Can Chandra resolve the remaining cosmic X-ray background?

The deepest extragalactic X-ray observation, the 2 Ms Chandra Deep Field North (CDF-N), resolves ~80% of the total extragalactic cosmic X-ray background (CXB) in the 1-2 keV band. Recent work has shown that 70% of the remaining CXB flux is associated with sources detected by the Hubble Space Telescope (HST). This paper uses the existing CDF-N data to constrain the X-ray flux distribution of these X-ray undetected HST sources, by comparing the number of 0.5-2 keV X-ray counts at the HST positions to those expected for model flux distributions. In the simple case where all the undetected HST X-ray sources have the same 0.5-2 keV flux, the data are best fit by 1.5-3 counts per source in 2 Ms, compared to a detection limit (at 10% completeness) of 9 counts. Assuming a more realistic power-law logN-logS distribution [N(>S) S^-alpha], the data favor a relatively steep flux distribution, with alpha=1.1^+0.5_-0.3 (limits are 99% confidence). This slope is very similar to that previously found for faint normal and starburst galaxies in the CDF-N. These results suggest deeper Chandra observations will detect a new population of faint X-ray sources, but extremely deep exposures are needed to resolve the remainder of the soft CXB. In the most optimistic scenario, when the HST sources have the flattest allowed flux distribution and all the sources without HST counterparts are detected, observations 5 times more sensitive than the existing ones would resolve at most ~60% of the remaining soft CXB.Comment: 9 emulateapj pages, 8 figures, v3: matches version to appear in ApJ (note correction to approximation of Poisson errors

Optical Constraints on an X-ray Transient Source in M31

We have detected a transient X-ray source in the M31 bulge through a continuing monitoring campaign with the Chandra ACIS-I camera. The source was detected at R.A.=00:42:33.428 +/- 0.11'', Dec.=+41:17:03.37 +/- 0.11'' in only a single observation taken 2004 May 23. Fortuitous optical HST/ACS imaging of the transient location prior to the X-ray outburst, along with follow-up HST/ACS imaging during and after the outburst, reveals no transient optical source brighter than B (equivalent) = 25.5. The location of the source and its X-ray properties suggest it is a low mass X-ray binary (LMXB). Assuming the transient is similar to many Galactic X-ray novae, the X-ray luminosity of (3.9 +/- 0.5) X 10$^{37}$ erg s$^{-1}$ and the upper-limit on the optical luminosity provide a prediction of <1.6 days for the orbital period of the binary system.Comment: 15 pages, 3 figures, accepted for publication in Ap

A 43-GHz Survey in the ELAIS N2 Area

We describe a survey in the ELAIS N2 region with the VLA at 43.4 GHz, carried out with 1627 independent snapshot observations in D-configuration and covering about 0.5 square degrees. One certain source is detected, a previously-catalogued flat-spectrum QSO at z=2.2. A few (<5) other sources may be present at about the 3sigma level, as determined from positions of source-like deflections coinciding with blue stellar objects, or with sources from lower-frequency surveys. Independently we show how all the source-like detections identified in the data can be used with a maximum-likelihood technique to constrain the 43-GHz source counts at a level of ~7 mJy. Previous estimates of the counts at 43 GHz, based on lower-frequency counts and spectral measurements, are consistent with these constraints, although the present results are suggestive of somewhat higher surface densities at the 7 mJy level. They do not provide direct evidence of intrusion of a previously unknown source population, although the several candidate sources need examination before such a population can be ruled out.Comment: 13 pages, 11 figures, 1 table; accepted for publication in Mon. Not R. Astr. So

Chandra and Hubble Study of a New Transient X-ray Source in M31

We present X-ray and optical observations of a new transient X-ray source in M31 first detected 23-May-2004 at R.A.=00:43:09.940 +/- 0.65'', Dec.=41:23:32.49 +/- 0.66''. The X-ray lightcurve shows two peaks separated by several months, reminiscent of many Galactic X-ray novae. The location and X-ray spectrum of the source suggest it is a low mass X-ray binary (LMXB). Follow-up HST ACS observations of the location both during and after the outburst provide a high-confidence detection of variability for one star within the X-ray position error ellipse. This star has $\Delta$B ~ 1 mag, and there is only a ~1% chance of finding such a variable in the error ellipse. We consider this star a good candidate for the optical counterpart of the X-ray source. The luminosity of this candidate provides a prediction for the orbital period of the system of 2.3$^{+3.7}_{-1.2}$ days.Comment: 17 pages, 3 figures, 4 tables, accepted for publication in Ap