MNRAS 000,1–14 (20252025) P ep in 11 No embe 2025 Compiled using MNRAS L
A
TEX s yle ile 3.3
EP250207b is no a collapsa as X- ay ansien . Is i due o a bina y
compac objec me ge ?
P. G. Jonke ,1,2A. J. Le an,1,3Xing Liu,4Dong Xu,4Yuan Liu,4Xinpeng Xu,4,5An Li,6N. Sa in,7,8
N.R. Tan i ,9G.P. Lamb,10 M.E. Ra asio,1J. Sánchez-Sie as,1J.A. Qui ola-Vásquez,1B.C. Rayson,9
J.N.D. an Dalen,1D.B. Malesani,11,12,1A.P.C. an Hoo ,1F. E. Baue ,13 J. Chacón,14 S.J. Sma ,15,16
A. Ma in-Ca illo,17 G. Co co an,17 L. Co e ,17 A. Rossi,18 F. Ono i,19 M. F ase ,17 P.T. O’B ien,9
R.A.J. Eyles-Fe is,9J. Hjo h,20 T.-W. Chen,21 G. Leloudas,22 L. Tomasella,23 S. Schulze,24 M. De Pasquale,25
F. Ca o enu o,26 J. B igh ,15,27 Chenwei Wang, 28 Shaolin Xiong, 28 Jinpeng Zhang, 28 Wangchen Xue, 28
Jiacong Liu, 28 Chengkui Li, 28 D. Ma a Sánchez,29,30 M.A.P. To es,29,30
1Depa men o As ophysics/IMAPP, Radboud Uni e si y, 6525 AJ Nijmegen, The Ne he lands
2SRON, Ne he lands Ins i u e o Space Resea ch, Niels Boh weg 4, 2333 CA Leiden, he Ne he lands
3Depa men o Physics, Uni e si y o Wa wick, Gibbe Hill Road, Co en y, CV4 7AL, UK
4Na ional As onomical Obse a o ies, Chinese Academy o Sciences, Beijing 100101, China
5School o As onomy and Space Science, Uni e si y o Chinese Academy o Sciences, Beijing 100049, China
6Beijing No mal Uni e si y, No.19, Xinjiekouwai S , Haidian Dis ic , Beijing, 100875, P.R.China
7Ka li Ins i u e o Cosmology, Uni e si y o Camb idge, Madingley Road, CB3 0HA, UK
8Ins i u e o As onomy, Uni e si y o Camb idge, Madingley Road, CB3 0HA, UK
9School o Physics and As onomy, Uni e si y o Leices e , Uni e si y Road, LE1 7RH, UK
10As ophysics Resea ch Ins i u e, Li e pool John Moo es Uni e si y, IC2 Li e pool Science Pa k, 146 B ownlow Hill, Li e pool, L3 5RF, UK
11Cosmic Dawn Cen e (DAWN), Denma k
12Niels Boh Ins i u e, Uni e si y o Copenhagen, Jag ej 128, 2200 Copenhagen N, Denma k
13Ins i u o de Al a In es igación, Uni e sidad de Ta apacá, Casilla 7D, A ica, Chile
14Ins i u o de As o ísica, Facul ad de Física, Pon i icia Uni e sidad Ca ólica de Chile, Campus San Joaquín, A . Vicuña Mackenna 4860, Macul San iago, Chile, 7820436
15Depa men o Physics, Uni e si y o Ox o d, Keble Road, Ox o d, OX1 3RH, UK
16As ophysics Resea ch Cen e, School o Ma hema ics and Physics, Queen’s Uni e si y Bel as , BT7 1NN, UK
17School o Physics and Cen e o Space Resea ch, Uni e si y College Dublin, Bel ield, Dublin 4, I eland
18Osse a o io di As o isica e Scienza dello Spazio, INAF, Via Pie o Gobe i 93/3, Bologna, 40129, I aly
19INAF-Osse a o io As onomico d’Ab uzzo, ia M. Maggini snc, I-64100 Te amo, I aly
20DARK, Niels Boh Ins i u e, Uni e si y o Copenhagen, Jag ej 155A, 2200 Copenhagen, Denma k
21G adua e Ins i u e o As onomy, Na ional Cen al Uni e si y, 300 Jhongda Road, 32001 Jhongli, Taiwan
22DTU Space, Depa men o Space Resea ch and Space Technology, Technical Uni e si y o Denma k, Elek o ej 327, 2800 Kgs. Lyngby, Denma k
23Is i u o Nazionale di As o isica INAF Osse a o io As onomico di Pado a ia dell’Osse a o io 8, 36012 Asiago
24Cen e o In e disciplina y Explo a ion and Resea ch in As ophysics (CIERA), No hwes e n Uni e si y, 1800 She man A e., E ans on, IL 60201, USA
25Uni e si y o Messina, MIFT Depa men , ia F. S. D’Alcon es 31, Messina, 98166, I aly
26 INAF-Osse a o io As onomico di Roma, Via F asca i 33, I-00078, Mon e Po zio Ca one (RM), I aly
27 B eak h ough Lis en, As ophysics, Depa men o Physics, The Uni e si y o Ox o d, Keble Road, Ox o d OX1 3RH, UK
28 S a e Key Labo a o y o Pa icle As ophysics, Ins i u e o High Ene gy Physics, Chinese Academy o Sciences, 19B Yuquan Road, Beijing 100049, China
29Ins i u o de As o ísica de Cana ias, IAC, E-38205, La Laguna, Tene i e, Spain
30Depa amen o de As o ísica, Uni . de La Laguna, E-38206, La Laguna, Tene i e, Spain
Accep ed XXX. Recei ed YYY; in o iginal o m ZZZ
ABSTRACT
Fas X- ay T ansien s (FXTs) a e sho -li ed ex a-galac ic X- ay sou ces. Recen p og ess h ough mul i-wa eleng h ollow-
up o Eins ein P obe disco e ed FXTs has shown ha se e al a e ela ed o collapsa s, which can also p oduce 𝛾- ay bu s s
(GRBs). In his pape we in es iga e he na u e o he FXT EP250207b. The VLT/MUSE spec a o a nea by (15.9 kpc in
p ojec ion) len icula galaxy e eal no signs o ecen s a o ma ion. I his galaxy is indeed he hos , EP250207b lies a a
edshi o 𝑧=0.082, implying a peak obse ed absolu e magni ude o he op ical coun e pa o M ′=−14.5. A he ime when
supe no ae (SNe) would peak, i is subs an ially ain e han all SN ypes. These esul s a e inconsis en wi h a collapsa o igin
o EP250207b. The p ope ies a ou a bina y compac objec me ge d i en o igin. The X- ay, op ical and adio obse a ions
a e compa ed wi h p edic ions o se e al ypes o ex a-galac ic ansien s, including a e glow and kilono a models. The da a
can be i wi h a sligh ly o -axis iewing angle a e glow. Howe e , he la e- ime (∼30 day) op ical/NIR coun e pa is oo b igh
o he a e glow and also o con en ional kilono a models. This could be emedied i ha la e emission is due o a globula
clus e o he co e o a ( idally dis up ed) dwa galaxy. I con i med, his would be he i s case whe e he mul i-wa eleng h
p ope ies o an FXT a e ound o be consis en wi h a compac objec me ge o igin, inc easing he pa allels be ween FXTs and
GRBs. We inally discuss i he sou ce could o igina e in a highe edshi hos galaxy.
Key wo ds: s a s: indi idual: EP250207b – supe no ae: gene al – ansien s: supe no ae – S a s: black holes –
©20252025 The Au ho s
2Jonke e al.
1 INTRODUCTION
The i s clea ex a-galac ic Fas X- ay T ansien s (FXTs) we e de-
ec ed se endipi ously (Sode be g e al. 2008;Jonke e al. 2013;
Baue e al. 2017). Sys ema ic sea ches h ough Chand a and XMM-
New on a chi alda a e ealed≈30 ex a-galac ic FXTs (Glennie e al.
