We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray Observatory as well as optical observations conducted ...during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18 super(h)13 super(m)39."04, decl. = -3346'22."3 (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power law (with photon index g = 1.41 c 0.06) plus blackbody (with kT = 0.95 c 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broadband spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star and therefore very unlikely to be responsible for the soft excess, making synchrotron emission a more reasonable candidate.
We report on the serendipitous discovery of a 442 Hz pulsar during a Rossi X-Ray Timing Explorer (RXTE) observation of the globular cluster NGC 6440. The oscillation is detected following a burstlike ...event which was decaying at the beginning of the observation. The timescale of the decay suggests we may have seen the tail end of a long-duration burst. Low-mass X-ray binaries (LMXBs) are known to emit thermonuclear X-ray bursts that are sometimes modulated by the spin frequency of the star, the so-called burst oscillations. The pulsations reported here are peculiar if interpreted as canonical burst oscillations. In particular, the pulse train lasted for similar to 500 s, much longer than in standard burst oscillations. The signal was highly coherent and drifted down by similar to 2 x 1O super(-3) Hz, much smaller than the similar to Hz drifts typically observed during normal bursts, but consistent with orbital motion of the neutron star. The pulsations are reminiscent of those observed during the much more energetic "superbursts"; however, the temporal profile and the energetics of the burst suggest that It was not the tail end nor the precursor feature of a superburst. Rather, it is likely that we caught a portion of an outburst from a new "intermittent" accreting millisecond pulsar, a phenomenon which until now had only been seen in HETE J1900.1-2455.
We report the discovery of 3.76 s pulsations from a new burst source near Sgr Asup * observed by the N mu STAR observatory. The strong signal from SGR J1745-29 presents a complex pulse profile ...modulated with pulsed fraction 27% + or - 3% in the 3-10 keV band. SGR J1745-29 joins a growing subclass of transient magnetars, indicating that many magnetars in quiescence remain undetected in the X-ray band or have been detected as high-B radio pulsars. The peculiar location of SGR J1745-29 has important implications for the formation and dynamics of neutron stars in the Galactic center region.
We report the detection with the Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA) of 530 Hz burst oscillations in a thermonuclear (type I) burst from the transient X-ray source ...A1744-361. This is only the second burst ever observed from this source, and the first to be seen in any detail. Our results confirm that A1744-361 is a low-mass X-ray binary (LMXB) system harboring a rapidly rotating neutron star. The oscillations are first detected along the rising edge of the burst, and they show evidence for frequency evolution of a magnitude similar to that seen in other burst sources. The modulation amplitude and its increase with photon energy are also typical of burst oscillations. The lack of any strong indication of photospheric radius expansion during the burst suggests a 9 kpc upper limit of the source distance. We also find energy-dependent dips, establishing A1744-361 as a high-inclination, dipping LMXB. The timescale between the two episodes of observed dips suggests an orbital period of 697 minutes. We have also detected a 2-4 Hz quasi-periodic oscillation (QPO) for the first time from this source. This QPO appears consistent with 61 Hz QPOs seen from other high-inclination systems. We searched for kilohertz QPOs and found a suggestive 2.3 s feature at 800 Hz in one observation. The frequency, strength, and quality factor are consistent with that of a lower frequency kilohertz QPO, but the relatively low significance argues for caution, so we consider this a tentative detection requiring confirmation.
SWIFT J1756.9-2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi ...X-Ray Timing Explorer during the 2007 outburst of the source. We detect strong (image35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (image0.1 Hz). This makes SWIFT J1756.9-2508 similar to the rest of AMPs and to other low-luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.
