EP240618a
GCN Circular 36690
Subject
EP240618a: EP-WXT detection of a fast X-ray transient and EP-FXT quick follow-up
Date
2024-06-18T12:34:18Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Sun, W. Chen (NAOC, CAS), H. Zhou (PMO, CAS), W. D. Zhang, J. W. Hu, D. Y. Li, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), J. Yang, C. Y. Dai (NJU) Y. F. Liang (PMO), 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 EP240618a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient triggered the WXT on-board processing unit at 2024-06-18T05:43:43 (UTC). The position of the source is R.A. = 281.627 deg, DEC = 23.820 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 100 seconds and has a peak unabsorbed flux of ~1.5 x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.2(+/-0.4) (with the column density fixed at the Galactic value of 1.98 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.9(+0.7/-0.6) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level 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.
Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP, which started at 2024-06-18T06:55:00 (UTC), about 1 hour after the WXT detection. An X-ray source was clearly detected at R.A. = 281.648 deg, DEC = 23.833 deg (R.A. = 18:46:35.4, Dec = 23:49:57.0; J2000), with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic), which is 1.4 arcmin away from the WXT position of EP240618a. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 1.98 x 10^21 cm^-2 and a photon index of 2.0(+/-0.1). The derived unabsorbed flux in 0.5-10 keV is 3.0(+/-0.3) x 10^-12 erg/s/cm2. During the observation, the flux of the source was decaying with time. Considering this source being within the error circle of the WXT transient, we suggest it being the late-time X-ray emission from EP240618a.
Further follow-up observations are encouraged to identify the nature of this X-ray transient.
The above observations were made with the WXT and FXT instruments 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 36701
Subject
EP240618a: BOOTES-6/DPRT optical upper limit
Date
2024-06-19T15:31:34Z (a year ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
legacy email
S.-Y. Wu, I. Perez-Garcia, E. Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martin-Carrillo and L. Hanlon (UCD, Ireland), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS) and C. J. Perez del Pulgar (UMA, Malaga), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP240618a by the Einstein Probe (Sun et al. GCNC 36690), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) observed the fast X-ray transient location starting on June 18, 20:52 UT (~ 15.1 h after trigger) in different optical bands. No new optical source is detected on the co-added images (clear-filter) within the EP/FXT 30” radius error box down to 20.2 mag.
We thank the staff at Boyden Observatory for their excellent support.
GCN Circular 36702
Subject
EP240618a: OHP/T193 MISTRAL i' and Y band upper limits
Date
2024-06-19T19:11:32Z (a year ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
Web form
C. Adami (LAM/Pytheas/AMU), E. Le Floc’h (CEA Paris-Saclay), F. Fortin (IRAP, Toulouse),
B. Schneider (MIT), J. Palmerio, A. Saccardi, S. Vergani (GEPI/Obs. de Paris), M. Dennefeld (IAP),
J. Schmitt, E. Schmitt, E. Molina (OHP/Pytheas), F. Schussler (DPhP CEA),
report on behalf of a larger collaboration:
We observed the field of EP240618a (GCN 36690; Sun et al., GCN 36701; Wu et al.) using the T193cm
telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We
obtained 5400s (9x600s with dithering) in the Y-band between 21h13 and 22h47 UT on the 2024-06-18
(~16h after trigger) and 3900s (13x300s with dithering) in the i'-band between 22h53 on the 2024-06-
18 and 00h18 UT on the 2024-06-19 (~18h after trigger).
We do not detect any significant new objects in our MISTRAL images within the X-ray position
uncertainty area, nor in i band (3-sigma detection limit of 20.9) or Y band (3-sigma detection limit
of 19.85). We however have a hint of a detection close to the ~2-sigma detection limit in the
MISTRAL Y-band image at an approximate magnitude of Y~20.3 (with a large error bar) and located at
18:46:36.3, +23:49:55.2. This putative source is not visible at all in the MISTRAL i-band image.
We encourage further deeper follow-ups of this possible source to assess its existence.
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the
magnitudes are not corrected for Galactic extinction.
We acknowledge the excellent support from Observatoire de Haute-Provence and in particular Jean
Pierre Troncin for the MISTRAL observations.