2015;Xue e al. 2019;Lin e al. 2019;Alp & La sson 2020;Qui ola-
Vásquez e al. 2022;Eappachen e al. 2023;Qui ola-Vásquez e al.
2023). A small numbe o e en s la e u ned ou o be caused by s el-
la la es om ac i e s a s in ou Milky Way (e.g. Eappachen e al.
2024 eclassi ied he XMM-New on-disco e ed e en XRT 140811
as a s ella la e). Howe e , o he as majo i y o iden i ied ex a-
galac ic FXTs, his scena io can be excluded. Ne e heless, wi hou
he de ec ion o a con empo aneous coun e pa a op ical o nea -
in a ed (NIR) wa eleng hs, hei o igin is di icul o de e mine.
A small bu c i ically impo an ac ion o he obse ed sou ces
in he ansien sky a e powe ed by he ac ion o a compac cen al
engine (black hole o highly magne ic neu on s a ). The p o o ype o
hese ex eme e en s is he la ge popula ion o 𝛾- ay bu s s (GRBs),
whose na u e has been he subjec o in ense s udy in he 50 yea s
since hei disco e y. GRBs ha e ypical du a ions spanning om a
ac ion o a second (sho GRBs), o minu es (long GRBs; Kou e-
lio ou e al. 1993), wi h only a iny mino i y ha ing du a ions up o
a ew hou s (so-called ul a-long GRBs; Le an e al. 2014). They
o igina e du ing he inal momen o a s a , ei he ia he collapse
o a massi e s ella co e (e.g. a collapsa ; Hjo h e al. 2003;S anek
e al. 2003) o he me ge o wo compac objec s (e.g. Tan i e al.
2013;Be ge e al. 2013;Abbo e al. 2017). Whe eas ini ially long
GRBs we e exclusi ely associa ed wi h collapsa s and sho GRBs
wi h bina y compac objec me ge s, ecen esul s show ha he
long-sho – collapsa -me ge dicho omy is no s ic (e.g. Ras inejad
e al. 2022).
Using he o en a csecond-p ecision knowledge o he FXT X-
ay sou ce posi ion on he sky, deep sea ches e eal candida e hos
galaxies (e.g.Lin e al. 2022;Eappachene al.2024;Qui ola-Vásquez
e al. 2024a). This in u n allows a (pho ome ic) edshi o be de-
i ed, which se s he luminosi y and ene gy scales in ol ed, c udely
cons aining he na u e o he FXT. Howe e , i is only since he
launch o he Eins ein P obe (EP) sa elli e (Yuan e al. 2022;Yuan
e al. 2025) on Jan. 9, 2024 which is de ec ing abou a 80 FXTs pe
yea (depending on he signal- o-noise limi adop ed) and announc-
ing hei disco e y apidly, ha mul i-wa eleng h coun e pa s o he
FXTs ha e been disco e ed egula ly (e.g. Gillande s e al. 2024;
Qui ola-Vasquez e al. 2025;Liu e al. 2025b;A yan e al. 2025).
The ollow-up obse a ions o coun e pa s led o he disco e y o a
b oad-lined Ic supe no a (SN) in he spec a and ligh cu e o se -
e al EP-disco e ed FXTs, in pa icula , EP240414a ( an Dalen e al.
2025;Sun e al. 2025;S i as a e al. 2025), EP250108a (Eyles-
Fe is e al. 2025;Ras inejad e al. 2025;S ini asa aga an e al.
2025), EP250304a (Co e e al. in p ep.) and possibly EP241021a
(Zheng e al. 2025;Gian agna e al. 2025;Busmann e al. 2025;
Yada e al. 2025; Qui ola-Vasquez e al. submi ed). Simila ly, o
some 20-30% o FXTs a (long) GRB is de ec ed (e.g. Liu e al. 2024;
Le an e al. 2024;Yin e al. 2024;Jiang e al. 2025), indica ing ha
a signi ican ac ion o he EP-disco e ed FXTs ha e a collapsa
o igin.
Howe e , o an impo an ac ion o Eins ein P obe-disco e ed
FXTs, no con empo aneous co-spa ial bu s o 𝛾- ay emission is
de ec ed (Ra asio e al. 2024,2025a,b, o lis bu a ew) despi e ob-
se a ions wi h su icien sensi i i y o de ec such bu s s o ypical
GRB spec al shapes. As se e al EP-disco e ed collapsa FXTs we e
also no de ec ed in 𝛾- ays, his is by no means e idence o a di e -
en na u e han a collapsa o igin o a signi ican ac ion o FXTs.
Howe e , he FXT EP240408a does no seem o o igina e om a
collapsa (Zhang e al. 2025).
By analogy wi h he me ge -d i en and collapsa d i en o igins o
GRBs, one could wonde i a ac ion o FXTs can be linked o me ge
d i en e en s, as has been sugges ed o many Chand a-disco e ed
FXTs, o ins ance on accoun o he pla eau ound in he X- ay ligh
cu e (e.g. Zhang 2013;Me zge & Pi o 2014;Ciol i 2016;Sun e al.
2017a;Sun e al. 2019a;Xue e al. 2019;Qui ola-Vásquez e al.
2024b). Wi h his in mind, we epo he e on he EP X- ay disco e y
o FXT EP250207b and ou X- ay, op ical, NIR, and adio ollow-up
obse a ions.
Th oughou his wo k, we assume he spa ially- la 6-pa ame e
ΛCDM Planck cosmology (Planck Collabo a ion e al. 2020) wi h
𝐻0=67.7km s−1Mpc−1and Ω𝑚=0.31. We p o ide all mag-
ni udes in he AB magni ude sys em. Fo NIR magni udes cali-
b a ed o 2MASS, which is in he Vega sys em, we use he Vega
o AB con e sions 𝐽AB =𝐽VEGA +0.91,𝐻𝐴𝐵 =𝐻VEGA +1.39,
𝐾𝑠,AB =𝐾𝑠,VEGA +1.85 (Blan on & Roweis 2007).
2 OBSERVATIONS & RESULTS
2.1 Eins ein P obe (EP) X- ay obse a ions
2.1.1 EP – Wide- ield X- ay Telescope (EP-WXT) obse a ions
A new FXT was disco e ed in EP-WXT obse a ions on Feb. 7,
2025, a T0=21:47:52.85 (UTC), which las ed mo e han 120 s and
had a epo ed WXT posi ion o Righ Ascension (R.A.) = 167.495
deg (J2000), and Declina ion (Dec) = -7.906 deg (J2000) wi h a 90%
con idence unce ain y o 2.7′in adius (Zhou e al. 2025). A e
backg ound sub ac ion, 27 sou ce pho ons we e de ec ed in he 0.5-4
keV ene gy band. See Fig. 1 o he ligh cu e. Requi ing ha each
bin con ains one o mo e pho ons and applying Poisson s a is ics in
he i , he a e age EP-WXT 0.5-4 keV spec um du ing his pe iod
can be i ed well (Cash-s a is ics [Cash 1979], 24.5 o 24 deg ees
o eedom [d.o. .]) by an abso bed powe law wi h a ( ixed; HI4PI
Collabo a ion e al. 2016) line-o -sigh Galac ic equi alen hyd ogen
column densi y o 4×1020 cm−2and a pho on index o 0.5±0.7.
The a e age unabso bed 0.5-4 keV lux is (6.5±3.6) × 10−10 e g
cm−2s−1(90% con idence le el). See Fig. 2 o he bes i o he EP-
WXT X- ay spec um. The EP X- ay da a was p ocessed using a da a
educ ionpipeline and he calib a ionda abase (CALDB)speci ically
designed o WXT (Liu e al. in p ep.). The CALDB inco po a es
esul s bo h om on-g ound and in-o bi calib a ion obse a ions
(Cheng e al. 2024). We used XSPEC e sion 12.15.0 o he i
(A naud 1996).
