2,724 research outputs found
The Great Pretenders Among the ULX Class
The recent discoveries of pulsed X-ray emission from three ultraluminous
X-ray (ULX) sources have finally enabled us to recognize a subclass within the
ULX class: the great pretenders, neutron stars (NSs) that appear to emit X-ray
radiation at isotropic luminosities ~erg~s~erg~s only because their emissions are strongly beamed toward
our direction and our sight lines are offset by only a few degrees from their
magnetic-dipole axes. The three known pretenders appear to be stronger emitters
than the presumed black holes of the ULX class, such as Holmberg II \& IX X-1,
IC10 X-1, and NGC300 X-1. For these three NSs, we have adopted a single
reasonable assumption, that their brightest observed outbursts unfold at the
Eddington rate, and we have calculated both their propeller states and their
surface magnetic-field magnitudes. We find that the results are not at all
different from those recently obtained for the Magellanic Be/X-ray pulsars: the
three NSs reveal modest magnetic fields of about 0.3-0.4~TG and beamed
propeller-line X-ray luminosities of ~erg~s,
substantially below the Eddington limit.Comment: To appear in Research in Astronomy and Astrophysic
Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud
Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray
pulsars. Together with discoveries from other satellites at least 45 SMC
pulsars are now known. One of these sources, a pulsar with a period of
approximately 7.8 seconds, was first detected in early 2002 and since discovery
it has been found to be in outburst nine times. The outburst pattern clearly
shows a period of 45.1 +/- 0.4 d which is thought to be the orbital period of
this system. Candidate outburst periods have also been obtained for nine other
pulsars and continued monitoring will enable us to confirm these. This large
number of pulsars, all located at approximately the same distance, enables a
wealth of comparative studies. In addition, the large number of pulsars found
(which vastly exceeds the number expected simply by scaling the relative mass
of the SMC and the Galaxy) reveals the recent star formation history of the SMC
which has been influenced by encounters with both the LMC and the Galaxy.Comment: 5 pages, 4 figures, AIP conference proceedings format. Contribution
to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA,
November, 200
The H1 Forward Track Detector at HERA II
In order to maintain efficient tracking in the forward region of H1 after the
luminosity upgrade of the HERA machine, the H1 Forward Track Detector was also
upgraded. While much of the original software and techniques used for the HERA
I phase could be reused, the software for pattern recognition was completely
rewritten. This, along with several other improvements in hit finding and
high-level track reconstruction, are described in detail together with a
summary of the performance of the detector.Comment: Minor revision requested by journal (JINST) edito
An ultrahigh-speed digitizer for the Harvard College Observatory astronomical plates
A machine capable of digitizing two 8 inch by 10 inch (203 mm by 254 mm)
glass astrophotographic plates or a single 14 inch by 17 inch (356 mm by 432
mm) plate at a resolution of 11 microns per pixel or 2309 dots per inch (dpi)
in 92 seconds is described. The purpose of the machine is to digitize the
\~500,000 plate collection of the Harvard College Observatory in a five year
time frame. The digitization must meet the requirements for scientific work in
astrometry, photometry, and archival preservation of the plates. This paper
describes the requirements for and the design of the subsystems of the machine
that was developed specifically for this task.Comment: 12 pages, 9 figures, 1 table; presented at SPIE (July, 2006) and
published in Proceeding
The binary period and outburst behaviour of the SMC X-ray binary pulsar system SXP504
A probable binary period has been detected in the optical counterpart to the
X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504
in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul
2002 and subsequently observed by XMM-Newton on 18 Dec 2003. The source is
coincident with an Optical Gravitational Lensing (OGLE) object in the
lightcurves of which several optical outburst peaks are visible at ~ 268 day
intervals. Timing analysis shows a period of 268.6 +/- 0.1 days at > 99%
significance. Archival Rossi X-ray Timing Explorer (RXTE) data for the 504s
pulse-period has revealed detections which correspond closely with predicted or
actual peaks in the optical data. The relationship between this orbital period
and the pulse period of 504s is within the normal variance found in the Corbet
diagram.Comment: Accepted by MNRAS. 1 LATEX page. 4 figure
Constraining the Nature of the Galactic Center X-ray Source Population
