TITLE: GCN CIRCULAR
NUMBER: 36690
SUBJECT: EP240618a: EP-WXT detection of a fast X-ray transient and EP-FXT quick follow-up
DATE: 24/06/18 12:34:18 GMT
FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn>
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.
View this GCN Circular online at https://gcn.nasa.gov/circulars/36690.
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TITLE: GCN CIRCULAR
NUMBER: 36689
SUBJECT: LIGO/Virgo/KAGRA S240618ah: Identification of a GW compact binary merger candidate
DATE: 24/06/18 08:06:03 GMT
FROM: simon.maenaut(a)kuleuven.be
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S240618ah during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2024-06-18 07:16:27.151 UTC (GPS time: 1402730205.151). The candidate was found by the CWB [1], GstLAL [2], and MBTA [3] analysis pipelines.
S240618ah is an event of interest because its false alarm rate, as estimated by the online analysis, is 6.5e-08 Hz, or about one in 5 months. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S240618ah
After parameter estimation by RapidPE-RIFT [4], the classification of the GW signal, in order of descending probability, is BBH (96%), Terrestrial (4%), NSBH (<1%), or BNS (<1%).
Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 4%.
Three sky maps are available at this time and can be retrieved from the GraceDB event page:
* bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 30 seconds after the candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 42 seconds after the candidate event time.
* bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 5 minutes after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 5360 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 7841 +/- 2600 Mpc (a posteriori mean +/- standard deviation).
For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.
[1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
[2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
[3] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
[4] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
[5] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
[6] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
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TITLE: GCN CIRCULAR
NUMBER: 36688
SUBJECT: LIGO/Virgo/KAGRA S240615ea: Upper limits from Swift/BAT-GUANO
DATE: 24/06/18 01:50:03 GMT
FROM: Samuele Ronchini at PSU <sjs8171(a)psu.edu>
Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Aaron Tohuvavohu (U Toronto) report:
Swift/BAT was observing 58% of the GW localization probability ([Bilby.offline0.multiorder.fits](https://gracedb.ligo.org/api/superevents/S2…) at merger time. A fraction 25% of the GW localization posterior is contained inside the BAT coded FoV.
The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.
We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:
|time_bin (s) | soft|normal| hard | GRB170817
|-|-|-|-|-|
|0.256 |9.60 | 6.43 | 5.86 | 6.98
|1.024 |4.89 | 3.27 | 2.98 | 3.55
|4.096 |2.62 | 1.75 | 1.60 | 1.90
|16.384 |1.60 | 1.07 | 0.98 | 1.16
The upper limits as function of sky position are plotted here, alongside the GW localization:
https://doi.org/10.5281/zenodo.12010849
The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.
GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be
found at: https://www.swift.psu.edu/guano/
View this GCN Circular online at https://gcn.nasa.gov/circulars/36688.
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TITLE: GCN CIRCULAR
NUMBER: 36687
SUBJECT: LIGO/Virgo/KAGRA S240615dg: Upper limits from Swift/BAT-GUANO
DATE: 24/06/18 01:49:53 GMT
FROM: Samuele Ronchini at PSU <sjs8171(a)psu.edu>
Samuele Ronchini (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea (PSU), James DeLaunay (PSU), Gayathri Raman (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT was observing >99% of the GW localization probability ([bayestar.multiorder.fits](https://gracedb.ligo.org/api/superevents/S240615d…) at merger time. A fraction >99% of the GW localization posterior is contained inside the BAT coded FoV.
The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.
We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:
|time_bin (s) | soft|normal| hard | GRB170817
|-|-|-|-|-|
|0.256 |2.37 | 2.66 | 2.51 | 2.78
|1.024 |1.21 | 1.36 | 1.28 | 1.42
|4.096 |0.66 | 0.74 | 0.70 | 0.77
|16.384 |0.42 | 0.47 | 0.44 | 0.49
The upper limits as function of sky position are plotted here, alongside the GW localization:
[https://doi.org/10.5281/zenodo.12008258]()
The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.
GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be
found at: [https://www.swift.psu.edu/guano/]()
View this GCN Circular online at https://gcn.nasa.gov/circulars/36687.
