaxy to date. The multi-wavelength morphology, X-ray spectrum and variability, and narrow-band optical imagings are consistent with a SNR. The SNR is a shell-shaped object with a diameter of about 10" (24 pc). The morphology of the SNR is consistent across the wavelengths while the Chandra spectrum can be well fitted with a nonequilibrium ionization model with an electron temperature of 2.8 keV and a 0.3-7 keV luminosity of 1.6e37 erg/s. The age of the SNR is estimated to be 1700-5800 years.Comment: 6 pages, 3 figures, accepted for publication in the Astronomical Journa

Discovery of an X-ray Nova in M31

We have obtained snapshot images of an X-ray nova in M31 from Chandra ACIS-I and the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The Chandra position of the X-ray nova was R.A.=00:44:06.68 +/- 1.74", Dec.=+41:12:20.0 +/- 2.31". A follow-up HST observation 24 days later revealed a source at R.A.=00:44:06.81, Dec.=+41:12:24.0 that was B=25.75 +/- 0.05. This optical source faded to B=27.1 +/- 0.1 in 3 months. During this time period, the X-ray flux decayed linearly from (3.6 +/- 0.2) X 10^{-4} to <(6.9 +/- 0.09) X 10^{-5} ct cm^{-2} s^{-1}. The HST identification of an optical source in the same region experiencing an obvious drop in brightness in concert with the X-ray nova suggests that this optical source is the counterpart of the X-ray nova. However, the precision of the X-ray position allows the possibility that the optical source is a nearby variable star. We discuss the implications of both possibilities.Comment: 20 pages, 3 figures, 5 tables, accepted for publication in Ap

A Possible Detection of M31* with Chandra

Two independent sets of Chandra and HST images of the nuclear region of M31 allow registration of X-ray and optical images to 0.1''. This registration shows that none of the bright (10^{37} erg/s) X-ray sources near the nucleus is coincident with the central super-massive black hole, M31*. A 50ks Chandra HRC image shows 2.5 sigma evidence for a faint (3 x 10^{35} erg/s), apparently resolved source which is consistent with the position of the M31*. The Bondi radius of M31* is 0.9'', making it one of the few super-massive black holes with a resolvable accretion flow. This large radius and the previous detections of diffuse, X-ray emitting gas in the nuclear region make M31* one of the most secure cases for a radiatively inefficient accretion flow and place some of the most severe constraints on the radiative processes in such a flow.Comment: 14 pages, 5 figures, submitted to Ap

A Potential Supernova Remnant/X-ray Binary Association in M31

The well-studied X-ray/Optical/Radio supernova remnant DDB 1-15 (CXOM31 J004327.8+411829; r3-63) in M31 has been investigated with archival XMM-Newton and Chandra observations. The timing data from XMM-Newton reveals a power density spectrum (PDS) characteristic of accreting compact objects in X-ray binaries (XRBs). The PDS shows features typical of Roche lobe overflow accretion, hinting that the XRB is low-mass. The Chandra observations resolve the SNR into a shell and show a variable count rate at the 94% confidence level in the northwest quadrant. Together, these XMM-Newton and Chandra data suggest that there is an XRB in the SNR r3-63 and that the XRB is located in the northwestern portion of the SNR. The currently-available X-ray and optical data show no evidence that the XRB is high-mass. If the XRB is low-mass, r3-63 would be the first SNR found to contain a low-mass X-ray binary.Comment: 30 pages, 3 tables, 11 figures, accepted for publication in Ap

An X-ray Transient and Optical Counterpart in the M31 Bulge

We have obtained snapshot images of a transient X-ray source in M31 from Chandra ACIS-I and the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The Chandra position of the X-ray nova was R.A.=00:42:56.038 +/-0.08'', Dec.=+41:12:18.50 +/-0.07''. The transient was active for at least 6 months. Previous observations set an upper limit before the X-ray outburst, demonstrating variability by a factor of >100 and confirming the transient nature of the source. For the first 6 months after the initial detection, the X-ray luminosity was ~6 X 10$^{37}$ erg/s; it then decayed to <5 X 10$^{36}$ erg/s over the following 2 months. An HST observation 29 days after the initial X-ray detection revealed a source at R.A.=00:42:56.042, Dec.=+41:12:18.45 that was B=24.52 +/- 0.07. This optical source faded to B=24.95 +/- 0.08$in 9 months. The HST identification of an optical source at the same position as the X-ray source, fading in concert with the X-ray source, indicates that this optical source is the counterpart of the X-ray transient. The lack of high-mass stars in the region suggests this source is a low-mass X-ray binary, and the X-ray and optical luminosities provide a rough orbital period estimate of 8$^{+12}_{-5}$ days for the system.Comment: 16 pages, 4 tables, 2 figures, accepted for publication in Ap

