21 research outputs found
X-ray, optical and infrared investigation of the candidate Supergiant Fast X-ray Transient IGR J18462-0223
We report on a broad-band X-ray study (0.5-60 keV) of the poorly known
candidate Supergiant Fast X-ray Transient (SFXT) IGR J18462-0223, and on
optical and near-infrared (NIR) followup observations of field objects. The
out-of-outburst X-ray state has been investigated for the first time with
archival INTEGRAL/IBIS, ASCA, Chandra and Swift/XRT observations. This allowed
us to place stringent 3 sigma upper limits on the soft (0.5-10 keV) and hard
(18-60 keV) X-ray emission of 2.9x10^-13 erg cm^-2 s^-1 and 8x10^-12 erg cm^-2
s^-1, respectively; the source was also detected during an intermediate soft
X-ray state with flux equal to 1.6x10^-11 erg cm^-2 s^-1 (0.5-10 keV). In
addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray
flares (18-60 keV) having a duration in the range 1-12 hours: the flaring
behavior was also investigated in soft X-rays (3-10 keV) with archival
INTEGRAL/JEM-X observations. The duty cycle (1.2%) and the dynamic ranges (>
1,380 and > 190 in the energy bands 0.5-10 keV and 18-60 keV, respectively)
were measured for the first time. Archival UKIDSS JHK NIR data, together with
our deep R-band imaging of the field, unveiled a single, very red object inside
the intersection of the Swift/XRT and XMM-Newton error circles: this source has
optical/NIR photometric properties compatible with a very heavily absorbed blue
supergiant located at about 11 kpc, thus being a strong candidate counterpart
for IGR J18462-0223. NIR spectroscopy is advised to confirm the association.
Finally, a hint of a possible orbital period was found at about 2.13 days. If
confirmed by further studies, this would make IGR J18462-0223 the SFXT with the
shortest orbital period among the currently known systems.Comment: accepted for publication in A&A, 9 pages, 7 figures, 2 table
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
Swift/XRT follow-up observation on IGR J17177-3656 at soft X-rays
Following the discovery of IGR J17177-3656 (Frankowski et al. 2011, ATel #3223) on 2011-03-15, we requested a Swift ToO, which was executed on 2011-03-16 from 18:39:24 to 20:41:01 UT with a total exposure of ~3.9 ks. 2266 photons have been extracted from the source region between 0.2 and 10 keV, corresponding to a net count rate of ~0.59 cts/s. Using the online Swift tool we find an enhanced position of RA = 259.428 (17h17m43s), Dec = -36.934 deg (-36:56:03s) (J2000), with a 90% confidence level error of 2.1 arcsec, consistent with the position measurements made by IBIS/ISGRI and JEM-X (ATel #3223)
IGR J18136-2739: A dwarf nova caught in outburst ?
IGR J18136-2739 is a new transient INTEGRAL/IBIS source which has been caught in outburst during only a short period of time (26-29 August 2003). Motivated by this finding, we acquired observations with the X-ray telescope (XRT, 0.3-10 keV) on board Swif
The impact of task structure on the use of vague expressions by EFL learners
The present study sets out to examine whether and how task structure affects the number and type of vague expressions used by a group of higher intermediate EFL learners. The participants were 50 Iranian EFL learners from 6 intact classes, all native speakers of Persian with limited opportunity to communicate with native speakers of English, and no experience in English-speaking countries. To elicit data, two picture description tasks were used. These picture-stories possessed the defining characteristics of structured and unstructured narrative tasks, respectively. Results revealed that (a) unstructured tasks were associated with the production of significantly more vague expressions and (b) the most notable differences between performances on the two task types concerned âvague nounsâ, âvague quantifiersâ, âvague deintensifiersâ and âvague subjectivisersâ. The results of the study have implications for both teachers and teacher educators in that they may help identify the kinds of tasks which induce language learners to use vague expressions more frequently