EP250830a, GRB 250830B

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea
(PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
MAXI-detected burst GRB 250830B. The observations now extend from
T0+11.1 ks to T0+223.0 ks and have a total exposure time of 6.0 ks. .
The source previously reported, "Source 1", is fading with 2.5 sigma
significance and thus is believed to be the GRB afterglow. Using 2349 s
of PC mode data and 2 UVOT images, we find an enhanced XRT position
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue): RA, Dec = 105.74037, -64.85289 which is equivalent
to:

RA (J2000): 07h 02m 57.69s
Dec(J2000): -64d 51' 10.4"

with an uncertainty of 3.0 arcsec (radius, 90% confidence). This
position is 2.3 arcmin from the EP position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.43 (+0.15, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.8, -0.5). The
best-fitting absorption column is  1.5 (+6.0, -0.4) x 10^21 cm^-2,
consistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 5.4 x 10^-11 (6.1 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.5 (+6.0, -0.4) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.8, -0.5)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021858.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021858.

This circular is an official product of the Swift-XRT team.

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea
(PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 250830B, collecting  2.4 ks of Photon Counting (PC) mode data
between T0+11.0 ks and T0+50.8 ks. This event also triggered Einstein
Probe around 25 s after MAXI, and was separately designated EP250830a.

One uncatalogued X-ray source has been detected within the estimated
3-sigma EP error region (296 arcsec), also consistent with the MAXI
error region; 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):  105.7399  =  07h 02m 57.59s
  Dec (J2000.0): -64.8525  =  -64d 51' 09.1"
  Error: 3.7 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0323 +/- 0.0046 ct s^-1   
  Distance: 134 arcsec from EP position.
  Flux: (1.74 +/- 0.25)e-12 erg cm^-2 s^-1 (observed, 0.3-10 keV)

A further uncatalogued X-ray source was also detected, however this was
too far from the EP position to be the afterglow.

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/00021858.

This circular is an official product of the Swift-XRT team.

V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250830a ( EP Team et al., GCN 41611
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 ) errorbox 9623 sec after trigger time at 2025-08-30 06:37:32 UT, with upper limit up to  20.1 mag. The observations began at zenith distance = 65 deg. The sun  altitude  is -54.1 deg. 

The galactic latitude b = -23 deg., longitude l = 275 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2980099

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

    9644 | 2025-08-30 06:37:32 |         MASTER-OAFA | (07h 03m 18.97s , -64d 47m 00.0s) |   C |    40 | 19.3 |        
    9684 | 2025-08-30 06:37:32 |         MASTER-OAFA | (07h 03m 18.97s , -64d 46m 60.0s) |   C |   120 | 20.1 |  Coadd 
    9690 | 2025-08-30 06:38:18 |         MASTER-OAFA | (07h 03m 18.83s , -64d 46m 58.5s) |   C |    40 | 19.3 |        
    9735 | 2025-08-30 06:39:04 |         MASTER-OAFA | (07h 03m 18.65s , -64d 46m 56.6s) |   C |    40 | 19.3 |        
    9781 | 2025-08-30 06:39:50 |         MASTER-OAFA | (07h 03m 18.51s , -64d 46m 55.0s) |   C |    40 | 19.3 |        
    9821 | 2025-08-30 06:39:50 |         MASTER-OAFA | (07h 03m 18.52s , -64d 46m 54.9s) |   C |   120 | 20.1 |  Coadd 
    9827 | 2025-08-30 06:40:35 |         MASTER-OAFA | (07h 03m 18.38s , -64d 46m 53.5s) |   C |    40 | 19.3 |        
    9872 | 2025-08-30 06:41:21 |         MASTER-OAFA | (07h 03m 18.20s , -64d 46m 52.4s) |   C |    40 | 19.3 |        
    9918 | 2025-08-30 06:42:07 |         MASTER-OAFA | (07h 03m 18.10s , -64d 46m 51.1s) |   C |    40 | 19.4 |        
    9964 | 2025-08-30 06:42:52 |         MASTER-OAFA | (07h 03m 17.96s , -64d 46m 49.8s) |   C |    40 | 19.4 |        
   10010 | 2025-08-30 06:43:38 |         MASTER-OAFA | (07h 03m 17.75s , -64d 46m 48.2s) |   C |    40 | 19.3 |        
   10056 | 2025-08-30 06:44:24 |         MASTER-OAFA | (07h 03m 17.57s , -64d 46m 46.8s) |   C |    40 | 19.3 |        
   10101 | 2025-08-30 06:45:10 |         MASTER-OAFA | (07h 03m 17.44s , -64d 46m 45.6s) |   C |    40 | 16.0 |        
   10147 | 2025-08-30 06:45:56 |         MASTER-OAFA | (07h 03m 17.20s , -64d 46m 44.0s) |   C |    40 | 19.1 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

R.D. Liang (NAOC), K. R. Ni (CCNU), X. Tian(GXU), W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250830a. The transient triggered EP-WXT (ID: 01709200995) at 2025-08-30T03:57:09 (UTC), whcih is about 25 s after the trigger of GRB 250830B by MAXI (GCN 41602
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 ). The WXT position of the source is R.A. = 105.723 deg, DEC = -64.816 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic), spatially consistent with GRB 250830B. The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with Galactic hydrogen column density fixed to 1.1 x 10^21 cm^-2 and a photon index of 1.6 (+/- 0.2), and a peak flux of approximately 1 x 10^-9 erg/s/cm^2.

We proposed a follow-up observation with the Swift XRT that was performed at 2025-08-30T07:00:13, approximately 3.05 h after the trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 105.73708 deg, DEC = -64.85286 deg (J2000) with an uncertainty of 5 arcsec in radius (90% C.L. statistical and systematic). The integrated 0.3-10 keV spectrum can be fitted with an absorbed powerlaw model with fixed identity Galactic hydrogen column density and a photon index of 0.9 (+/-0.5), and an averaged flux of 2.5 x 10^-12 erg/s/cm^2. 

We gratefully acknowledge the support of the Swift team.

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).

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the MAXI-detected event
GRB 250830B. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021858
Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the MAXI event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

This circular is an official product of the Swift-XRT team.

T. Usuki (Ehime U.), H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, H. Nishio  (Nihon U.),
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, 
Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, 
Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, 
Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), 
K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), 
S. Yamada (Tohoku U),report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source 
at 03:56:44 UT on August 30, 2025. Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (105.209 deg, -64.628 deg) = (07 00 50, -64 37 40) (J2000)
with a 90% C.L. statistical error of 0.33 deg and an additional systematic uncertainty of 
0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 225 +- 36 mCrab (4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (105.751, -63.592) deg = (07 03 00, -63 35 31) (J2000)
(R.A., Dec) = (104.300, -63.666) deg = (06 57 11, -63 39 57) (J2000)
(R.A., Dec) = (104.713, -65.344) deg = (06 58 51, -65 20 38) (J2000)
(R.A., Dec) = (106.255, -65.266) deg = (07 05 01, -65 15 57) (J2000)
There was no significant excess flux in the previous transit at 02:23 UT 
and in the next transit at 05:30 UT with an upper limit of 20 mCrab for each.

We requested Swift/XRT followup observation.