Two New X-ray/Optical/Radio Supernova Remnants in M31

We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to Local Group Survey narrow-band optical data and Very Large Array (VLA) radio data of the same region. Our precisely registered images reveal two new optical shells with X-ray counterparts. These shells have sizes, [S II]/H-alpha flux ratios, and X-ray spectral properties typical of supernova remnants (SNRs) with ages of 9$^{+3}_{-4}$ and 17$^{+6}_{-9}$ kyr. Analysis of complementary VLA data reveals the radio counterparts, further confirming that they are SNRs. We discuss and compare the properties and morphologies of these SNRs at the different wavelengths.Comment: 18 pages, 5 figures, accepted for publication in Ap

Modeling Patient-Specific Dose-Function Response for Enhanced Characterization of Personalized Functional Damage

PURPOSE: Functional-guided radiation therapy (RT) plans have the potential to limit damage to normal tissue and reduce toxicity. Although functional imaging modalities have continued to improve, a limited understanding of the functional response to radiation and its application to personalized therapy has hindered clinical implementation. The purpose of this study was to retrospectively model the longitudinal, patient-specific dose-function response in non-small cell lung cancer patients treated with RT to better characterize the expected functional damage in future, unknown patients. METHODS AND MATERIALS: Perfusion single-photon emission computed tomography/computed tomography scans were obtained at baseline (n = 81), midtreatment (n = 74), 3 months post-treatment (n = 51), and 1 year post-treatment (n = 26) and retrospectively analyzed. Patients were treated with conventionally fractionated RT or stereotactic body RT. Normalized perfusion single-photon emission computed tomography voxel intensity was used as a surrogate for local lung function. A patient-specific logistic model was applied to each individual patient's dose-function response to characterize functional reduction at each imaging time point. Patient-specific model parameters were averaged to create a population-level logistic dose-response model. RESULTS: A significant longitudinal decrease in lung function was observed after RT by analyzing the voxelwise change in normalized perfusion intensity. Generated dose-function response models represent the expected voxelwise reduction in function, and the associated uncertainty, for an unknown patient receiving conventionally fractionated RT or stereotactic body RT. Differential treatment responses based on the functional status of the voxel at baseline suggest that initially higher functioning voxels are damaged at a higher rate than lower functioning voxels. CONCLUSIONS: This study modeled the patient-specific dose-function response in patients with non-small cell lung cancer during and after radiation treatment. The generated population-level dose-function response models were derived from individual patient assessment and have the potential to inform functional-guided treatment plans regarding the expected functional lung damage. This type of patient-specific modeling approach can be applied broadly to other functional response analyses to better capture intrapatient dependencies and characterize personalized functional damage

Why are X-ray sources in the M31 Bulge so close to Planetary Nebulae?

We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to deep [O III] Local Group Survey data of the same region. Through precision image alignment using globular cluster X-ray sources, we are able to improve constraints on possible optical/X-ray associations suggested by previous surveys. Our image registration allows us to rule out several emission-line objects, previously suggested to be the optical counterparts of X-ray sources, as true counterparts. At the same time, we find six X-ray sources peculiarly close to strong [O III] emission-line sources, classified as planetary nebulae (PNe) by previous optical surveys. Our study shows that, while the X-rays are not coming from the same gas as the optical line emission, the chances of these six X-ray sources lying so close to cataloged PNe is only ~1%, suggesting that there is some connection between these [O III] emitters (possibly PNe) and the X-ray sources. We discuss the possibility that these nebulae are misidentified supernova remnants, and we rule out the possibility that the X-ray sources are ejected X-ray binaries. There is a possibility that some cases involve a PN and an LMXB that occupy the same undetected star cluster. Beyond this unconfirmed possibility, and the statistically unlikely one that the associations are spatial coincidences, we are unable to explain these [O III]/X-ray associations.Comment: 21 pages, 3 figures, 2 tables, accepted for publication in A