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TITLE: GCN CIRCULAR
NUMBER: 36686
SUBJECT: GRB 240615A: J-band upper limits from WINTER
DATE: 24/06/17 15:22:28 GMT
FROM: Viraj Karambelkar at Indian Inst of Tech,Bombay <karambelkarvraj21197(a)gmail.com>
Viraj Karambelkar (Caltech), Geoffrey Mo (MIT), Robert Stein (Caltech),
Danielle Frostig (MIT), Tomas Ahumada (Caltech), Nathan Lourie (MIT),
Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the Swift-GUANO localization region of short GRB 240615A (GCN
36671, 36672, 36673, 36676, 36680, 36682, 36683) in the near-infrared
J-band with the Palomar 1-m telescope, equipped with the 1-square degree
WINTER camera (Lourie et al. 2020).
Our observations began at 2024-06-16T06:06:34 UTC (~12 hours after the GRB
trigger) and lasted for an hour. The images were processed using the WINTER
data reduction pipeline (https://github.com/winter-telescope/mirar,
https://doi.org/10.5281/zenodo.10888436), with image subtraction performed
relative to J-band images from the UKIRT Hemisphere survey (Dye et al.,
2017).
We do not detect the XRT source (reported by GCN 36683) in our stacked and
subtracted images to a depth of J~19 mag (AB). No other new sources are
identified in our subtracted images, to a depth of J~19 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between
MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF
AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute
for Astrophysics and Space Research.
View this GCN Circular online at https://gcn.nasa.gov/circulars/36686.
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TITLE: GCN CIRCULAR
NUMBER: 36685
SUBJECT: GRB 240615A: Fermi GBM Observation
DATE: 24/06/17 14:47:05 GMT
FROM: Oliver J Roberts at USRA/NASA <oliver.roberts(a)nasa.gov>
O.J. Roberts (USRA/NASA-MSFC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 17:51:45.05 UT on 15 June 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 240615A (trigger 740166710/240615744), which was
also detected by Swift BAT GUANO (DeLaunay et al. 2024, GCN 36672),
KONUS-WIND (Frederiks et al. 2024, GCN 36677) and GECAM (Tan et al. 2024, GCN 36682).
The Fermi-GBM Final Real-time location (Fermi GBM Team 2024, GCN 36671)
is consistent with the Swift BAT-GUANO position.
The angle from the Fermi LAT boresight is 23 degrees.
The GBM light curve consists of bright, overlapping emission episodes with a
duration (T90) of about 0.10 s (50-300 keV). The time-averaged spectrum
from T0-0.03 to T0+0.09 s is best fit by a power law function with an
exponential high-energy cutoff. The power law index is -0.52 +/- 0.07
and the cutoff energy, parameterized as Epeak, is 933 +/- 105 keV.
A Band function fits the spectrum equally well with Epeak= 858 +/- 110 keV,
alpha = -0.48 +/- 0.08 and beta = -3.04 +/- 0.76.
The event fluence (10-1000 keV) in this time interval is
(1.56 +/- 0.06)E-06 erg/cm^2. The 64-ms peak photon flux measured
starting from T0+0.0 s in the 10-1000 keV band is 45 +/- 2 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
View this GCN Circular online at https://gcn.nasa.gov/circulars/36685.
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TITLE: GCN CIRCULAR
NUMBER: 36684
SUBJECT: LIGO/Virgo/KAGRA S240615ea: Updated Sky localization
DATE: 24/06/17 14:41:48 GMT
FROM: Charlie Hoy at University of Portsmouth <charlie.hoy(a)port.ac.uk>
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S240615ea (GCN Circular 36670). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits, distributed via GCN Notice, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S240615ea
For the Bilby.offline0.multiorder.fits sky map, the 90% credible region is 653 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3590 +/- 1058 Mpc (a posteriori mean +/- standard deviation).
For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.
[1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. (2023) arXiv:2307.13380
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TITLE: GCN CIRCULAR
NUMBER: 36683
SUBJECT: GRB 240615A: Swift-XRT observations
DATE: 24/06/17 09:09:22 GMT
FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk>
K.L. Page (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), M. Perri (SSDC & INAF-OAR), J. D. Gropp (PSU), S.
Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester) and
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
Swift/BAT-GUANO-detected burst GRB 240615A, collecting 5.0 ks of Photon
Counting (PC) mode data between T0+78.9 ks and T0+92.0 ks.
One uncatalogued X-ray source has been detected consistent with being
within 394 arcsec of the Swift/BAT-GUANO position, it is below the RASS
limit and shows no definitive signs of fading. Therefore, at the
present time we cannot confirm this as the afterglow. Details of this
source are given below:
Source 1:
RA (J2000.0): 326.0968 = 21:44:23.22
Dec (J2000.0): +38.5681 = +38:34:05.1
Error: 8.9 arcsec (radius, 90% conf.)
Count-rate: (3.6 [+9.7, -3.6])e-4 ct s^-1
Distance: 157 arcsec from Swift/BAT-GUANO position.
Flux: (8.5 [+23.2, -8.5])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)
Four uncatalogued sources were also detected too far from the GRB
position to be likely afterglow candidates.
The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021696.
This circular is an official product of the Swift-XRT team.
View this GCN Circular online at https://gcn.nasa.gov/circulars/36683.
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TITLE: GCN CIRCULAR
NUMBER: 36682
SUBJECT: GRB 240615A: GECAM detection
DATE: 24/06/17 07:34:20 GMT
FROM: tanwj(a)ihep.ac.cn
Wen-Jun Tan, Ce Cai,Shao-Lin Xiong report on behalf of the GECAM team:
GECAM-C was triggered by a short burst, GRB 240615A, during the routine ground search at 2024-06-15T17:51:45.100 UTC (T0), which was also detected by Fermi/GBM (GCN #36671), Swift/BAT-GUANO arcminute localization(GCN #36672) and Konus-Wind (GCN #36677) .
According to the GECAM-C light curve, this burst shows one single pulse with a total duration of ~0.1 sec.
We note that these results are very preliminary. Refined analysis will be reported later.
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
View this GCN Circular online at https://gcn.nasa.gov/circulars/36682.
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TITLE: GCN CIRCULAR
NUMBER: 36681
SUBJECT: X-ray transient EP240414a: Chandra late time observation
DATE: 24/06/17 06:54:06 GMT
FROM: P.G. Jonker at Radboud University <p.jonker(a)astro.ru.nl>
P.G. Jonker (Radboud Univ. & SRON), A.J. Levan (Radboud Univ. & Warwick Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. Quirola-Vásquez (Radboud Univ.), Maria E. Ravasio (Radboud Univ.), F. E. Bauer (PUC), D. Mata Sanchez (IAC), M.A.P. Torres (IAC), report on behalf of a larger collaboration:
We observed the location of the X-ray counterpart of the fast X-ray transient EP240414a reported by the Einstein Probe WXT instrument and FXT (Lian et al. GCN 36091; Guan et al. GCN 36129) using the Chandra X-ray Observatory. Observations started at 2024-06-16:05:35 UT and lasted for 10.7 ksec. The VFAINT observing mode was used. In a preliminary analysis we do not detect an X-ray counterpart to the
transient. Two X-ray photons are detected in a circular region with a 1" radius, centred on the position of the optical transient reported in Aryan et al. (GCN 36094). Using Kraft et al. (1991) this leads to a 90% upper limit on the source count of ~4E-4 cnt/s. Using an absorbed power law model with index 1.7 for the source (N_H=3.35E20 cm-2) following the detected parameters of Guan et al. (GCN 36129), we obtain a flux upper limit of 8E-15 erg cm-2 s-1 (0.5-10 keV). This corresponds to an upper limit on the source luminosity of L_X<5E42 erg/s for the z=0.4 redshift of the source.
The AGN at the centre of the host galaxy of EP240414a with (RA, Dec)=(12:46:02.0, -09:43:09.16) is detected with a count rate of 1.7E-3 cnt/s, which, for the same spectral parameters as above, yields a luminosity of 2E43 erg/s (0.5-10 keV). Care should be taken to remove the contribution of the AGN to earlier X-ray detections of EP240414a obtained with satellites with lower spatial resolution than Chandra.
We like to thank the CXC director and team for approving and executing this observation.
View this GCN Circular online at https://gcn.nasa.gov/circulars/36681.
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