0 Assigned ID: 2527 ----- Attempting Submission 1 (Sun Sep 14 00:43:57 GMT 2014) ----- 0 ----- Attempting Submission 1 (Sat Oct 25 16:46:51 GMT 2014) ----- Swift J1910.2-0546 is a newly discovered transient X-ray binary that exhibited a ~1 yr long outburst in 2012-2013. Owing to its low hydrogen column density and fair proximity, it is one of the few sources for which X-ray/UV quiescent studies are feasible. This provides the opportunity to verify the nature of the compact primary (black hole versus neutron star), and to investigate the properties of the quiescent accretion flow. Such a study would provide an important perspective to understand the intriguing outburst phenomena observed for this source. To this end, we request a 45 ks Chandra/ACIS-S observation and 2 orbits of HST/WFC3-UVIS time for Swift J1910.2-0546. UV emission from quiescent LMXBs is thought to be the prime tracer of the quiescent accretion stream that provides important insight into the physics of quiescent accretion flows. Such studies are highly challenging, however, given that most LMXBs are located in the Galactic plane and hence their intrinsically faint UV emission further suffers from substantial interstellar extinction. Owing to its low extinction, Swift J1910.2-0546 is one of the few LMXBs for which quiescent UV studies are feasible. To make a direct comparison with other LMXBs studied in quiescence in the UV band (see e.g., Hynes & Robinson 2012, ApJ 749, 3), we chose to perform near-UV photometry (~2000-4000 A). The most sensitive imaging instrument in this range is the WFC3/UVIS. To obtain the spectral-energy distribution, we propose to use three filters (similar to those used in studies of other LMXBs): F225W (lambda_c~2359 A), F275W (lambda_c~2704 A), and F336W (lambda_c~3355 A). We propose to perform the science exposures in sequences of F336W-F275W-F225W-F336W, so that we can average over possible variability (and are able to search for such variability if the source is bright enough). We used the online WFC3/UVIS exposure time calculator (v. 22.2). Our target is located at ~6.9 kpc and the reddening is E(B-V)=0.66. A recent XMM-Newton/OM observation (dd 2014 October 5; PI Degenaar) did not detect the source in the UV band, providing an upper limit of <1E-17 erg/cm2/s/A (measured at lambda_c ~ 2298 A). The faintest quiescent LMXBs observed in the UV have luminosities of ~1E31 erg/s (Hynes & Jones 2012). At the distance of our target this translates into a flux of ~3E-18 erg/cm2/s/A. Based on UV studies of other quiescent LMXBs (e.g., Hynes & Jones 2012), we model our point source as a blackbody with a temperature of 10,000 K, where we normalized the continuum flux to our lowest expected level of 3E-18 erg/cm2/s/A. We impose a S/N ratio of 25 at this minimum flux level. We use dither patterns (2 dithers for exposures <1000s, 3 dithers for longer exposures) and lamp flashes are applied to ensure that the minimum background for proper CTE is reached (12 e-/pixel). The exposure times range from ~700-2000 s per filter. A total of two orbits are required, which are combined into a single visit (only one instrument is used). Our target is a transient X-ray source, but its outburst recurrence time is very low: only one outburst (in 2012-2013) has been detected so far in the history of X-ray astronomy. The maximum UV flux reached during that outburst was ~18 mag at lambda_c~2246 A (~3E-16 erg/cm2/s/A; Degenaar et al. 2014, ApJ 784, 122). The probability that the source goes into outburst in the next year is very low. Nevertheless, should the source go into outburst, that will be readily picked up by all-sky monitors such as Swift/BAT (15-50 keV) and MAXI (2-20 keV), which cover the position of our target every day. The rise of accretion outbursts in transient LMXBs typically takes a few days. HOT STARS Default false false false false false SUPPORTED STAR ACCRETION DISK LMXB X-RAY TRANSIENT This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 <1E-17 erg/cm/s/A at lambda_c ~ 2298 A WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 2 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 3 false 1414099553532 /Users/degenaar/Work/Proposals/CHANDRA/A16/SwiftJ1910/HST_PHASE_II/OP-cache/swiftj1910-v01.xml The scheduling data was generated by CASM 22.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13939 :visit-id "01" :proposal-version -1 :trans-version "20140605" :date "23-Oct-2014 17:25:53.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2887504 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13939 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13939 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2889 :al-overhead-at-end 123 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3222 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13939 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3000 :al-overhead-at-end 123 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3222 :pcs-gap 0 ) true 2 0