GCN Circular 36710
Subject
EP240618a: TRT optical upper limit
Date
2024-06-20T02:58:48Z (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, J. An, X. Liu, S.Y. Fu, Z.P. Zhu, 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 EP240618a (Sun et al., GCN 36690) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatory, California, USA. Observations started at 07:37:06.059 UTC on 2024-06-18, i.e., 1.89 hr after the EP trigger, and 15 x 120 s R-band frames were obtained.
No uncatalogued optical transient is detected in the stacked R-band image within the 30-arcsec EP/FXT error circle (Sun et al., GCN 36690), down to a 5-sigma limiting magnitude of R ~ 20.3, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for any catalogued source within the EP/FXT error circle.
GCN Circular 36712
Subject
EP240618a: GRANDMA Optical Upper Limits
Date
2024-06-20T09:24:27Z (a year ago)
From
Dalya Akl at American Uni. SHJ <dalyaakl.d@gmail.com>
Via
Web form
D. Akl (AUS), M. Masek (FZU), C. Andrade (UMN), E. G. Elhosseiny (NRIAG), H. Peng (THU), L. Wang (XAO), S. Antier (OCA), S. Karpov (FZU), M. Coughlin (UMN), I. Tosta e Melo (UniCT-DFA), P. Hello (IJCLAB), P-A Duverne (APC), D. Turpin (CEA-Saclay/Irfu), N. Guessoum (AUS), T. Pradier (Unistra/IPHC), J.-G. Ducoin (CPPM), M. Prouza (FZU), A. Klotz (CNRS-OMP-IRAP), M. Boer, A. S. Gervasoni, C. Limonta (OCA), Y.Rajabov, O.Burkhonov (UBAI), Y.Tillayev, S.Ehgamberdiev (UBAI, NUUz), T.Sadibekova (UBAI-CEA), M. Freeberg (KNC), M. Serrau (KNC), M. Odeh (AKO), C. Galdies (KNC), A. LeCalloch (Berkeley), N. Leroy (IJCLAB) on behalf of the GRANDMA collaboration:
We performed observations with GRANDMA and its citizen science project Kilonova-catcher (KNC) on the X-ray transient EP240618a detected by Einstein Probe (EP) WXT on T0 = 2024-06-18T05:43:43 (GCN 36690). We monitored the source with 8 telescopes, including TAROT-TCH, TAROT-TCA, FRAM-CTA-N, and UBAI-NT60, from 2024-06-18T16:10:05 to 2024-06-19T09-57-01 UTC, starting 0.43 days after the EP trigger.
We did not find any new optical counterpart in any of our frames within 30 arcsec radius centered around Ra/Dec: 281.648/ 23.833 deg, consistent with the reports from (Tinyanont et al., GCN 36710; Adami et al., GCN 36702; Wu et al., GCN 36701).
All our observations and upper limits, which correspond to the depth of the full image (given in the AB system), can be found here:
https://skyportal-icare.ijclab.in2p3.fr/public/sources/EP240618a/version/8f2a92bbe70faf40c582c7454103e505
Below is the version that can be downloaded from the link above:
+--------------+----------+----------------+-------------------+
| MJD | Filter | Lim.Mag | Instrument |
+==============+==========+================+===================+
| 60479.687743 | I | 18.96 | UBAI/NT-60 |
+--------------+----------+----------------+-------------------+
| 60479.942361 | sdssg | 17.82 | TAROT/TCA |
+--------------+----------+----------------+-------------------+
| 60479.942361 | sdssg | 17.83 | TAROT/TCH |
+--------------+----------+----------------+-------------------+
| 60479.912306 | R | 17.97 | FRAM-CTA-N |
+--------------+----------+----------------+-------------------+
| 60480.406678 | R | 19.29 | KNC |
+--------------+----------+----------------+-------------------+
| 60479.993831 | V | 20.26 | KNC |
+--------------+----------+----------------+-------------------+
| 60479.759005 | I | 19.54 | KNC |
+--------------+----------+----------------+-------------------+
| 60479.683044 | R | 19.54 | KNC |
+--------------+----------+----------------+-------------------+
| 60480.027174 | sdssg | 20.15 | KNC |
+--------------+----------+----------------+-------------------+
| 60479.983422 | sdssr | 19.93 | KNC |
+--------------+----------+----------------+-------------------+
| 60479.664595 | V | 19.65 | KNC |
+--------------+----------+----------------+-------------------+
| 60479.949853 | G | 20.40 | KNC |
+--------------+----------+----------------+-------------------+
We note that in the Skyportal page above, bessellv, bessellr, and besselli filters correspond to V, R, and I Johnson Cousin filters, respectively.
