arXiv subjects

S. Wachter

Publications and source records attributed to S. Wachter.

43 records · Page 3Linked to original sources

arXiv2004

Far Infrared Imaging of NGC 55

We present images of the galaxy NGC 55 at 24, 70, and 160 micron obtained with the Multiband Imaging Photometer for Spitzer (MIPS) instrument aboard the Spitzer Space Telescope. The new images display the far infrared emission in unprecedented detail and demonstrate that the infrared morphology differs dramatically from that at shorter wavelengths. The most luminous emission region in the galaxy is marginally resolved at 24 micron and has a projected separation of nearly 520 pc from the peak emission in the optical and near infrared. This region is responsible for ~9% of the total emission at 24 micron and is likely a young star formation region. We show that this and other compact sources account for more than 1/3 of the total 24 micron emission. We compute a total infrared luminosity for NGC 55 of 1.2*10^9 L_sun. The star formation rate implied by our measurements is 0.22 M_sun/yr. We demonstrate that the cold dust is more extended than the warm dust in NGC 55--the minor-axis scale heights are 0.32, 0.43, and 0.49 kpc at 24, 70 and 160 micron, respectively. The dust temperature map shows a range of temperatures that are well-correlated with the 24 micron surface brightness, from 20 K in low-surface-brightness regions to 26 K in high-surface-brightness regions.

astro-ph↗

arXiv2004

Radio detections of the neutron star X-ray binaries 4U 1820-30 and Ser X-1 in soft X-ray states

We present the analysis of simultaneous X-ray (RXTE) and radio (VLA) observations of two atoll-type neutron star X-ray binaries: 4U 1820-30 and Ser X-1. Both sources were steadily in the soft (banana) X-ray state during the observations. We have detected the radio counterpart of 4U 1820-30 at 4.86 GHz and 8.46 GHz at a flux density of ~0.1 mJy. This radio source is positionally coincident with the radio pulsar PSR 1820-30A. However, the pulsar's radio emission falls rapidly with frequency (proportional to ν^(-3)) and we argue that the X-ray binary's radio emission is dominant above ~2 GHz. Supporting this interpretation, comparison with previous observations reveals variability at the higher radio frequencies that is likely to be due to the X-ray binary. We have detected for the first time the radio counterpart of Ser X-1 at 8.46 GHz, also at a flux density of ~0.1 mJy. The position of the radio counterpart has allowed us to unambiguously identify its optical counterpart. We briefly discuss similarities and differences between the disc-jet coupling in neutron star and black hole X-ray binaries. In particular, we draw attention to the fact that, contrary to other states, neutron star X-ray binaries seem to be more radio loud than persistent black hole candidates when the emission is 'quenched' in the soft state.

astro-ph↗

arXiv2003

Unraveling the cooling trend of the Soft Gamma Repeater, SGR 1627-41

SGR 1627-41 was discovered in 1998 after a single active episode which lasted \~6 weeks. We report here our monitoring results of the decay trend of the persistent X-ray luminosity of the source during the last 5 years. We find an initial temporal power law decay with index 0.47, reaching a plateau which is followed by a sharp (factor of ten) flux decline ~800 days after the source activation. The source spectrum is best described during the entire period by a single power law with high absorption (N_H=9.0(7)x10^(22) cm^(-2)); the spectral index, however, varies dramatically between 2.2-3.8 spanning the entire range for all known SGR sources. We discuss the cooling behavior of the neutron star assuming a deep crustal heating initiated by the burst activity of the source during 1998.

astro-ph↗

arXiv2003

The Peculiar X-ray Transient IGR 16358-4726

The new transient IGR 16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627-41 with the Chandra X-ray Observatory at the 1.7 x 10^{-10} ergs s^{-1} cm^{-2} flux level (2-10 keV) with a very high absorption column (N_H=3.3(1) x 10^{23} cm^{-2}) and a hard power law spectrum of index 0.5(1). We discovered a very strong flux modulation with a period of 5880(50) s and peak-to-peak pulse fraction of 70(6)% (2-10 keV), clearly visible in the x-ray data. The nature of IGR 16358-4726 remains unresolved. The only neutron star systems known with similar spin periods are low luminosity persistent wind-fed pulsars; if this is a spin period, this transient is a new kind of object. If this is an orbital period, then the system could be a compact Low Mass X-ray Binary (LMXB).

astro-ph↗

arXiv2002

Solid immersion lens-enhanced micro-photoluminescence: principle and applications

We demonstrate the combination of a hemispherical solid immersion lens with a micro-photoluminescence setup. Two advantages introduced by the SIL, an improved resolution of 0.4 times the wavelength in vacuum and a 5 times enhancement of the collection efficiency, make it an ideal system for spatially resolved spectroscopy applications. The influence of the air gap between the SIL and the sample surface is investigated in detail. We confirm the tolerance of the set--up to an air gap of several micrometers. Such a system is proven to be ideal system in the studies of exciton transport and polarization dependent single quantum dot spectroscopy.

physics.optics↗

arXiv2001

A Closer Look at the Soft X-ray Transient X1608-52: Long-term Optical/X-ray Observations

We report long-term simultaneous optical and (RXTE) X-ray observations of the soft X-ray transient and low mass X-ray binary X1608-52 spanning from 1999 to 2001. In addition to the usual X-ray outburst and quiescent states, X1608-52 also exhibits an extended low intensity state during which the optical counterpart, QX Nor, is found to be about two magnitudes brighter than during quiescence. We detect optical photometric variability on a possible period of 0.5370 days with a semi-amplitude of ~0.27 mag in the I band. The modulation could be orbital but is also consistent with a scenario invoking a superhump with decreasing period. Observations of QX Nor during quiescence indicate an F to G type main sequence secondary while theoretical considerations argue for an evolved mass donor. Only an evolved mass donor would satisfy the condition for the occurrence of superhumps.

astro-ph↗

arXiv2001

Infrared Properties of Cataclysmic Variables from 2MASS: Results from the 2nd Incremental Data Release

Because accretion-generated luminosity dominates the radiated energy of most cataclysmic variables, they have been ``traditionally'' observed primarily at short wavelengths. Infrared observations of cataclysmic variables contribute to the understanding of key system components that are expected to radiate at these wavelengths, such as the cool outer disk, accretion stream, and secondary star. We have compiled the J, H, and Ks photometry of all cataclysmic variables located in the sky coverage of the 2 Micron All Sky Survey (2MASS) 2nd Incremental Data Release. This data comprises 251 systems with reliably identified near-IR counterparts and S/N > 10 photometry in one or more of the three near-IR bands.

astro-ph↗