EP240506a
GCN Circular 36405
Subject
EP240506a: EP-WXT detection of a new fast X-ray transient
Date
2024-05-07T07:01:44Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D.Y. Li (NAOC, CAS), J. Yang (NJU), A. Li (BNU), W. Yuan, Z. X. Ling, Y. Liu, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, H. Sun, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang (NAOC,CAS), Y. Chen, S. M. Jia, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team
We report on the detection of a fast X-ray transient, designated EP240506a, at 2024-05-06T05:01:39 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 213.978 deg, DEC = -16.715 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for about 50 seconds and has a peak flux of 1 x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. The averaged 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.0(+/- 0.5) (with the column density fixed at the Galactic value of 9.0 x 10^20 cm^-2), giving an unabsorbed flux of 1.1(+0.3, -0.2) x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The 1-sigma uncertainties are given for the above parameters.
No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. Further multi-wavelength follow-up observations are encouraged to identify the nature of this X-ray flare.
The above observation was made with the WXT instrument during the commissioning phase of EP. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
GCN Circular 36408
Subject
EP240506a: Optical upper limit with Kinder observations
Date
2024-05-07T17:19:29Z (a year ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan (NCUIA), S. Yang (HNAS), T.-W. Chen, C.-S. Lin, C.-C. Ngeow, W.-J. Hou, H.-Y. Hsiao, C.-H. Lai, A. Sankar.K, M.-H. Lee, Y.-C. Pan, H.-C. Lin, J.-K. Guo (all NCUIA), H. F. Stevance, S. J. Smartt (Oxford/QUB), S. Srivastav, J. Gillanders (Oxford), M. Fulton, T. Moore, C. Angus, and A. Aamer (all QUB) report:
We observed the field of the fast X-ray transient EP240506a (Li et al., GCN 36405) using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen et al., AstroNote 2021-92). The first SLT epoch of observations started at 13:07 UT on 07th of May 2024 (MJD = 60437.547), 1.34 days after the EP trigger. We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and subtract the stacked images with the Legacy Survey DR10 template images.
We detect the presence of a variable star in the subtracted image at R.A. = 14:15:57.85 and DEC = -16:43:17.30, which corresponds to SDSS J141557.82-164317.2 with an r-band magnitude of 17.52 mag.
Besides this, we do not detect any optical counterpart in the errorbox of 3 arcminute around the position of the EP source specified by Li et al., (GCN 36405). The details of the observations and the corresponding 2.5 sigma upper limit (in the AB system) in our combined image are as follows:
Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
SLT | r | 60437.547 | 1.34 days | 300 sec * 29 | > 21.82 | 2".2 | 1.35
The reported upper limit is calibrated against nearby stars from SDSS and is not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.09 mag in the direction of the EP source (Schlafly & Finkbeiner 2011).
GCN Circular 36412
Subject
EP240506a: BOOTES Network optical upper limit
Date
2024-05-08T06:12:28Z (a year ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
legacy email
I. Perez-Garcia, E. Fernandez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu and A. J. Castro-Tirado (IAA-CSIC, Granada), Y.-D. Hu (INAF-OAB), D. Hiriart, W. H. Lee (UNAM), I. M. Carrasco-Garcia (SMA), I. H. Park (SKKU), L. Hernandez-Garcia (Univ. de Valparaiso), C. Perez del Pulgar, A. Castellon, A. Reina (Univ. de Malaga), B.-B. Zhang (Nanjing Univ.), A. Maury (Space, San Pedro de Atacama), D. R. Xiong, Y. F. Fan, J. M. Bai, C. J. Wang, Y. X. Xin, and X. H. Zhao (Yunnan Observatories of CAS), on behalf of a larger collaboration, report:
Following the detection of EP240506a by EP-WXT (Li et al., GCNC 36405), several telescopes of the Global BOOTES Network pointed to its position on the sky.
The BOOTES-5/JGT 0.6m robotic telescope at Observatorio Astronomico Nacional in San Pedro Martir (Mexico) observed the fast X-ray transient location on May. 7, 07:02 UT (1.08 days after trigger). In the co-added frame (60 x 59 s, clear filter), no source is detected within the error position down to 20.6 mag.
The BOOTES-7 0.6m robotic telescope at San Pedro de Atacama (Chile) also observed the transient position on May, 7 at 07:12 UT (1.09 days after trigger). In the co-added image (60 x 32 s, clear filter), no optical counterpart is detected down to 20.3 mag.
Later on, we also triggered the BOOTES-4/MET 0.6m robotic telescope at Lijiang Astronomical Observatory (China), also resulting on a non-detection. All the above-mentioned upper limits are in agreement with other reports (cf. Aryan et al., GCCN 36408).
We thank the staff at the BOOTES sites for their excellent support.
GCN Circular 36413
Subject
EP240506a: TRT optical upper limit
Date
2024-05-08T10:19:12Z (a year ago)
From
S. Tinyanont at National Astronomical Research Institute of Thailand <samaporn@narit.or.th>
Via
Web form
S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Q. Jiang, Z.P. Zhu, X. Liu, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration:
We observed the field of EP240506a (Li et al., GCN 36405) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Springbrook Observatory, New South Wales, Australia. Observations started at 09:53:36.57 UT on 2024-05-07, i.e., 1.20 day after the EP trigger, and 6 x 300 s R-band and 6 x 300 s I-band frames were obtained, respectively.
No uncatalogued optical transient is detected in the stacked R-band and I-band images within the 3 arcmin EP/WXT error circle (Li et al., GCN 36405), down to 3-sigma limiting magnitudes of R ~ 21.6 and I ~ 21.0, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for the catalogued sources within the error circle.
