Transient LMXBs in Globular Clusters

Science Keywords: Select from 2 to 5 items. Science Keywords is a required field. 0 Assigned ID: 2555 ----- Attempting Submission 1 (Wed Sep 15 12:15:14 GMT 2021) ----- Joint Chandra/HST M15 was observed on Oct 3 2022. Observations in orbit1 of Visit 1 (named Visit-Transient1) were successful, observations in orbit 2 failed. The observations of orbit 2 will be repeated. See Visit-Transient1_repeat, which is a duplicate of Visit-Transient1. Note that *only* observations in orbit 2 of this visit need to be repeated. 955822485 ----- Attempting Submission 1 (Thu Oct 14 07:22:43 GMT 2021) ----- ----- Attempting Submission 2 (Thu Sep 15 23:13:38 GMT 2022) ----- HST Phase II Proposal 16839 successfully submitted. Receipt: # 16839-2 ----- Attempting Submission 3 (Tue Sep 20 06:57:13 GMT 2022) ----- ----- Attempting Submission 4 (Sun Oct 09 22:07:19 GMT 2022) ----- Transient LMXBs in Globular Clusters Since the discovery of globular cluster LMXBs in the 1970s, it was long assumed there was only one luminous LMXB per cluster. Deep Chandra observations of globular clusters have revealed that they contain numerous quiescent LMXB systems, any of which could go into outburst. Past Chandra studies have shown that globular clusters can indeed harbor multiple transients and that more quiescent LMXBs are hiding under the sensitivity limits of even relatively deep Chandra exposures. Here we propose a Chandra program to precisely localize new transients in globular clusters, with the aim of gaining better insight into globular cluster LMXB populations. As part of this effort we further request HST observations to identify the optical counterparts of these transients. * General background for the program The goal of this joint Chandra/HST Target-of-Opportunity (ToO) proposal is to measure accurate coordinates of the next two X-ray transients in a globular cluster with Chandra, and to use HST subsequently to look for their optical counterparts inside the Chandra positional error circles. The Chandra error circle is large enough (about 0.6" radius) that it likely contains multiple globular-cluster stars. Comparison of the photometry that is extracted from the observations requested here with photometry from archival HST data reveals highly variable objects in the error circle, and hence the identity of the true counterpart. Our choice for the ACS/WFC is driven by the existence of a large body of archival globular-cluster images taken with this camera. We have been allocated time to do Chandra/HST follow-up for two X-ray transients. For the HST component we were granted 4 HST orbits in total (2 orbits per transient). * The first trigger (September 2022) Early September 2022, MAXI and subsequently Swift detected an outburst of an X-ray transient in the globular cluster M15. This X-ray transient has not been confidently identified with any of the known low-mass X-ray binaries in the cluster. We have triggered our Chandra ToO program to obtain an accurate X-ray position of the source. The Chandra observations will be performed on **September 19, 2022**. We have triggered the HST component of our program to search for the optical counterpart. * Visit 1 Our program for Visit 1 to observe M15 consists of observations in the F435W and F814W filters as there are already archival ACS data in these filters that we can use for comparison. The observations are organized in a 4-point dither-box pattern to improve the resolution. We aim for a S/N>10 for a star of mag 25 (F435W, F814W). We also include short observations to get photometry of bright stars, and of stars near bright stars. To fit all observation in one orbit, we make use of subarrays (aperture WFC1B-1K). The positional uncertainty on the X-ray transient fits well within this field-of-view. *** Note added on Oct 9 2022: M15 was observed on Oct 3. The observations in orbit 1 of Visit 1 (named Visit-Transient1) were successful, but the observations in orbit 2 failed. The observations in orbit 2 will be repeated, see Visit-Transient1_repeat. N/A Stellar Physics and Stellar Types 6 false false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER NO Our target is a to-be-identified star in the globular cluster M15. The counterpart that we aim to identify is expected to have a magnitude in F435W m(F435W) >~ 16. J2000 STAR X-RAY TRANSIENT next X-ray transient discovered in a Galactic globular cluster, whose position can only be accurately determined with Chandra second trigger of globular-cluster X-ray transient program ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W POS-TARG DITHER 0.262 LINE 0.192 false true 68.14 4 18.39 ACS-WFC-DITHER-BOX 1

false

1663656883865 The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. 1664792377000 96.04207151308788 (Tic-Visit :proposal-id 16839 :visit-id "01" :proposal-version -1 :trans-version "20220630" :date "20-Sep-2022 08:54:43.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 844880 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16839 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16839 :visit-id "01" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2763 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16839 :visit-id "01" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2751 :al-overhead-at-end 0 :internal-dur-at-end 344 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) true 2 0 1665360538402 The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. /private/var/folders/46/4jpf0wpx40jbz16h87362w240001d2/T/OPVisit8526168740051228993.xml (Tic-Visit :proposal-id 16839 :visit-id "03" :proposal-version -1 :trans-version "20220630" :date "09-Oct-2022 20:08:58.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 422448 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16839 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16839 :visit-id "03" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2764 :al-overhead-at-end 0 :internal-dur-at-end 344 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) true 1 0 1663104588824 The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. (Tic-Visit :proposal-id 16839 :visit-id "02" :proposal-version -1 :trans-version "20220630" :date "13-Sep-2022 23:29:48.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16839 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16839 :visit-id "02" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2742 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3142 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16839 :visit-id "02" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2703 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3142 :pcs-gap 0 ) true 2 0 Target of Opportunity