X-ray Variability from the Compact Source in the Supernova Remnant RCW 103

A new ASCA observation of 1E 161348-5055, the central compact X-ray source in the supernova remnant RCW 103, reveals an order-of-magnitude decrease in its 3 - 10 keV flux since the previous ASCA measurement four years earlier. This result is hard to reconcile with suggestions that the bulk of the emission is simple quasi-blackbody, cooling radiation from an isolated neutron star. Furthermore, archived EINSTEIN and ROSAT datasets spanning 18 years confirm that this source manifests long-term variability, to a lesser degree. This provides a natural explanation for difficulties encountered in reproducing the original EINSTEIN detection of 1E 161348-5055. Spectra from the new data are consistent with no significant spectral change despite the decline in luminosity. We find no evidence for a pulsed component in any of the data sets, with a best upper limit on the pulsed modulation of 13 percent. We discuss the phenomenology of this remarkable source.Comment: 5 pages with 2 embedded figures, LaTex, emulateapj.sty. To appear in the Astrophysical Journal Letter

Simultaneous Absolute Timing of the Crab Pulsar at Radio and Optical Wavelengths

The Crab pulsar emits across a large part of the electromagnetic spectrum. Determining the time delay between the emission at different wavelengths will allow to better constrain the site and mechanism of the emission. We have simultaneously observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with $\mu$s time resolution and at 2 GHz using the Nan\c{c}ay radio telescope with an instrument doing coherent dedispersion and able to record giant pulses data. We have studied the delay between the radio and optical pulse using simultaneously obtained data therefore reducing possible uncertainties present in previous observations. We determined the arrival times of the (mean) optical and radio pulse and compared them using the tempo2 software package. We present the most accurate value for the optical-radio lag of 255 $\pm$ 21 $\mu$s and suggest the likelihood of a spectral dependence to the excess optical emission asociated with giant radio pulses.Comment: 8 pages; accepted for publication in Astronomy and Astrophysic

First results of a cryogenic optical photon counting imaging spectrometer using a DROID array

Context. In this paper we present the first system test in which we demonstrate the concept of using an array of Distributed Read Out Imaging Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam 3 detector the next step in the development of a cryogenic optical photon counting imaging spectrometer under the S-Cam project is to increase the field of view using DROIDs. With this modification the field of view of the camera has been increased by a factor of 5 in area, while keeping the number of readout channels the same. Methods. The test has been performed using the flexible S-Cam 3 system and exchanging the 10x12 Superconducting Tunnel Junction array for a 3x20 DROID array. The extra data reduction needed with DROIDs is performed offline. Results. We show that, although the responsivity (number of tunnelled quasiparticles per unit of absorbed photon energy, e- /eV) of the current array is too low for direct astronomical applications, the imaging quality is already good enough for pattern detection, and will improve further with increasing responsivity. Conclusions. The obtained knowledge can be used to optimise the system for the use of DROIDs.Comment: 7 pages, 9 figures, accepted for publicaiton in A&

Sonic-Point Model of Kilohertz Quasi-Periodic Brightness Oscillations in Low-Mass X-ray Binaries

Strong, coherent, quasi-periodic brightness oscillations (QPOs) with frequencies ranging from about 300 Hz to 1200 Hz have been discovered with the Rossi X-ray Timing Explorer in the X-ray emission from some fifteen neutron stars in low-mass binary systems. Two simultaneous kilohertz QPOs differing in frequency by 250 to 350 Hertz have been detected in twelve of the fifteen sources. Here we propose a model for these QPOs. In this model the X-ray source is a neutron star with a surface magnetic field of 10^7 to 10^10 G and a spin frequency of a few hundred Hertz, accreting gas via a Keplerian disk. The frequency of the higher-frequency QPO in a kilohertz QPO pair is the Keplerian frequency at a radius near the sonic point at the inner edge of the Keplerian flow whereas the frequency of the lower-frequency QPO is approximately the difference between the Keplerian frequency at a radius near the sonic point and the stellar spin frequency. This model explains naturally many properties of the kilohertz QPOs, including their frequencies, amplitudes, and coherence. We show that if the frequency of the higher-frequency QPO in a pair is an orbital frequency, as in the sonic-point model, the frequencies of these QPOs place interesting upper bounds on the masses and radii of the neutron stars in the kilohertz QPO sources and provide new constraints on the equation of state of matter at high densities. Further observations of these QPOs may provide compelling evidence for the existence of a marginally stable orbit, confirming a key prediction of general relativity in the strong-field regime.Comment: 67 pages, including 15 figures and 5 tables; uses aas2pp4; final version to appear in the Astrophysical Journal on 1 December 199