2.1.2 EP-Follow-up X- ay Telescope (EP-FXT) obse a ions
The EP-Follow-up X- ay Telescope ins umen wi h ac onym EP-
FXT1consis s o wo elescopes each wi h i s own de ec o , sys em
A and B, and he de ec o s a e sensi i e o e he 0.3–10 keV band.
Th ee EP-FXT ollow-up obse a ions we e pe o med. The i s one
s a ed on Feb. 8, 2025, a 14:50:57 (UTC) wi h an exposu e ime
o 3025 s (a 𝑡=0.71 d a e he EP-WXT igge ). The second one
s a ed on Feb. 9, 2025 a 18:09:49 (UTC) wi h an exposu e ime o
1No e ha we always e e o he Eins ein P obe Follow-up X- ay Telescope
ins umen as "EP-FXT" and o a Fas X- ay T ansien as "FXT" o y o
a oid con usion be ween he wo.
MNRAS 000,1–14 (20252025)
EP250207b a compac bina y me ge ? 3
−200 −150 −100 −50 0 50 100 150 200
Time since T0(s)
0
2
4
6
Coun s
T0=2025-02-07 21:47:56 (UTC)
Figu e 1. The backg ound sub ac ed EP250207b disco e y ligh cu e ob-
ained by he EP-WXT ins umen . Time is in seconds a e T0and he bin
size is 6 s. A pe iod o ≈200 s be o e he FXT s a is shown o assess he
numbe o e en s a he sou ce loca ion be o e he FXT onse .
6×10 12×1003×100
Ene gy (keV)
10 2
10 1
100
Coun s s 1 keV 1
WXT-EP
100
6×10 12×1003×100
Ene gy (keV)
2.5
0.0
2.5
(d-m)/
Figu e 2. Top panel: The EP250207b disco e y spec um (0.5–4 keV) ob-
ained by he EP-WXT ins umen a e aged o e 120 s. The bes - i powe
law model a ec ed by Galac ic ex inc ion is shown. Bo om panel: The da a,
minus he bes i model, di ided by he e o in he da a poin is shown. No
signi ican de ia ions wi h espec o he bes - i model a e p esen .
5044 s (a 𝑡=1.85 d a e he EP-WXT igge ). The esul s o hese
i s wo obse a ions ha e also been announced in Zhou e al. (2025)
wi h a bes - i sou ce J2000 posi ion o : (R.A.,Dec) = (167.5130, -
7.8695) wi h an unce ain y o 10′′ ( adius, 90% con idence le el).
A hi d obse a ion s a ed on Feb. 10, 2025 a 13:16:43 (UTC) wi h
an exposu e ime o 9045 s (a 𝑡=2.65 d a e he EP-WXT igge ).
The spec a o hese EP-FXT obse a ions (see Fig. 3) ha e been i
simul aneously using an abso bed powe law wi h a Galac ic equi -
alen hyd ogen column densi y ixed o he mean line-o -sigh alue
p o ided in HI4PI Collabo a ion e al. (2016) o 4×1020 cm−2and
pho on indices o 1.6±0.4, 2.3±0.9, and 2.3±0.9, o he i s , sec-
ond, and hi d obse a ions, espec i ely. In he i , he EP-FXT-A
and EP-FXT-B de ec o s ha e hei own esponse ma ices. The i
has a Cash-s a is ic o 156 o 147 d.o. . The unabso bed 0.3–10 keV
luxes a e (3.3+1.4
−0.9) ×10−13,(4+3
−1) ×10−14,(3+2
−1) ×10−14 e g cm−2
s−1(90% con idence le el), espec i ely. No signi ican change in
he powe law spec um is obse ed o e he h ee obse ing epochs
while he lux dec eased, a leas be ween he i s and second epochs.
Ene gy (keV)
10 5
10 4
10 3
10 2
10 1
100
Coun s s 1 keV 1
EP-FXT-A (Epoch 1)
EP-FXT-B (Epoch 1) EP-FXT-A (Epoch 2)
EP-FXT-B (Epoch 2) EP-FXT-A (Epoch 3)
EP-FXT-B (Epoch 3)
100101
Ene gy (keV)
2.5
0.0
2.5
(d-m)/
Figu e 3. Top panel: Shown is he EP-FXT spec um o each o he h ee
obse a ions o each FXT elescope uni A and B sepa a ely. In addi ion,
he bes - i powe law model a ec ed by Galac ic ex inc ion is shown using
a ma ching-colou d awn line. Epoch 1 EP-FXT-A is shown in black and
EP-FXT-B in ed, while epoch 2 EP-FXT-A is shown in g een and EP-FXT-B
in blue, and inally, epoch 3 EP-FXT-A is shown in ligh blue and EP-FXT-
B in pu ple. No la ge change in he sou ce spec al shape is obse ed in
be ween he h ee obse ing epochs while he lux dec eased be ween he
i s and he second epoch. Bo om panel: The da a, minus he bes i model,
di ided by he e o in he da a is shown o each o he EP-FXT-A and
B uni elescope–de ec o sys em o each o he h ee obse ing epochs.
No signi ican de ia ions wi h espec o he bes - i model a e p esen . The
colou s ep esen he same da a/epoch as in he op panel.
We also in es iga ed he da a o he EP-FXT-A and EP-FXT-B de ec-
o s o each o he h ee epochs o sea ch o a iabili y (e.g. la es),
bu none we e ound. No e ha each de ec o only de ec ed be ween
17–61 coun s du ing hese obse a ions.
We con e ed he 0.5-4 keV EP-WXT (unabso bed) lux o a 0.3–
10 keV (unabso bed) lux o acili a e compa ison wi h he EP-FXT
unabso bed lux using W3PIMMS aking he bes - i abso bed powe
law model o he EP-WXT spec um (see Sec ion 2.1.1) as inpu . We
ob ain an EP-WXT unabso bed 0.3–10 keV lux o (3+2
−1.4)×10−9e g
cm−2s−1. We show he X- ay ligh cu e combining he EP-WXT
and EP-FXT measu emen in Fig 4. O e -plo ed is he bes - i powe
law decay unc ion wi h 𝐹𝑋=𝐶× ( 𝑡
1 d)𝑚, he bes - i powe law
index 𝑚=−1.5and 𝐶=1.4×10−13 e g cm−2s−1.
2.2 GECAM limi s on gamma- ay emission
The G a i a ional wa e high-ene gy Elec omagne ic Coun e pa
All-sky Moni o (GECAM) is a cons ella ion o ou gamma- ay all-
sky moni o s, including GECAM-A/B (Li e al. 2022), GECAM-C
(Zhang e al. 2023), and GECAM-D (Wang e al. 2024). Th oughou
he bu s du a ion o EP250207b, only GECAM-B was con inuously
collec ing da a wi h a good co e age o he loca ion o EP250207b.
Howe e , no signi ican signal was de ec ed nei he in- ligh (Zhao e
al. 2024) no on-g ound (Cai e al. 2025). We pe o med a a ge ed
sea ch (Cai e al. 2021) o he de ec ion o a sou ce using GECAM-
B da a om 2025-02-07 21:47:36 o 2025-02-07 21:56:16 (UTC).
No signi ican sou ce is ound. We calcula ed an uppe limi o he
de ec ion o a sou ce conside ing h ee ypical GRB spec al models
(i.e. so , no mal and ha d Band unc ions ( ollowing e.g. Zheng
e al. 2025) and h ee imescales (0.1 s, 1 s, 10 s). The 3 sigma
MNRAS 000,1–14 (20252025)
4Jonke e al.