GCN Circular 36414
Subject
EP240506a: Archival radio searching
Date
2024-05-08T10:50:46Z (a year ago)
From
Ning Chang at Xinjiang Astronomical Observatory, CAS <changning@xao.ac.cn>
Via
Web form
Ning Chang (XAO), Lang Cui (XAO), Yongfeng Huang (NJU), Tao An (SHAO), Jie Liao (XAO, UCAS), Pengfei Jiang (XAO), Wancheng Xu (XAO) and Dongyue Li(NAOC) report:
We report an archival radio source located within the error circle of the fast X-ray transient EP240506a. EP240506a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 36405). Aryan et al., (GCN 36408) observed the field of the EP240506a using the 40cm SLT, and only found a variable star which corresponds to SDSS J141557.82-164317.2 within the error circle of 3 arcmin. Perez-Garcia et al. (GCN 36412) reported that no source was detected within the error position of the field of EP240506a using the telescopes of the Global BOOTES Network. Additionally, Tinyanont et al. (GCN 36413) observed the field of EP240506a using the 0.7-m telescope of TRT and found no uncatalogued optical transient.
We have searched for a radio counterpart using the data from the Rapid ASKAP Continuum Survey (RACS, Hale et al. 2021 PASA 38, e058). We only found one radio detection in the RACS images within the uncertainty of 3 arcmin reported in GCN 36405. We also inspected the VLA Sky Survey data (VLASS, Gordon et al. 2021 ApJS 255, 30) but found no detections.
The fluxes and position of this radio source from RACS are:
2019-04-27: 2.74 mJy (0.8875 GHz, from RACS low, with RA=14h15m56.348s, DEC=-16d41m22.947s)
2021-02-12: 2.46 mJy (1.3675 GHz, from RACS mid, with RA=14h15m56.187s, DEC=-16d41m22.673s)
2022-01-09: 1.42 mJy (1.655 GHz, from RACS high, with RA=14h15m56.12s, DEC=-16d41m23.16s)
The power-law spectral index is -1.47. This radio source is about 1.6 arcmin away from the X-ray transient EP240506a position, and about 2 arcmin away from the variable star position. We further inspected the AllWISE Source Catalog and found that WISEA J141556.28-164124.4 is close to the radio source about 2 arcsec away. We infer that the radio source could be a background galaxy.
The figures of this radio source can be accessed from https://drive.google.com/drive/folders/1Yy9nHYF9JKhmnQhD27fqg5vQkaBpl4Nr?usp=sharing
Multi-wavelength follow-up observations are strongly encouraged to help further reveal the nature of the new X-ray transient.
GCN Circular 36417
Subject
EP240506a: CrAO ZTSH optical upper limit
Date
2024-05-08T18:48:21Z (a year ago)
From
Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru>
Via
legacy email
EP240506a: CrAO ZTSH optical upper limit
N. Pankov (HSE, IKI), A. Pozanenko (IKI), V. Rumyantsev (CrAO), S. Belkin (IKI) report on behalf of GRB-IKI-FuN collaboration:
We observed the EP240506a detected by EP-WXT (GCN 36405) with 2.6-meter ZTSh telescope of CrAO, equipped with CCD-photometer and the R-filter. The observations started on 2024-05-07 at 20:43 (UT), i.e. ~1.6 day after EP trigger. The series of 31x120s images were taken.
We do not detect any optical object within EP-WXT localization area. The preliminary photometry is following:
Date UT start MJD Exp. Filter OT Err. UL(3sigma)
(mid, days) (s)
2024-05-07 20:58:29 60437.88266 28x120 R n/d n/d 22.5
The result is in agreement with previous observations (Aryan et al., GCN 36408; Perez-Garcia et al., GCN 36412; Tinyanont et al., GCN 36413).
We also do not detect any optical source consistent with the location of WISEA J141556.28-164124.4 reported by Chang et al. (GCN 36414).
The variable star SDSS J141557.82-164317.2 mentioned in (Chang et al., GCN 36414) does not show any variability in comparison with USNO-B1.0 (R2=17.02) for which we measure the magnitude of R=16.92 +/- 0.10.
The magnitudes were calibrated using nearby USNO-B1.0 stars (R2 magnitude). No correction has been made for a Galactic extinction towards the EP240506a.
GCN Circular 36419
Subject
EP240506a: Xinglong optical upper limit
Date
2024-05-09T02:55:13Z (a year ago)
From
Dr Jie Zheng at Xinglong Observatory, National Astronomical Observatories ( NAOC), CAS <jiezheng@nao.cas.cn>
Via
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J.-J. Jin(NAOC), H.-Y. Mu(NAOC), Z. Fan(NAOC), H. Wu(NAOC), J. Zheng(NAOC), S. Liu(NAOC) report on behalf of a large collaboration:
Following the detection of EP240506a by EP-WXT (Li et al., GCN 36405), we observed the field of EP240506a using the 2.16-m telescope at Xinglong Observatory, NAOC. We obtained 3x600s clear-band frames with a median time of 2024-05-07T15:16:34 i.e., 10 hr after the EP trigger.
No uncatalogued optical transient is detected in the stacked images within the 3 arcmin EP/WXT error circle (Li et al., GCN 36405), down to 3-sigma limiting magnitudes of g ~ 23.1, calibrated with Pan-STARRS sources in the field. Also, there is no apparent brightening for the cataloged sources within the error circle.