INTEGRAL observations of the black hole candidate H 1743-322 in outburst

INTEGRAL made 3 observations in 2003 April of the black hole candidate H 1743-322 during the rising part, and close to the maximum, of an outburst. H 1743-322 was previously observed in outburst in 1977-1978. The source is located in a crowded region of the sky (l = 357 deg, b = -2 deg) and at least 18 sources are clearly detected in the field of view of the gamma-ray imager during a 277 ksec exposure. These are well known persistent X-ray binaries and 3 transient sources in outburst. The combined 5-200 keV JEM-X and SPI spectrum of H 1743-322 is well fit with an absorbed ((2.5 10E22 atom/cm2) soft (photon index 2.70 +/- 0.09) power-law model consistent with J 1743-322 being in a high/soft state.Comment: 5 pages. Figs. 2 and 3 are best viewed in color. To appear in INTEGRAL special edition of A&A

Evidence for Doppler-Shifted Iron Emission Lines in Black Hole Candidate 4U 1630-47

We report the first detection of a pair of correlated emission lines in the X-ray spectrum of black hole candidate 4U 1630-47 during its 1996 outburst, based on RXTE observations of the source. At the peak plateau of the outburst, the emission lines are detected, centered mostly at $\sim$5.7 keV and $\sim$7.7 keV, respectively, while the line energies exhibit random variability $\sim$5%. Interestingly, the lines move in a concerted manner to keep their separation roughly constant. The lines also vary greatly in strength, but with the lower-energy line always much stronger than the higher-energy one. The measured equivalent width ranges from $\sim$50 eV to $\sim$270 eV for the former, and from insignificant detection to $\sim$140 eV for the latter; the two are reasonably correlated. The correlation between the lines implies a causal connection --- perhaps they share a common origin. Both lines may arise from a single $K_{\alpha}$ line of highly ionized iron that is Doppler-shifted either in a Keplerian accretion disk or in a bi-polar outflow or even both. In both scenarios, a change in the line energy might simply reflect a change in the ionization state of line-emitting matter. We discuss the implication of the results and also raise some questions about such interpretations.Comment: To appear in Ap

XMM-Newton observations of the INTEGRAL X-ray transient J17544-2619

On 2003 September 17 INTEGRAL discovered a bright transient source 3 degrees from the Galactic Center, IGR J17544-2619. The field containing the transient was observed by XMM-Newton on 2003 March 17 and September 11 and 17. A bright source, at a position consistent with the INTEGRAL location, was detected by the European Photon Imaging Camera (EPIC) during both September observations with mean 0.5-10 keV unabsorbed luminosities of 1.1x10^35 and 5.7x10^35 erg s-1 for an (assumed) distance of 8 kpc. The source was not detected in 2003 March, with a 0.5-10 keV luminosity of < 3.8x10^32 erg s-1. The September 11 and 17 EPIC spectra can be represented by a power-law model with photon indices of 2.25+/-0.15 and 1.42+/-0.17, respectively. Thus, the 0.5-10 keV spectrum hardens with increasing intensity. The low-energy absorption during both September observations is comparable to the interstellar value. The X-ray lightcurves for both September observations show energy dependent flaring which may be modeled by changes in either low-energy absorption or power-law index.Comment: Six pages, five figures; to appear in Astronomy and Astrophysic

Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1

We show that dramatic spectral evolution associated with dips occurring near phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is ~10% of that of the unabsorbed bright component. A prominent Fe emission line at ~6.5 keV is evident during the dips. The absolute line flux outside the dips is similar to that during the dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial- covering model could explain ASCA spectra of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an Fe K-alpha fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through it. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages, including 11 figures

Chandra Observations of G11.2-0.3: Implications for Pulsar Ages

We present Chandra X-ray Observatory imaging observations of the young Galactic supernova remnant G11.2-0.3. The image shows that the previously known young 65-ms X-ray pulsar is at position (J2000) RA 18h 11m 29.22s, DEC -19o 25' 27.''6, with 1 sigma error radius 0.''6. This is within 8'' of the geometric center of the shell. This provides strong confirming evidence that the system is younger, by a factor of ~12, than the characteristic age of the pulsar. The age discrepancy suggests that pulsar characteristic ages can be poor age estimators for young pulsars. Assuming conventional spin down with constant magnetic field and braking index, the most likely explanation for the age discrepancy in G11.2-0.3 is that the pulsar was born with a spin period of ~62 ms. The Chandra image also reveals, for the first time, the morphology of the pulsar wind nebula. The elongated hard-X-ray structure can be interpreted as either a jet or a Crab-like torus seen edge on. This adds to the growing list of highly aspherical pulsar wind nebulae and argues that such structures are common around young pulsars.Comment: 16 pages, 3 figures, Accepted for publication in ApJ. For a full resolution version of Fig 1, see http://www.physics.mcgill.ca/~vkaspi/G11.2-0.3/f1.ep