10−310−210−1100101
Time since T0(days [obse e ame])
10−14
10−13
10−12
10−11
10−10
10−9
X- ay lux (0.3-10 keV: e g cm−2s−1)
Mul i-wa eleng h ligh cu e EP250207b
23.0
23.5
24.0
24.5
25.0
25.5
26.0
26.5
27.0
Appa en magni ude (AB mag)
i : m=-1.5, c=1.4E-13
X- ay
J
Ks
/F606W
z
F105W
F125W
F160W
Figu e 4. The EP 0.3–10 keV X- ay ligh cu e o EP250207b. The i s da a
poin is he EP-WXT a e age lux and he nex h ee da a poin s a e om he
EP-FXT obse a ions. The blue dashed line indica es he bes i powe law
unc ion o 𝐹𝑋=𝐶× ( 𝑡
1 d )𝑚. The loca ion o he i s da a poin is aken
o be a 𝑡=10−3d (≈86 s), e lec ing ha he i s da a poin is an a e age
o e ≈150 s. The igh hand y-axis shows he obse ed op ical and NIR
magni udes. Clea ading is de ec ed in he di e en il e s (see Sec ions 2.3
&2.4 ).
uppe limi s (Zhang e al. 2025) on he GRB lux (10-1000 keV)
a y be ween ≈ (1−10) × 10−7e g cm−2s−1 o he longes ha d
o he sho es so assumed GRBs. We also checked o a po en ial
𝛾- ay coun e pa o EP250207b in Fe mi/GBM. Un o una ely, he
loca ion o he sou ce was Ea h-occul ed o Fe mi/GBM o he
en i e du a ion o he e en , so no simul aneous Fe mi/GBM uppe
limi can be epo ed. Finally, we checked whe he he EP250207b
loca ion was obse ed by Swi /BAT. While he loca ion o he sou ce
was no Ea h-occul ed o Swi /BAT, he posi ion ell ou side he
coded-mask ield o iew. As a esul we do no epo a Swi /BAT
uppe limi .
2.3 Op ical and nea -in a ed g ound based obse a ions
The pho ome y epo ed below is unco ec ed o ex inc ion, which
aking he NH=4×1020 cm−2 om he X- ay spec al i s, would
co espond o an AV=0.18 mag ollowing Gü e & Özel (2009).
This is sligh ly highe han he Galac ic AV=0.14 mag de i ed
om he dus eddening in SDSS s a s (Schla ly & Finkbeine 2011,
assuming AV
E(B−V)=3.1). Gi en he in insic sca e in he ela ion,
his is likely consis en wi h he Galac ic AV alue (c . Zhu e al.
2017).
2.3.1 NOT/ALFOSC + NOT/NOTCam
The ield o EP250207b was obse ed wice using he No dic Op ical
Telescope (NOT). Ini ially, he ALFOSC ins umen was used o
ob ain 4 exposu es o 200 s in he 𝑟′- il e . The mid- ime o hese
exposu es was 2025-02-09 03:16:13 UTC, i.e. 1.23 d a e he EP-
WXT s a ime o EP250207b. A e s anda d bias sub ac ion and
co ec ion o la ield, a sou ce was disco e ed ha is no p esen
in deepe Legacy Su ey images (Dey e al. 2019) o he ield. We
sub ac he Legacy 𝑟′-band image om he new NOT image o
Figu e 5. The NOT/ALFOSC disco e y𝑟′-band image o he op ical coun e -
pa o he FXT EP250207b combining he wo bes -seeing images o he ou .
The whi e ci cle indica es he EP-FXT sou ce localiza ion unce ain y egion
(Zhou e al. 2025). The new ain op ical sou ce (𝑟′=23.3±0.16 mag)
is indica ed by he ick ma ks. I lies in p ojec ion close o he galaxy
WISEA J111002.65−075211.9.
emo e backg ound ligh a he posi ion o he coun e pa due o he
nea by (candida e hos ) galaxy using he ZOGY algo i hm (Zackay
e al. 2016) implemen ed in PyZOGY (Gue el e al. 2021). F om
he sub ac ed image, we ob ain a magni ude o 𝑟′=23.3±0.16
o he new sou ce (calib a ed agains Pan-STARRS). This is b oadly
consis en wi h he magni ude o 𝑟′≈23.7 quo ed in he o iginal
wo k epo ing on his da a (Liu e al. 2025a). In Fig. 5we show he
NOT coun e pa 𝑟′-band disco e y image.
A a mid- ime o 2025-02-11 03:07:13 UTC, i.e. 3.22 d a e he
onse o he FXT, 30 exposu es o 60 s each we e ob ained in he 𝐽-
band il e using he NOTCam de ec o . No sou ce was de ec ed in he
combined image a he loca ion o he candida e op ical coun e pa
o he FXT down o 𝐽AB >22.8 mag (3 𝜎).
In o de o de e mine he bes posi ion o he sou ce, we combined
he i s and las o ou ou ALFOSC images as hey we e aken unde
he bes seeing condi ions. The bes i posi ion o he ansien sou ce
has a R.A. (J2000) = 11:10:03.22 and Dec. (J2000) = -07:52:07.25
wi h an es ima ed unce ain y o ≈0.5′′ (68 % con idence), which
alls well inside he EP-FXT e o egion o EP250207b (see Fig. 5).
2.3.2 Gemini No h and Sou h Mul i-Objec Spec og aph
obse a ions (GMOS)
The ield o he FXT was obse ed using he GMOS ins umen ,
on bo h he Gemini-Sou h (GS) elescope loca ed a Ce o Pachon,
Chile, as well as he Gemini-No h (GN) elescope loca ed a Mauna
Kea, Hawaii, U.S. GMOS was used in imaging mode du ing h ee
epochs ( wo a GN a ∼2.54 d [6 exposu es o 60 s each] and 3.57 d [5
exposu es o 60 s each]), and one a GS 4.36 d a e he disco e y o
EP250207b [12 exposu es o 60 s each] using p og ams GS-2024B-
Q-131 and GS-2024B-FT-112 (PI Baue ). Two GMOS obse a ions
we e execu ed using he 𝑧′- il e and one using he 𝑔′- il e . In he i s
GMOS obse a ion, a ain sou ce was de ec ed a he posi ion o he
candida e op ical coun e pa wi h 𝑧′=24.7±0.2mag. The second
𝑧′- il e obse a ion yielded a non-de ec ion wi h 𝑧′>24.1mag,
whilea deepe inal GS/GMOSobse a ion p o ideda ain de ec ion
wi h 𝑧′=25.1±0.3mag. The pho ome y was calib a ed agains
Pan-STARRS.
Fu he mo e, we used GS/FLAMINGOS 2 (F2) a wo di e en
epochs o obse e he ield o EP250207b in he 𝐽and 𝐾𝑠- il e s.
The 𝐽-band obse a ions we e ob ained on 2025-02-13, he i s and
MNRAS 000,1–14 (20252025)
EP250207b a compac bina y me ge ? 5
las exposu es s a ed a 04:46:41 (UTC) and 05:12:35 (UTC), e-
spec i ely. We combined 27 exposu es o 40 s o sea ch o emission
om he coun e pa . No sou ce was de ec ed a he posi ion o he
coun e pa , wi h a 3 𝜎uppe limi o 𝐽AB >24.2 mag using he
Pho ome y Sans F us a ion py hon ool (Nicholl e al. 2023). In
addi ion, on 2025-02-14 s a ing a 04:39:23 (UTC) 90 exposu es o
15 s each we e ob ained using he 𝐾𝑠- il e . The las exposu e s a ed
a 05:33:11 (UTC). We de i e an uppe limi a he sou ce posi ion
o 𝐾𝑠,AB >23.15 mag (3 𝜎).
2.4 Hubble Space Telescope obse a ions
The ield o he op ical coun e pa o EP250207b was obse ed
wice using he Wide Field Came a 3 (WFC3) onboa d he Hubble
Space Telescope (HST) unde p og am GO-17806 (PI Tan i ). Ob-
se a ions we e ob ained in ou di e en wide-band il e s and wo
de ec o s. Du ing he i s epoch 4×505 s exposu es we e ob ained
o alling 2020 s in he F606W il e using he ul a iole - isible
(UVIS) de ec o and 4 exposu es o 552.94 s we e ob ained o alling
≈2212 s in F105W, F125W and F160W each we e ob ained using
he IR de ec o . All he second epoch obse a ions had an iden ical
se -up and exposu e ime as ha o he i s epoch. Howe e , du ing
he second epoch o F606W obse a ions a cosmic ay hi e y close
o he ansien ’s posi ion was p esen in wo o he ou exposu es.