Generic telescopes in the Skyportal page correspond to KNC telescopes.
All the data have been reduced by a single data processing pipeline STDPIPE (Karpov et al., 2022).
Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog while images taken with Sloan filters were calibrated using the Pan-STARRS DR1 Catalog.
We note that some amateur telescopes use green filters, which were calibrated with the Gaia catalog.
We use the Skyportal application (skyportal.io) to monitor our observational campaign.
We thank the Einstein Probe team for useful communication.
GRANDMA is a worldwide telescope network (https://grandma.lal.in2p3.fr/) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 36714
Subject
EP240618a: Abastumani optical observationsEP240618a
Date
2024-06-20T17:11:44Z (a year ago)
From
XXXX at IKI <alex@cgrsmx.iki.rssi.ru>
Via
legacy email
N. Pankov (HSE, IKI), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO) report on behalf of GRB-IKI-FuN collaboration:
We observed the refined error circle of EP240618a (Sun et al., GCN 36690) with AS-32 telescope of Abastumani Observatory (AbAO) equipped with CCD photometer and the R-filter to search for an optical counterpart. The observations were taken on dates 2024-06-18 and 2024-06-19. The first series is consist of 66x60 sec images, while latter one is 120x60 sec. Using image differencing, we find no new source within the localization circle or any brightness excess of catalog sources. The obtained results are consistent with observations by other telescopes (Wu et al., GCN 36701; Adami et al., GCN 36702; Tinyanont et al., GCN 36710; Akl et al., GCN 36712). Preliminary upper limits are following:
Date UT start t-T0 Exp. Filter OT Err. UL(3 sigma)
(mid, days) (s)
2024-06-18 18:18:51 0.524398 64x60 R n/d n/d 19.6
2024-06-19 21:45:02 1.667581 120x60 R n/d n/d 20.1
The difference image has an upper limit of 19.9 mag. The magnitudes were calibrated using nearby stars from USNO-B1.0 (R2 magnitude) catalog. The magnitudes are not corrected for the Galactic extinction towards EP240618a, which is A(V) = 0.4294 mag (S&F 2011).
GCN Circular 36723
Subject
EP240618a: Swift/XRT upper limit
Date
2024-06-21T10:21:36Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
W. Chen, H. Sun (NAOC, CAS), H. Zhou (PMO, CAS), W. D. Zhang, D. Y. Li, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, H. Q. Cheng, 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, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (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
Following the detection of the fast X-ray transient EP240618a by the Einstein Probe (Sun et al., GCN 36690), we triggered a Swift target of opportunity observation (ObsID: 00016670001). The XRT onboard Swift began the observation at 2024-06-20T15:56:06 UTC, about 58 hours after the WXT detection, with an exposure time of 2.0 ks in the Photon Counting mode.
No significant X-ray source was detected within the 30 arcsec region around the reported FXT position. Assuming an absorbed power-law model with a Galactic column density of 1.98 x 10^21 cm^-2 and a photon index of 2, the estimated flux upper limit in 0.3-10 keV at the 90% confidence level is 1.2 x 10^-13 ergs/cm^2/s.
We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible.
GCN Circular 36725
Subject
X-ray transient EP240618a: Fermi/GBM non-detection
Date
2024-06-21T15:56:04Z (a year ago)
From
mariaedvige.ravasio@ru.nl
Via
Web form
M. E. Ravasio (Radboud University) and P.G. Jonker (Radboud University) report on behalf of a larger collaboration:
Following the detection of the fast X-ray transient EP240618a by the Einstein Probe WXT and FXT (Sun et al., GCN 36690), we inspected the Fermi/GBM daily data for the presence of a possible gamma-ray counterpart. Fermi was operational (it was not in the SAA) and the location of the source was visible (i.e. it was not Earth-occulted).
