15,414 research outputs found
Missing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472
4U 1630-472 is a recurrent X-ray transient classified as a black-hole
candidate from its spectral and timing properties. One of the peculiarities of
this source is the presence of regular outbursts with a recurrence period
between 600 and 730 d that has been observed since the discovery of the source
in 1969. We report on a comparative study on the spectral and timing behaviour
of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all
the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory
(INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years
of activity. We show that, in spite of having a similar spectral and timing
behaviour in the energy range between 3 and 30 keV, these three outbursts show
pronounced differences above 30 keV. In fact, the 2010 outburst extends at high
energies without any detectable cut-off until 150-200 keV, while the two
previous outbursts that occurred in 2006 and 2008 are not detected at all above
30 keV. Thus, in spite of a very similar accretion disk evolution, these three
outbursts exhibit totally different characteristics of the Compton electron
corona, showing a softening in their evolution rarely observed before in a low
mass X-ray binary hosting a black hole. We argue the possibility that the
unknown perturbation that causes the outbursts to be equally spaced in time
could be at the origin of this particular behaviour. Finally we describe
several possible scenarios that could explain the regularity of the outbursts,
identifying the most plausible, such as a third body orbiting around the binary
system.Comment: April 2015: accepted for publication in MNRAS. May 2015: in pres
Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence
We present the first optical and infrared polarimetric study of the low mass
transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed
to search for an intrinsic linear polarisation component in the system emitted
radiation that might be due, e.g., to synchrotron emission from a compact jet,
or to Thomson scattering with free electrons in an accretion disc. Multiband
(BVRI) optical polarimetric observations were obtained during two nights in
2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These
observations cover about the 30% of the 15.1 hours orbital period. J-band
observations were obtained in 2007 with the NICS (TNG) instrument at La Palma,
for a totality of 1 hour observation. We obtained 3-sigma upper limits to the
polarisation degree in all the optical bands, with the most constraining one
being in the I-band (P<0.5%). No phase-correlated variability has been noticed
in all the filters. The J-band observations provided a 6% upper limit on the
polarisation level. The constraining upper limits to the polarisation in the
optical allowed us to evaluate the contribution of the possible emission of a
relativistic particles jet to the total system radiation to be less then the
10%. This is in agreement with the observation of a spectral energy
distribution typical of a single black body of a K-spectral type main sequence
star irradiated from the compact object. Due to the low S/N ratio it was not
possible to investigate the possible dependency of the polarisation degree from
the wavelength, that could be suggestive of polarisation induced by Thomson
scattering of radiation with free electrons in the outer part of the accretion
disc. Observations with higher S/N ratio are required to examine in depth this
hypothesis, searching for significant phase-correlated variability.Comment: 7 pages, 9 figures, accepted for publication in section 7. Stellar
structure and evolution of Astronomy and Astrophysic
Further Constraints on Thermal Quiescent X-ray Emission from SAX J1808.4-3658
We observed SAX J1808.4-3658 (1808), the first accreting millisecond pulsar,
in deep quiescence with XMM-Newton and (near-simultaneously) Gemini-South. The
X-ray spectrum of 1808 is similar to that observed in quiescence in 2001 and
2006, describable by an absorbed power-law with photon index 1.74+-0.11 and
unabsorbed X-ray luminosity L_X=7.9+-0.7*10^{31} ergs/s, for N_H=1.3*10^{21}
cm^{-2}. Fitting all the quiescent XMM-Newton X-ray spectra with a power-law,
we constrain any thermally emitting neutron star with a hydrogen atmosphere to
have a temperature less than 30 eV and L_{NS}(0.01-10 keV)<6.2*10^{30} ergs/s.
A thermal plasma model also gives an acceptable fit to the continuum. Adding a
neutron star component to the plasma model produces less stringent constraints
on the neutron star; a temperature of 36^{+4}_{-8} eV and L_{NS}(0.01-10
keV)=1.3^{+0.6}_{-0.8}*10^{31} ergs/s. In the framework of the current theory
of neutron star heating and cooling, the constraints on the thermal luminosity
of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these
neutron stars.
We compile data from the literature on the mass transfer rates and quiescent
thermal flux of the largest possible sample of transient neutron star LMXBs. We
identify a thermal component in the quiescent spectrum of the accreting
millisecond pulsar IGR J00291+5934, which is consistent with the standard
cooling model. The contrast between the cooling rates of IGR J00291+5934 and
1808 suggests that 1808 may have a significantly larger mass. This can be
interpreted as arising from differences in the binary evolution history or
initial neutron star mass in these otherwise similar systems.Comment: ApJ in press, 7 pages, 2 color figure
The Structure of 1-\u3cem\u3etert\u3c/em\u3e-butyl-2,2,3,4,4-pentamethylphosphetane 1-oxide, C\u3csub\u3e12\u3c/sub\u3eH\u3csub\u3e25\u3c/sub\u3eOP
Structure of l-Benzyl-2,2,3,3-tetramethyl-l-phenylphosphetanium Bromide, a Cyelobutane Derivative
Interplanetary trajectory design in high-fidelity model: Application to deep-space CubeSatsâ cruises
Poetic Tracks and Treading on Indigenous Lands: Examining Marlatt and Warlandâs and Akiwenzie-Dammâs Literary Travels to Australia and Aotearoa
This paper considers the work of poets who travel from the area of the Indigenous land of Turtle Island now known as Canada to the Indigenous territories of Australia and Aotearoa. The poets engage in different forms of movement on the land that reveal varying degrees of awareness of and respect for Indigenous sovereignty. In particular, I put â17:00 / coming into Port Pirieâ and â30/5 8:50 / past Menindeeâ from Daphne Marlatt and Betsy Warlandâs 1988 Double Negative, an understudied collection of poetry in which the lesbian poets traverse Australia by train while reflecting on travelling through â(ab) original countryâ (19), into conversation with Kateri Akiwenzie-Dammâs (Chippewas of Nawash Unceded First Nation/Anishnaabe) âfrom turtle island to aotearoa,â the final poem in Akiwenzie-Dammâs 1993 collection My Heart is a Stray Bullet, in which she enacts a nation-to-nation relationship with the MÄori people through âslow cautious stepsâ (50) upon their lands. Using Eve Tuck (UnangaxÌ) and K. Wayne Yangâs influential piece âDecolonization is not a Metaphor,â I discuss how the poets variously use language of movement to elide complicity in settler colonialism and settler mobility or, as is the case with Akiwenzie-Damm, honour Indigenous forms of movement and relationships to their lands
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