The e o e, we used only 2×505 s o alling 1010 s o exposu e in he
F606W il e .
The obse a ions we e aken o e he pe iods 7.4–8.7 d and 28.8–
29.0 d a e he WXT igge . Images we e aligned o sou ces in
common o each ame, and subsequen ly d izzled o inal pixel
scales o 0.05 and 0.07 a cseconds pe pixel o he UVIS and IR
il e obse a ions, espec i ely. A sou ce is de ec ed in all HST il e
obse a ions in bo h epochs a he posi ion o he NOT-op ical coun-
e pa . We combined all he il e obse a ions and bo h epochs in o
a single image d izzled o a pixel scale o 0.15′′ (see he op panel o
Fig. 6). An o e -densi y o s a s seemingly connec ing he len icula
galaxy wi h he loca ion o he ansien is ound and indica ed wi h
a g een ellipse o guide he eye in he igu e. In addi ion, in Fig. 6
we show he esul an i s epoch F606W image (Bo om le panel),
he di e ence image ob ained sub ac ing he second epoch F105W
image om he i s epoch (Bo om middle panel), and he di e ence
image ob ained sub ac ing he second epoch F125W image om he
i s epoch (Bo om igh panel).
F om he di e ence images i is clea ha he sou ce aded be ween
he i s and second epochs o HST obse a ions in he F105W and
F125W bands, while he magni ude measu emen s in he F606W
bands also show i aded in F606W. Due o he la ge measu emen
unce ain ies in he F160W band obse a ions (see Table A1) he e
is only ma ginal e idence o ading be ween HST epoch 1 and 2 in
ha band. We colla e all he op ical and NIR pho ome y in Table A1
and we show he ligh cu es in Fig. 4.
2.4.1 Ve y La ge Telescope/MUSE
We obse ed he candida e hos galaxy and he loca ion o
EP250207b on 2025-03-03 using he Mul i Uni Spec oscopic Ex-
plo e (MUSE) moun ed on Ve y La ge Telescope (VLT) Uni Tele-
scope 4. The obse a ions a e pa o he p og am wi h ID: 111.259Q
(PI Jonke ) and s a ed a 02:47:36 (UTC) and las ed un il 03:50:08
(UTC). Fou exposu es o 697 s each we e ob ained wi h small posi-
ional o se s be ween he exposu es, howe e , he seeing de e io a ed
signi ican ly while he exposu es we e ob ained. The e o e, we only
used he i s wo exposu es wi h he bes seeing o ≈1′′. The da a
a e educed using he ESO Re lex pipeline (Weilbache e al. 2020;
ESO CPL De elopmen Team 2015).
The MUSE cube da a o he candida e hos galaxy is spa ially
Vo onoi binned o a a ge signal- o-noise a io (S/N)=30 pe bin
using heVo Bin me hodand so wa eo Cappella i&Copin (2003).
Each spec oscopic bin in each spaxel wi h a 𝑆/𝑁 < 5is ejec ed
o emo e esidual-domina ed spec a be o e binning. Ou o he 46
spa ial spec a, one is con amina ed by ligh o an objec o he Sou h-
Eas o he galaxy, he e o e his spa ial bin is emo ed, lea ing 45
(see Fig. 7).
The candida e hos galaxy, WISEA J111002.65−075211.9 (Le an
e al. 2025) is classi ied in NED as an i egula spi al galaxy, howe e
in he HST images i isually esembles a len icula o ellip ical
galaxy. In Fig. 7, we show he spa ial dis ibu ions o he adial
eloci y V, he eloci y dispe sion 𝜎, he o al me allici y [M/H],
and age o he s ella popula ion. We no e ha he spa ial a ia ion
de ec ed in V, shown in he op-le panel o Fig. 7is ypical o ha
obse ed in a len icula galaxy (e.g., Emsellem e al. 2004).
We also ob ain he a e age spec um o he whole galaxy com-
bining he 45 spa ial bins. We use he penalized pixel i ing me hod
(pPXF;Cappella i 2017) o i he spec um. We use Flexible S ella
Popula ion Syn hesis ( sps 3.2; Con oy e al. 2009,2010;Con oy &
Gunn 2010) as he empla e bank o ou s ella popula ion syn hesis
model. As inpu pa ame e s, we use he edshi o z=0.082, 𝑣=0km
s−1(wi h espec o his edshi ), and s ella eloci y dispe sion o
𝜎=200 km s−1as ini ial guesses. The bes i o he a e age spec-
um o he galaxy is displayed in Fig. 8. The a e age spec um is
shown in black, wi h he bes - i galaxy empla e o e -plo ed in ed.
The esiduals o he i a e shown in g een.
2.5 Mee KAT Radio obse a ions
We obse ed he posi ion o EP250207b wi h he Mee KAT adio
elescope (Camilo e al. 2018;Jonas 2018), as pa o p og am SCI-
20241101-FC-01 (PI Ca o enu o). We conduc ed h ee obse a ions
log-spaced in ime, each wi h he same o al on-sou ce ime o 42
minu es. The i s obse a ion s a ed on Feb ua y 13, 2025 a 01:02
UTC (≈5.1 days a e he i s X- ay de ec ion). The second and
hi d obse a ions we e pe o med, espec i ely, on Ma ch 3, 2025 a
00:47 UTC (≈23 days a e he i s X- ay de ec ion) and on Ma ch
23, 2025 a 00:41 UTC (≈43 days a e he i s X- ay de ec ion). We
obse ed a a cen al equency o 3.06 GHz (S-band, S4), wi h a o al
bandwid h o 875MHz. PKS J1939–6342 and PKS 1128–047 we e
used as lux and complex gain calib a o s, espec i ely. The da a we e
educed wi h he OxKAT pipeline (Heywood 2020), which pe o ms
s anda d lagging, calib a ion and imaging using icolou (Hugo
e al. 2022), CASA (CASA Team e al. 2022) and WSCLEAN (O inga
e al. 2014), espec i ely. In he imaging s ep, we adop ed a B iggs
weigh ing scheme wi h a −0.3 obus pa ame e , yielding a ∼3.6′′
beam and a 8𝜇Jy beam−1 ms noise in he a ge ield. We do no
de ec adio emission a he posi ion o he op ical coun e pa o
EP250207b, and we place a 3𝜎uppe limi s on he peak lux densi y
o he a ge a 27, 24, and 27 𝜇Jy beam−1, o he i s , second,
and hi d obse a ions, espec i ely. To sea ch o pe sis en adio
emission a he loca ion o he sou ce, we also s acked he da a o he
h ee epochs. No adio sou ce was de ec ed down o a 3𝜎uppe limi
o 15 𝜇Jy beam−1.
MNRAS 000,1–14 (20252025)
6Jonke e al.
Figu e 6. Top panel: Fi s and second epoch o ou HST WFC3 F606W+F105W+F125W+F160W il e s obse a ions o he ield o EP250207b combined.