The early optical emission of the moderately high redshift (2 = 3.08) GRB 060607A shows a remarkable broad and strong peak with a rapid rise and a relatively slow power-law decay. It is not ...coincident with the strong early-time flares seen in the X-ray and gamma-ray energy bands. There is weak evidence for variability superposed on this dominant component in several optical bands that can be related to flares in high-energy bands. While for a small number of gamma-ray bursts (GRBs), well-sampled optical flares have been observed simultaneously with X-ray and gamma-ray pulses, GRB 060607A is one of the few cases where the early optical emission shows no significant evidence for correlation with the prompt emission. In this work we first report in detail the broad-band observations of this burst by Swift. Then by applying a simple model for the dynamics and the synchrotron radiation of a relativistic shock, we show that the dominant component of the early emissions in optical wavelengths has the same origin as the tail emission produced after the main gamma-ray activity. The most plausible explanation for the peak in the optical light curve seems to be the cooling of the prompt after the main collisions, shifting the characteristic synchrotron frequency to the optical bands. The fact that the early emission in X-ray does not show a steep decay, like what is observed in many other GRBs, is further evidence for slow cooling of the prompt shell within this GRB. It seems that the cooling process requires a steepening of the electron energy distribution and/or a break in this distribution at high energies. From simultaneous gamma-ray emission during the first flare, the behaviour of hardness ratio, and the lack of spectral features, we conclude that the X-ray flares are due to the collision of late shells rather than late reprocessing of the central engine activities. The sharp break in the X-ray light curve at few thousands of seconds after the trigger, is not observed in the infrared/optical/ultraviolet bands, and therefore cannot be a jet break. Either the X-ray break is due to a change in the spectrum of the accelerated electrons or the lack of an optical break is due to the presence of a related delayed response component.
The Symbiotic System SS73 17 Seen with Suzaku Smith, Randall K; Mushotzky, Richard; Mukai, Koji ...
Publications of the Astronomical Society of Japan,
02/2008, Letnik:
60, Številka:
sp1
Journal Article
Recenzirano
Odprti dostop
We observed with Suzaku the symbiotic star SS73 17, motivated by the discovery by the INTEGRAL satellite and the Swift BAT survey that it emits hard X-rays. Our observations showed a highly-absorbed ...X-ray spectrum with
$N_{\rm H} \gt 10^{23}$
cm
$^{-2}$
, equivalent to
$A_{V} \gt 26$
, although the source has B magnitude 11.3 and is also bright in UV. The source also shows strong, narrow iron lines including fluorescent FeK as well as FeXXV and FeXXVI. The X-ray spectrum can be fit with a thermal model including an absorption component that partially covers the source. Most of the equivalent width of the iron fluorescent line in this model can be explained as a combination of reprocessing in a dense absorber plus reflection off a white dwarf surface, but it is likely that the continuum is partially seen in reflection as well. Unlike other symbiotic systems that show hard X-ray emission (CH Cyg, RT Cru, T CrB, GX1
$+$
4), SS73 17 is not known to have shown nova-like optical variability, X-ray flashes, or pulsations, and has always shown faint soft X-ray emission. As a result, although it is likely a white dwarf, the nature of the compact object in SS73 17 is still uncertain. SS73 17 is probably an extreme example of the recently discovered and relatively small class of hard X-ray emitting symbiotic systems.
We present the X-ray timing and spectral evolution of the Galactic Center magnetar SGR J1745-2900 for the first ~4 months post-discovery using data obtained with the Nuclear Spectroscopic Telescope ...Array and Swift observatories. Our timing analysis reveals a large increase in the magnetar spin-down rate by a factor of 2.60 + or - 0.07 over our data span. We further show that the change in spin evolution was likely coincident with a bright X-ray burst observed in 2013 June by Swift, and if so, there was no accompanying discontinuity in the frequency. We find that the source 3-10 keV flux has declined monotonically by a factor of ~2 over an 80 day period post-outburst accompanied by a ~20% decrease in the source's blackbody temperature, although there is evidence for both flux and kT having leveled off. We argue that the torque variations are likely to be magnetospheric in nature and will dominate over any dynamical signatures of orbital motion around Sgr A*.
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