We searched for infrared counterparts to the cluster of X-ray point sources
discovered by Chandra in the Galactic Center Region (GCR). While the sources
could be white dwarfs, neutron stars, or black holes accreting from stellar
companions, their X-ray properties are consistent with magnetic Cataclysmic
Variables, or High Mass X-ray Binaries (HMXB) at low accretion-rates. A direct
way to decide between these possibilities and hence between alternative
formation scenarios is to measure or constrain the luminosity distribution of
the companions. Using infrared (J, H, K, Br-gamma) imaging, we searched for
counterparts corresponding to typical HMXB secondaries: spectral type B0V with
K<15 at the GCR. We found no significant excess of bright stars in Chandra
error circles, indicating that HMXBs are not the dominant X-ray source
population, and account for fewer than 10% of the hardest X-ray sources.Comment: 4 pages, 3 figures, 1 table, accepted in ApJ Letters for publicatio
High Speed Visible Light Communication Using Blue GaN Laser Diodes
GaN-based laser diodes have been developed over the last 20 years making them desirable for many security and defence applications, in particular, free space laser communications. Unlike their LED counterparts, laser diodes are not limited by their carrier lifetime which makes them attractive for high speed communication, whether in free space, through fiber or underwater. Gigabit data transmission can be achieved in free space by modulating the visible light from the laser with a pseudo-random bit sequence (PRBS), with recent results approaching 5 Gbit/s error free data transmission. By exploiting the low-loss in the blue part of the spectrum through water, data transmission experiments have also been conducted to show rates of 2.5 Gbit/s underwater. Different water types have been tested to monitor the effect of scattering and to see how this affects the overall transmission rate and distance. This is of great interest for communication with unmanned underwater vehicles (UUV) as the current method using acoustics is much slower and vulnerable to interception. These types of laser diodes can typically reach 50-100 mW of power which increases the length at which the data can be transmitted. This distance could be further improved by making use of high power laser arrays. Highly uniform GaN substrates with low defectivity allow individually addressable laser bars to be fabricated. This could ultimately increase optical power levels to 4 W for a 20-emitter array. Overall, the development of GaN laser diodes will play an important part in free space optical communications and will be vital in the advancement of security and defence applications
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 Halpha
emission line. Additionally, 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 day.
Doppler images during the decline phase of the Halpha emission line show
evidence for a hotspot and emission from the gas stream: the hotspot 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 Halpha
emission line is offset by about -100 km/s from the systemic velocity which
suggests that the disk continues to precess. A Halpha tomogram reveals emission
from near the donor star after subtraction of the ring-like contribution from
the accretion disk which we attribute to chromospheric emission. No hotspot 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 Halpha profile of this system also
exhibits a behaviour consistent with a precessing disk.Comment: 11 pages, 5 figures, accepted by Ap
X-Ray and Optical Observations of XTE J0052-723 a Transient Be/X-Ray Pulsar in the Small Magellanic Cloud
On December 27th 2000 during our regular SMC monitoring program with Rossi
X-ray Timing Explorer, strong pulsations were detected with a period of 4.78
seconds. Subsequent slew observations performed on Jan 9th and 13th across the
field of view allowed localisation of the pulsar's position to RA: 0 52 17,
Dec: 72 19 51 (J2000). The outburst continued until Jan 24th, 7 PCA
observations were obtained during this period, yielding a maximum X-ray
luminosity ~10^38 ergs/s. Following calculation of the pulsar position, optical
observations of the RXTE error box were made on Jan 16th 2001 with the 1m
telescope of the South African Astronomical Observatory (SAAO) while the source
was still in X-ray outburst. Candidate Be stars identified from their
photometric colours were subsequently observed with the SAAO 1.9m on Nov 7th
2001 to obtain spectra. Only one of the photometrically identified stars
[MA93]537 showed prominent H emission, with a double peaked
line-profile (EW= -43.3+/-0.7 A, separation velocity= 200+/-15 km/s) confirming
the presence of a substantial circumstellar disk.Comment: 8 pages, 8 figures, Accepted for publication in MNRA
- …