The ed ci cle shows he loca ion o he ansien i s iden i ied in ou NOT obse a ions. In addi ion, a galaxy o he No h o he len icula galaxy is indica ed
wi h "N". This galaxy has a edshi o 𝑧=2.18 (see ex ). The igu e also clea ly shows he p oximi y o he op ical coun e pa o EP250207b o he candida e
(len icula ) hos galaxy WISEA J111002.65−075211.9 a 𝑧=0.082. Finally, an o e -densi y o s a s seemingly connec ing he len icula galaxy wi h he loca ion
o he ansien is indica ed wi h a g een ellipse o guide he eye. Bo om le panel: Fi s epoch o ou HST WFC3 F606W il e obse a ions o he ield o
EP250207b, showing he loca ion o he ansien i s iden i ied in ou NOT obse a ions a a magni ude o F606W=26.31±0.08, o be compa ed wi h he
𝑟′=23.3-band magni ude a de ec ion. Bo om middle panel: The esidual image esul ing om sub ac ing he second epoch o ou HST WFC3 F105W il e
obse a ion om he i s . In he egion indica ed by he ed ci cle a posi i e esidual is p esen , indica ing he coun e pa aded be ween he i s and second
epoch o ou HST F105W obse a ions. Bo om igh panel: The esidual image esul ing om sub ac ing he second epoch o ou HST WFC3 F125W il e
obse a ion om he i s . The ading u he solidi ies he associa ion be ween he coun e pa and he FXT EP250207b. In all hese h ee panels he size o he
ci cle is he same (0.5′′) as ha in he op panel.
3 DISCUSSION
A e he EP disco e y o EP250207b, we ob ained X- ay, op-
ical, NIR, and adio obse a ions o ollow he sou ce e olu-
ion. We could no de i e a di ec edshi o he ansien . How-
e e , he ansien ’s loca ion, nex o he la ge len icula galaxy
WISEA J111002.65−075211.9, p o ides s a is ical e idence h ough
he low Pchance <0.5% (Malesani e al. 2025) o hei associa ion.
Fu he mo e, ou combined, deep, HST obse a ions show ha he
sou ce posi ion is consis en wi h he ou ski s o his galaxy (Fig. 6).
This image u he e eals he p esence o enhanced emission seem-
ingly b idging he len icula galaxy and he posi ion o he ansien
(see he g een ellipse in he op panel o Fig. 6), u he s eng h-
ening he sugges ed physical connec ion. Finally, in ou deep HST
imaging, we ind no e idence o ex ended emission a he sou ce
posi ion, such as could be he case had he sou ce o igina ed in a
no - oo-dis an backg ound galaxy. The e o e, we i s discuss he
na u e and he p ope ies o he sou ce unde he assump ion ha
i lies a he same edshi as he len icula galaxy (𝑧=0.082; Le an
e al. 2025; and his wo k, see Sec ion 2.4.1).
A a edshi o 𝑧=0.082, he obse ed b igh es absolu e magni-
ude o 𝑟′
ABS =−14.5is a he ain when compa ed o he peak
absolu e magni ude o se e al EP-disco e ed FXTs (e.g., see Fig. 9).
Howe e , while i is oo ain o be explained as an a e glow o a
long GRB, i is consis en wi h he peak absolu e magni ude and ligh
cu e e olu ion o some ain e , me ge -d i en sho GRBs (Fig. 9).
MNRAS 000,1–14 (20252025)
EP250207b a compac bina y me ge ? 7
Figu e 7. Spa ial dis ibu ions o he he line-o -sigh eloci y V, he eloci y
dispe sion 𝜎, he me allici y [M/H] and age in he 45 Vo onoi bins o he
candida e hos galaxy WISEA J111002.65−075211.9 obse ed wi h MUSE.
The axes a e gi en in xand ydis ances om he cen al pixel o he 𝑧=0.082
galaxy in kilopa secs. No e ha he loca ion o he ansien s lies ou side he
sky egion shown.
Fu he mo e, he o se o 10′′ (Liu e al. 2025a), co esponding o
15.9 kpc in p ojec ion, is well wi hin he ange o ypical hos galaxy
o se s obse ed o sho GRBs (e.g., Bloom e al. 2002a;Fong e al.
2022) and simula ed me ge o igin GRB popula ion s udies (Mand-
hai e al. 2022). In addi ion, he high age o he s a s in he len icula
galaxy ha we de i e om ou VLT/MUSE obse a ions (see Fig. 7)
is inconsis en wi h a collapsa o igin, bu is consis en wi h a me ge -
d i en (sho ) GRB scena io. Finally, he a e o decay obse ed in
he 𝑟′and F606W-band (see Fig. 4) seems o decele a e. This could
be consis en wi h pe sis en con ibu ions om a globula clus e o
he co e o a ( idally dis up ed) dwa galaxy hos o EP250207b. A
a edshi o 𝑧=0.082, a globula clus e nea he peak o he absolu e
magni ude dis ibu ion, i.e., wi h an absolu e magni ude o ≈ −10
(Ha is 2010), would co espond o F606W=27.84 AB mag and hus
would con ibu e abou 30% o he obse ed lux a ou obse a ional
epoch 𝑡≈28 d. Clea ly, an e en b igh e globula clus e could be e-
sponsible o nea ly all he op ical/NIR ligh in his inal epoch. The
absolu e magni ude dis ibu ion o ul a compac dwa s (UCDs),
which migh be he co es o a idally dis up ed dwa galaxies, o e -
laps ha o he b igh end o he absolu e magni ude dis ibu ion o
globula clus e s (e.g. Mieske, Hilke , & In an e 2002), so he sou ce
obse ed a la e ime could also be explained by such a UCD. A
idal s eam om a idally dis up ed dwa galaxy could also explain
he enhanced emission linking WISEA J111002.65−075211.9 and
he loca ion o he ansien . Fu u e HST o JWST obse a ions o
EP250207b can es he scena io ha pa o all o he ligh in he las
epoch o HST obse a ions is due o a globula clus e o a dis up ed
dwa galaxy. Finally, o a edshi o 𝑧=0.082, he absolu e mag-
ni udes on es ame imescales o ≈5 o 25 d a e he disco e y
o EP250207b ule ou he p esence o a (b oad-lined) Type Ic (o
indeed any) supe no a.
The 3.06 GHz adio Mee KAT non-de ec ions a 𝑡=5.6, 23, and
43 d a e he disco e y a lux le els o ≈25𝜇Jy imply a adio lumi-
nosi y uppe limi o 𝐿3.06 GHz<
∼1.3×1037 e g s−1, o <
∼4×1027 e g
s−1Hz−1. Such a adio luminosi y is low when compa ed o he a-
dio emission de ec ed o (on-axis) sho GRBs (see e.g. igu e 13
in Fong e al. 2021). We used edback (Sa in e al. 2024) o com-
pa e ou adio, op ical (𝑟′/V-band), and X- ay ligh cu es wi h hose
es ima ed ia he a e glow_models.gaussian_ edback s uc u ed je
model. This model is iden ical o ha used o he only con i med
‘o -axis’ iewed me ge o igin GRB 170817A (e.g., Lyman e al.
2018;Lamb e al. 2019), and assumes a Gaussian-shaped je s uc-
u e. The ex e nal medium o he a e glow is assumed o be uni o m,
and he je unde goes sideways expansion as desc ibed by G ano &
Pi an (2012) o hei 𝑎=1case. The model p io s and pos e io dis-
ibu ions a e p esen ed in Table 1and he i is indica i e o a mildly
‘o -axis’ iewed GRB a e glow in a low-densi y ambien medium,
see Figu e10. No e ha Wiche n e al. (2024) showed ha an o -
axis scena io is di icul o econcile wi h he p ope ies o he ull
sample o Chand a-disco e ed FXTs, sugges ing pe haps ha (some)
EP-disco e ed FXT ha e a di e en na u e han hose. Finally, he
op ical emission de ec ed in he second HST epoch o obse a ions is
oo b igh o ou a e glow model, sugges ing ha addi ional emis-
sion mechanisms o sou ces (such as he possible globula clus e o
dwa galaxy men ioned abo e) could con ibu e o he op ical ligh .