Inspecting CSPEC data of the 4 best source-angle detectors (n7, n6, nb, b1) in the energy range 8-900 keV (for n7, n6, nb) and 300-40000 keV (for b1), with a time resolution of 4 s, we found no significant gamma-ray source within the time interval [T0; T0+100] s (same duration reported by Sun et al., GCN 36690, where T0 = 2024-06-18T05:43:43).
Assuming a power-law model with a photon index of 2, the estimated upper limit to the flux in the energy range 10-1000 keV at the 90% confidence level over the time interval mentioned above is 8.5 x 10^-9 erg/cm^2/s.
GCN Circular 36731
Subject
EP240618a: GSP optical upper limit
Date
2024-06-22T09:48:46Z (a year ago)
From
Wenxiong Li <liwenxiong1992@gmail.com>
Via
Web form
W. X. Li, S. J. Xue, (NAOC), M. Andrews, J. Farrah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP240618a by the Einstein Probe (Sun et al., GCN 36690), we initiated observations of the fast X-ray transient location starting on June 18 at 7:43 UT (~2 hours after the trigger) in the r band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at McDonald Observatory in USA.
No new optical source was detected in the co-added images within the EP/FXT error box down to ~21.5 mag.
These observations were taken as part of the Global Supernova Project.
GCN Circular 36736
Subject
EP240618a: NOT optical counterpart candidate
Date
2024-06-22T18:53:13Z (a year ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
X. Liu, S.Y. Fu, J. An, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC), J. Terwel (NOT) report on behalf of a large collaboration:
We observed the field of EP240618a detected by EP/WXT (Sun et al., GCN 36690) using the the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 00:07:28 UT on 2024-06-20, i.e., ~ 1.77 day after the EP trigger, and 12x120 s Sloan z-filter images were obtained.
An uncatalogued optical transient (OT), within the 30 arcsec EP/FXT error circle (Sun et al., GCN 36690), is detected in the stacked z-band image at coordinates:
R.A. (J2000)= 18:46:35.78
Dec. (J2000)= +23:50:13.17
with an uncertainty of ~ 0.2 arcsec. The OT is 1.3 arcsec away from a nearby 18.4 magnitude star, and its preliminary photometry is z ~ 21 mag at 1.78 day since the EP trigger, calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction.
The OT is not detected by early TRT observation (Tinyanont et al., GCN 36710) down to R ~ 20.3 mag, and not by LCO observation (Li et al., GCN 36731) down to r ~ 21.5 mag.
If EP240618a is a GRB, the z-r color might indicate high redshift.
A finding chart of the candidate is available at
https://drive.google.com/file/d/1pPAKrFyVvk3D7QnpgCfBQHim1ieGQ8zq/view?usp=sharing
A second epoch in z-band at the NOT has been scheduled for the night of June 22.
GCN Circular 36741
Subject
EP240618a: NOT further optical observations
Date
2024-06-23T02:56:10Z (a year ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
X. Liu, S.Y. Fu, J. An, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC), M. Turkki (NOT) report on behalf of a large collaboration:
Following the first epoch of the observations of EP240618a (Sun et al., GCN 36690) at the the 2.56-m Nordic Optical Telescope (NOT, Liu et al., GCN 36736), we have carried out the second epoch at the NOT.
Observations started at 2024-06-23T00:13:37, i.e., ~ 4.8 day after the EP trigger, and again 12x120 s Sloan z-filter images were obtained.
The candidate in the 1st epoch is clearly detected in the 2nd epoch with comparable brightness in z-band. Specifically, it has z = 21.03 +/- 0.05 in the 1st epoch and z = 21.05 +/- 0.06 mag in the 2nd epoch, more or less affected by the nearby bright star ~1.3 arcsec away and the photometric method.
As the candidate is probably mixed with the bright star in the Pan-STARRS image, we also measure the candidate+star in the NOT images. Both NOT epochs yield the same magnitude of z ~ 18.4 mag, being consistent with the value by Pan-STARRS.
Hence, we conclude that the candidate is likely not the counterpart of EP240618a.
Meanwhile, we searched for other possible candidates by image subtraction of the two NOT epochs within the 30-arcsec EP/FXT error circle (Sun et al., GCN 36690). Neither uncatalogued optical transient nor apparent brightening of catalogued source is found.