Paul J. Callanan

New Evidence for Black Hole Event Horizons from Chandra

Previously we claimed that black hole X-ray novae (BHXNs) in quiescence are much less luminous than equivalent neutron star X-ray novae (NSXNs). This claim was based on the quiescent detection of a single short-period BHXN (A0620-00, Porb = 7.8 hr) and two longer period BHXNs (GRO J1655-40, Porb = 62.9 hr; V404 Cygni, Porb = 155.3 hr), along with sensitive upper limits. Here we announce the detection of two more short-period BHXNs (GRO J0422+32, Porb = 5.1 hr; GS 2000+25, Porb = 8.3 hr), an upper limit for a third that is improved by 2 orders of magnitude (4U 1543-47, Porb = 27.0 hr), and a new, much lower quiescent measurement of GRO J1655-40. Taken together, these new Chandra Advanced CCD Imaging Spectrometer measurements confirm that the quiescent X-ray luminosities of BHXNs are significantly lower than those of NSXNs. We argue that this provides strong evidence for the existence of event horizons in BHXNs.

A black hole in the superluminal source sax j1819.3-2525 (v4641 sgr)

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best-fitting period of Pspect = 2.81678 ± 0.00056 days and a semiamplitude of K2 = 211.0 ± 3.1 km s-1. The optical mass function is f(M) = 2.74 ± 0.12 M☉. We find a photometric period of Pphoto = 2.81730 ± 0.00001 days using a light curve measured from photographic plates. The folded light curve resembles an ellipsoidal light curve with two maxima of roughly equal height and two minima of unequal depth per orbital cycle. The secondary star is a late B-type star that has evolved off the main sequence. Using a moderate resolution spectrum (R = 7000) we measure Teff = 10500 ± 200 K, log g = 3.5 ± 0.1, and Vrot sin i = 123 ± 4 km s-1 (1 σ errors). Assuming synchronous rotation, our measured value of the projected rotational velocity implies a mass ratio of Q ≡ M1/M2 = 1.50 ± 0.08 (1 σ). The lack of X-ray eclipses implies an upper limit to the inclination of i ≤ 707. On the other hand, the large amplitude of the folded light curve (≈0.5 mag) implies a large inclination (i 60°). Using the above mass function, mass ratio, and inclination range, the mass of the compact object is in the range 8.73 ≤ M1 ≤ 11.70 M☉ and the mass of the secondary star is in the range 5.49 ≤ M2 ≤ 8.14 M☉ (90% confidence). The mass of the compact object is well above the maximum mass of a stable neutron star, and we conclude that V4641 Sgr contains a black hole. The B-star secondary is by far the most massive, the hottest, and the most luminous secondary of the dynamically confirmed black hole X-ray transients. We find that the α-process elements nitrogen, oxygen, calcium, magnesium, and titanium may be overabundant in the secondary star by factors of 2-10 times with respect to the Sun. Finally, assuming E(B-V) = 0.32 ± 0.10, we find a distance 7.40 ≤ d ≤ 12.31 kpc (90% confidence). This large distance and the high proper motions observed for the radio counterpart make V4641 Sgr possibly the most superluminal galactic source known, with an apparent expansion velocity of 9.5c and a bulk Lorentz factor of Γ 9.5, assuming that the jets were ejected during one of the bright X-ray flares observed with the Rossi X-ray Timing Explorer.

Observations of the X-ray Nova GRO J0422+32. 1: Outburst and the decay to quiescence

We present optical photometry and spectroscopy and Burst and Transient Source Experiment (BATSE) observations of the X-ray nova GRO J0422+32, obtained during outburst and its subsequent decay to quiescence. Although the X-ray and optical properties of GRO J0422+32 are broadly similar to those of other X-ray novae, it is unique in several respects. The unusually protracted decay to quiescence of the optical light curve has been punctuated by at least two minioutbursts of approximately 4 mag. The BATSE and optical outbursts are each separated by approximately 120 days. We find that the optical luminosity of GRO J0422+32 during the primary outburst is dominated by reprocessing of E greater than 10 keV X-rays. In contrast, the optical minioutbursts are most likely generated by an intrinsically bright disk rather than X-ray reprocessing: they do not appear to have any X-ray counterparts. Extremely broad (up to 6000 km/s FWZI) absorption lines have also been observed during both primary outbursts and minioutbursts. During the second minioutburst, H-alpha and H-beta emission was observed superposed on redshifted absorption features. We find that the interoutburst light curve of GRO J0422+32 may be inconsistent with an accretion disk instability as the origin of the minioutbursts. Finally, a transient 5.1/10.2 hr modulation, which may be related to the orbital period, has been observed during roughly half of our observations. However, confirmation of the orbital period must await observations in quiescence.

MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence*

We report on the analysis of new and previously published MMT optical spectra of the black hole binary XTE J1118+480 during the decline from the 2000 outburst to true quiescence. From cross-correlation with template stars, we measure the radial velocity of the secondary to derive a new spectroscopic ephemeris. The observations acquired during approach to quiescence confirm the earlier reported modulation in the centroid of the double-peaked Hα emission line. In addition, our data combined with the results presented by Zurita et al. (2002) provide support for a modulation with a periodicity in agreement with the expected precession period of the accretion disk of ~52 days. Doppler images during the decline phase of the Hα emission line show evidence for a hot spot and emission from the gas stream: the hot spot is observed to vary its position, which may be due to the precession of the disk. The data available during quiescence show that the centroid of the Hα emission line is offset by about -100 km s-1 from the systemic velocity, which suggests that the disk continues to precess. An Hα tomogram reveals emission from near the donor star after subtraction of the ringlike contribution from the accretion disk, which we attribute to chromospheric emission. No hot spot is present, suggesting that accretion from the secondary has stopped (or decreased significantly) during quiescence. Finally, a comparison is made with the black hole XRN GRO J0422+32: we show that the Hα profile of this system also exhibits a behavior consistent with a precessing disk.

Spectroscopic Identification of Probable Cataclysmic Variables in the Globular Cluster NGC 6397

The existence of compact binaries in dense clusters is of considerable interest for testing theories of binary production and evolution, as well as the dynamical evolution and survival of the clusters in which they reside. Here, using the newly repaired Hubble Space Telescope to observe the collapsed core of the nearby globular cluster NGC 6397, we report the first spectra of stars well below the main-sequence turnoff near the center of a dense globular star cluster. The spectra confirm our WFPC1 photometry that these objects are almost certainly the long-sought population of cataclysmic variables (CVs) in globulars, and that they are the likely counterparts for the recently discovered (ROSAT) low-luminosity X-ray sources in this cluster. The spectra suggest the CVs are magnetic (DQ Her type), raising the intriguing possibility that magnetic white dwarfs are preferentially produced in globular cluster cores.

The light curve of the companion to PSR B1957+20

We present a new analysis of the light curve for the secondary star in the eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100 per cent coverage in the R-band. We also have a number of new K s -band data points, which we use to constrain the infrared magnitude of the system. We model this with the eclipsing light-curve (ELC) code. From the modelling with the ELC code we obtain colour information about the secondary at minimum light in BVRI and K. For our best-fitting model we are able to constrain the system inclination to 65° ± 2° for pulsar masses ranging from 1.3 to 1.9 M⊙. The pulsar mass is unconstrained. We also find that the secondary star is not filling its Roche lobe. The temperature of the unirradiated side of the companion is in agreement with previous estimates and we find that the observed temperature gradient across the secondary star is physically sustainable.

Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4−3658, obtained during the 2008 September/October outburst. Doppler tomography of the N iii λ4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 ± 15 km s 1 ; applying a “K-correction”, we find the velocity of the secondary star projected onto the line of sight to be 370 ± 40 km s 1 . Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044 +0.005 0.004, and the mass function for the pulsar to be 0.44 +0.16 0.13 M⊙. Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4−3658 is more massive than the canonical value of 1.4 M⊙. Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca ii H and K interstellar absorption lines suggest that the distance to the source is ∼2.5 kpc. This is consistent with previous distance estimates based on type-I Xray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.

A Catalog of Transient X-Ray Sources in M31

From 1999 October to 2002 August, 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 light curves for these sources covering this 2.8 yr 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 eight of the transients and range from 34% to 2%; upper limits to the duty cycles are estimated for an additional 15 transients and cover a similar range. We find five transients that have rapid decay times and may be ultracompact X-ray binaries. Spectra of three of the transients suggest that 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.

Power-Law Decays in the Optical Counterparts of GRB 970228 and GRB 970508

We report on RC- and K-band observations of the optical counterpart to GRB 970508 with the Fred Lawrence Whipple Observatory (FLWO) 1.2 m telescope. Eleven RC-band observations were obtained on 1997 May 12, and three on May 14. The counterpart clearly faded between the two nights. On May 12 there was no evidence for variability (!9%) on 10‐70 minute timescales based on 11Rc-band observations. On May 19 a 1 hr observation set a limit on the K magnitude of . Comparison of these data points with those obtained by other authors K 1 18.6 shows that the decay of the optical counterpart can be well fitted by a power law of the form , where 2a f » t with occasional fluctuations superposed. We note that the decay of the optical counterpart to a 5 1.22 5 0.03 another burst, GRB 970228, can also be well fitted with a power law with exponent with occasional 10.2 a 5 1.020.5 fluctuations superposed. These two decay light curves are remarkably similar in form to that predicted by cosmicfireball models. Subject heading: gamma rays: bursts

Spectroscopic observations of the candidate sgB(e)/X-ray binary CI Camelopardalis

We present a compilation of spectroscopic observations of the sgB(e) star CI Cam, the optical counterpart of XTE J0421+560. This includes data from before, during, and after its 1998 outburst, with quantitative results spanning 37 years. The object shows a rich emission line spectrum originating from circumstellar material, rendering it dicult to determine the nature of either star involved or the cause of the outburst. We collate all available pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with outburst and post-outburst data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with Feii and (Nii) lines still a factor of2 above the pre-outburst level and Hei ,H eii ,a nd Nii lines suppressed by a factor of 2-10. We find that the spectral properties of CI Cam are similar to other sgB(e) stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other ob- jects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. Based on a comparison of the properties of CI Cam with the other sgB(e) stars we suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. The enhanced outburst emission most likely originated either directly from this supercritical accretion, or from the interaction of the expanding remnant with the equatorial material, or from a combination of both mechanisms.