The a e age 0.3–10 keV X- ay luminosi y a he EP-WXT disco -
e y is 𝐿X,𝑎𝑣𝑒 =1×1046 e g s−1. This X- ay luminosi y is on he
high end, bu s ill consis en wi h, he X- ay luminosi y heo ized
o be emi ed h ough he spin-down o a ms massi e (>2M⊙)
magne a o med in a bina y neu on s a me ge (e.g. Zhang 2013;
Me zge & Pi o 2014;Ciol i 2016;Sun e al. 2017b,2019b;Qui ola-
Vásquez e al. 2024b). No e ha unde his model he X- ay emission
is quasi-iso opic and no powe ed by a je . Howe e , he X- ay ligh
cu e can be well desc ibed by a simple powe -law decay, i.e., no
clea , ks-las ing pla eau in he ligh cu e is de ec ed, such as has
been p e iously in oked o FXTs ha a e sugges ed o be magne a -
powe ed (e.g. Zhang 2013;Me zge & Pi o 2014;Ciol i 2016;Sun
e al. 2017a;Sun e al. 2019a;Xue e al. 2019;Qui ola-Vásquez e al.
2024b). Al hough, as he EP-WXT obse a ion was cu sho ( he
ansien was s ill ongoing when he sa elli e s a ed mo ing), he
measu ed FXT du a ion is a lowe limi . Ne e heless, i is possible
ha ou iewing angle o EP250207b is such ha mos o he pla eau
emission is no obse ed ( he so-called " apped zone"; Sun e al.
e.g., 2019a). Only a e he X- ay emission has ionised he ejec a in
ou line o sigh he X- ays escape, leading o he de ec ion o , in his
case, a small pa o he pla eau (desc ibing he EP-WXT ligh cu e
wi h a cons an ; see Fig. 1) and he powe law decay phase (c . Sun
e al. 2019a; see Fig. 4).
The index o he bes - i powe law decay o he X- ay ligh cu e
o −1.5is in-be ween he p edic ions o he decay o 𝐿𝑋∝𝑡−1and
𝐿𝑋∝𝑡−2 o imes la ge han he cha ac e is ic imescale o magne-
a spin down due o he emission o g a i a ional wa e adia ion and
o imes la ge han he cha ac e is ic imescale o magne a spin
down caused by elec omagne ic adia ion, espec i ely (c . Qui ola-
Vásquez e al. 2024b). Howe e , he powe -law index does no need
o be −1o −2, i he elec omagne ic adia ion om a ms magne a
is no only om acuum dipole adia ion. Mo eo e , he spin-down
mechanism e ol es wi h ime. The e o e, gi en ha he alue o
≈ −1.5is measu ed o e a days- o week-long ime pe iod, i can also
e lec his e olu ion (see e.g., Lasky e al. 2017,Sa in e al. 2020).
We ha e used se e al kilono a (KN) models implemen ed in Red-
back (Sa in e al. 2024) o calcula e possible KN op ical (F606W)
and NIR (F105W, F125W, F160W) ligh cu es o compa e wi h he
HST da a poin s (see Fig.11). We ind ha many o he KN models
om Kasen e al. (2017) o e -p edic he obse ed HST magni udes
a obse e imes 7.3–8.7 d a e he sou ce disco e y. Only KN
emission p oduced by me ge s gi ing ise o a ela i ely low ejec a
mass o 0.005 M⊙is consis en wi h he obse a ions (g een line
in Fig. 11). The assumed lan hanide ac ion is 10−1. Such ejec a
masses a e low compa ed o he ejec a masses ypically ound om
modelling he obse a ions o GW 170817 (e.g. Villa e al. 2017,
MNRAS 000,1–14 (20252025)
8Jonke e al.
Figu e 8. Bes - i pPXF (Cappella i 2017) model o he MUSE spec um o he hos galaxy. The obse ed spec um is shown in black, wi h he lexible s ella
popula ion syn hesis empla e in ed. Model-sub ac ed esiduals a e shown in g een, wi h blue and he g ey shaded egions showing he emo ed ou lie s o he
model.
Pa ame e P io Pos e io Desc ip ion
𝜃obse e ( ad) [sin]0↔𝜋/2 0.11 ±0.04 Obse e s line-o -sigh angle
log 𝐸K,iso (log e g) 44 ↔54 51.7±0.7Iso opic equi alen kine ic ene gy
𝜃co e ( ad) 0.01 ↔0.1 0.08 ±0.02 Je co e hal opening angle
𝜃edge ( ad) 0.1↔0.2 0.16 ±0.03 Angula ex en o s uc u ed je
log 𝑛ism (log cm−3)−5↔2−4.3+0.8
−0.5Ambien medium numbe densi y
𝑝1.4↔3.1 2.92 ±0.06 Elec on spec al ene gy densi y index
log 𝜖𝑒−5↔0†−0.4+0.2
−0.4F ac ion o ene gy in elec ons
log 𝜖𝐵−5↔0†−2.9±1.4F ac ion o ene gy in magne ic ield
𝜀𝑁0.1↔1.0 0.34+0.3
−0.17 Synch o on pa icipa ion ac ion
Γ040 ↔400 260 ±110 Ini ial bulk Lo en z ac o
Table 1. The model pa ame e s o ou a e glow model i da a shown in Figu e 10. No e ha we excluded he op ical and NIR da a ob ained a e 𝑡 > 10 d om
he i , as his emission seems o be coming om a componen ha is no an a e glow. The p io ange and, whe e app op ia e, he dis ibu ion unc ion a e gi en
(else he dis ibu ion is la in he ange indica ed), and he model pos e io median and 1−𝜎con idence in e al. A co ne plo o he pos e io dis ibu ion is
included in AppendixB.†We also se he equi emen log 𝜖𝑒>log 𝜖𝐵.
Waxman e al.2018) andsamples o sho GRBs (e.g. Ras inejade al.
2021). Also, while he e a e no many spec oscopically con i med
KNe known, such low ejec a masses a e a ely seen in nume ical sim-
ula ions which p oduce hype massi e o longe -li ed neu on s a s
(Kawaguchi e al. 2022;Kawana e al. 2018), i.e., he magne a s ha
ha e been in oked o explain some FXTs, and mo e likely poin s
owa ds a bina y neu on s a me ge whe e he emnan p omp ly
collapsed in o a black hole (Nedo a e al. 2022). The la e end o
p oduce less ejec a, bo h dynamically and om he disk-winds (e.g.,
Siegel 2019;Sa in & Lasky 2021).
The KN models om Me zge (2017) p oduce a b igh KN signal
ha apidly decays, and can jus be consis en wi h he obse a ions;
e.g., o an ejec a mass o 0.01 M⊙, ejec a eloci y o 0.25 𝑐, lan-
hanide ac ion 𝜒=0.1, eloci y index 𝛽=3 o 𝑚e𝑗𝑒𝑐𝑡 𝑎 ∝𝑣−𝛽, and
a g ey-opaci y o 𝜅=0.5co esponding o lan hanide-poo , “blue”
ejec a. Magne a enhanced KN models om Sa in e al. (2022) o
ypical pa ame e alues o e -p edic he op ical and NIR magni-
udes ( ed line in Fig. 11). The magne a enhanced models can only
be made consis en wi h he HST obse a ions o a he cons ain-
ing, p obably e en implausible, pa ame e s (magen a line in Fig. 11).
This model equi es signi ican g a i a ional-wa e emission, which
educes he a ailable o a ional ene gy budge a ailable o elec o-
magne ic adia ion, has a high 𝛾- ay opaci y o u he educe he
a ailable ene gy in op ical, and inally, we o ce he opaci y o be
high, in con as o expec ed opaci ies in he case o long-li ed neu-
on s a emnan s (Me zge 2019).
Finally o a edshi o 𝑧=0.082, we calcula e using bilby (Ash-
on e al. 2019) ha any bina y neu on s a me ge like GW 170817
would p obably p o ide a ma ginal g a i a ional wa e (GW) signal
de ec ion wi h a signal- o-noise a io (SNR) o ≈9in LIGO-Vi go
Kag a (LVK), i.e., below he con en ional igge h eshold o an
SNR 12. I he e en was ins ead due o a black hole – neu on s a
me ge , he black hole mass and spin would ha e o ha e been such
ha he e was (ample) o ma e ial ou side he black hole’s e en
ho izon, implying ha as he GW signal would be s onge han o a
BNS me ge , i migh well ha e yielded a de ec ion. The lack o such
a de ec ion sugges s he e o e ha a black hole – neu on s a me ge
is less likely as an o igin o EP250207b. Combining he ime and
sky loca ion o EP250207b wi h GW da a will boos he con idence
in any GW signal (c . Sa in e al. 2023).
Nex , we conside an al e na i e scena io whe e he associa ion
wi h he candida e hos galaxy, WISEA J111002.65−075211.9 is
MNRAS 000,1–14 (20252025)
EP250207b a compac bina y me ge ? 9
10 1100101102
Res - ame ime since bu s (days)
25
20
15
10
Absolu e magni ude
z
=2.182
z
=0.082
SN 1998bw
SN 2006aj
long GRBs
sho GRBs
EP 240315a
EP 240801a
EP 250108a
EP 241021a
EP 240414a
EP 250207b
Figu e 9. Kann plo showing he app oxima e 𝑟′-band ligh cu e o
EP250207bin ed oge he wi h hose o a sample o EP-disco e ed FXTs
(EP240414a [blue; an Dalen e al. 2024], EP241021a [o ange; Qui ola-
Vásquez e al. submi ed], EP240801 [pu ple; Jiang e al. 2025], EP240315a
[da k blue; Le an e al. 2024], EP250108a [magen a; Eyles-Fe is e al. 2025;
Ras inejad e al. 2025]) and sho GRBs ( hin pu ple lines) and long GRBs
( hin black lines). The GRB ligh cu es a e ob ained om Kann e al. (2006,
2010,2011); Nicuesa Guelbenzu e al. (2012). The absolu e magni udes o
he da a-poin s in he ligh cu e om EP250207b a 𝑧=0.082 a e consis en
wi h he ain -end o ha o he sho GRB dis ibu ion. I he sou ce edshi
is 𝑧=2.18 ins ead (see Discussion), he ligh cu e is consis en wi h ha o
he b igh end o sho GRBs. The e olu ion as a unc ion o ime is consis en
wi h ha seen in sho GRBs also, al hough he decay a e seems o decele a e
a e he i s HST obse a ion (nea 𝑡=7−8d). Fo a edshi o 𝑧=0.082,
he p esence o a (b oad-lined) Type Ic (o indeed any) supe no a can be uled
ou .
a me e chance alignmen and he FXT mus o igina e om (nea )
ano he hos galaxy. The ligh de ec ed in he second-epoch o HST
obse a ions could be due o an un esol ed backg ound galaxy. A a
magni ude o F606W ≈27 and an o se o <
∼1a csecond, he chance
alignmen p obabili y o such a galaxy is ≈10% (Bloom e al.
2002b), signi ican ly la ge han he chance alignmen p obabili y o
<
∼0.5% o WISEA J111002.65−075211.9 (Malesani e al. 2025).
Ne e heless, i we assume he second epoch HST de ec ions o
he sou ce a e in ac due o he un esol ed hos galaxy, and we i
BAGPIPES (Ca nall e al. 2018) and P ospec o (Johnson e al. 2021)
models o i we ob ain a pho ome ic edshi o 𝑧BP =3.0±0.5,
and 𝑧P osp =3.5±0.7. We ha e added a igu e showing he bes -
i P ospec o galaxy model o he Appendix (C). Fo a edshi o
𝑧=3, he EP-WXT 0.3–10 keV X- ay luminosi y would become
2×1050 e g s−1, which is a ypical alue o long GRBs (e.g.,
see Daino i e al. 2016). The absolu e magni ude would become
es ame 𝑔′-band ≈ −21.5, no ou o he o dina y o a long GRB
hos galaxy a such a edshi (e.g., Schneide e al. 2022). Thus,
besides he highe chance alignmen , his scena io canno be uled
ou .
Finally, we also b ie ly conside as hos o EP250207b he galaxy
ma ked wi h a "N" in Fig. 6 ha lies in p ojec ion o he No h
o he candida e hos len icula galaxy. Using he Legacy Su ey
(Dey e al. 2019) magni ude con e ed o Vega mag and co ec ing
o Galac ic ex inc ion we ha e R = 23.7 a a sepa a ion o 5.8”
wi h espec o he FXT. This leads o chance associa ion p obabili y
o 32% o he galaxy. E en hough his is hus a likely chance
alignmen , we de i ed he edshi om i s MUSE spec um. Two
0.1 1 10 100
Time since explosion [days]
10−7
10−6
10−5
10−4
10−3
10−2
10−1
100
Flux densi y [mJy]
3.06 GHz
z’
’
F606W
1 keV
Figu e 10. We used edback (Sa in e al. 2024) o i he op ical (g ey, g een
and pink) and X- ay (ligh blue) a e glow da a (excluding he da a ob ained
a e 𝑡 > 10 d), including he adio (blue) uppe -limi s a an assumed edshi ,
𝑧=0.082. The a e glow model used was gaussian_ edback ollowing Lamb
e al. (2018) and including synch o on sel -abso p ion e ec s (see Lamb &
Kobayashi 2019, o de ails). The shaded egions indica e he 90% c edible
in e al o he model i s o he da a, whe e we used nessai (Williams e al.
2024) as he sample wi h a Gaussian likelihood. The model i s e u n an ou -
o -je -co e iewing angle, essen ially pe haps sligh ly ‘o -axis’, o ∼6˚.3 o a
je co e angle o ∼4˚.6, howe e , he unce ain ies on bo h pa ame e s a e also
consis en wi h an ’on-axis’ scena io. Fu he mo e, he Gaussian s uc u ed
je ‘wings’ ex end o ∼9˚.2, al hough he ene gy a he wide angles will
con ibu e insigni ican ly o he obse ed emission. A e he ini ial decay
om he p omp emission ( he p omp emission is no modelled he e) he
X- ay obse a ions om EP-FXT, he op ical/NIR da a a 𝑡 < 10 d, and adio
uppe limi s a e consis en wi h mildly o -axis a e glow emission.
clea emission lines a e ound a obse edwa eleng hs o 8132.2±0.3
and 8138.6±0.4Å. The wa eleng hs o hese lines a e consis en wi h
hose o he [OII] double a a edshi o 𝑧=2.1824. I EP250207b
and i s coun e pa a e in ac associa ed wi h his galaxy, he o se
o he op ical coun e pa o he cen e o his galaxy o ≈5.78′′
co esponds o an o se o abou 50 kpc in p ojec ion, s ill consis en
wi h he dis ibu ion o o se s ound o sho GRBs (see e e ences
abo e) bu inconsis en wi h he cumula i e dis ibu ion o long
GRBs(Blancha de al.2016).Theobse ed peak absolu emagni ude
in he 𝑟′-band would become −21.7, nea he b igh -end o he sho
GRB dis ibu ion (see Fig. 9) and he a e age X- ay luminosi y a he
EP-WXT disco e y 𝐿X=2×1049 e g s−1 o his highe edshi .
The adio 𝐿3.06 GHz would become <
∼3×1040 e g s−1, o <
∼9×
1030 e g s−1Hz−1. These alues o he adio luminosi y a e in
line wi h hose ound o o he sho GRBs (see e.g. igu e 13 in
Fong e al. 2021). Howe e , he peak X- ay luminosi y in his case
becomes oo high o be a magne a -powe ed FXT and we mus be
obse ing he je - ela ed p omp X- ay emission. Concluding, e en i
he galaxy ma ked "N" is he hos galaxy, his EP-disco e ed FXT can
be explained as due o a me ge d i en e en and no as a collapsa -
d i en e en .
ACKNOWLEDGEMENTS
We hank he e e ee o hei commen s which helped imp o e he
Manusc ip .
We a e deeply g a e ul o Tom Ma sh o de eloping he molly
MNRAS 000,1–14 (20252025)
