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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17276, 17277, 17278
Message-Id: <20250708071808.CB40528804C1@server.astronomerstelegram.org>
Date: Tue,  8 Jul 2025 03:18:08 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17276							     ATEL #17276

Title:	Spectral Slope and Colors of Interstellar Comet 3I/ATLAS with
		the Palomar 200-inch Palomar Next Generation Spectrograph
Author:	Matthew Belyakov (Caltech),  Bryce T. Bolin (Eureka Scientific),
		Christoffer Fremling (Caltech), Matthew Graham (Caltech)
Queries:	mattbel@caltech.edu
Posted:	7 Jul 2025; 12:09 UT
Subjects:Comet, Solar System Object

We report characterizing observations of interstellar comet 3I/ATLAS taken
with the Palomar 200-inch (P200)/Next Generation Palomar Spectrograph (NGPS)
on 2025 July 3 UTC. Imaging with the NGPS shows that the comet has an extended
appearance, confirming the results of ATEL #17263 by Jewitt & Luu, ATEL#17264
by Alacron et al., and ATEL #17275 by Minev et al. (see attached g, r,
and i band image composite). The comet possesses a coma with a FWHM of
~2 arcsec in non-sidereal tracked image vs. background stars in sideral
tracked images with a FWHM of ~1.6 arcsec. The g, r, and i band observations
show that the comet has g-r  = 0.43 +/- 0.02,  r-i = 0.16+/-0.02, and g-i
= 0.59+/-0.03 measured with an aperture size of 4 arcsec, equivalent to
10,000 km at the distance of the comet. The NGPS spectroscopy covering
the 580 - 1020 nm wavelength range reveals a flat spectrum with a spectral
slope of 3% per 100 nm and no notable emission lines. Subsequent observations
will monitor the comet's activity as it approaches perihelion over the
next several months.

Images of 3I/ATLAS taken in g, r, and i bands taken on 2025 July 3 UTC
from the P200/NGSP: 
https://i0.wp.com/mattbel.io/wp-content/uploads/2025/07/i3_panel-1.png

Our work is based on observations taken at Palomar Observatory. The authors
would like to thank the Palomar observing staff for their assistance with
the observations. The authors also wish to recognize and acknowledge the
cultural significance that Palomar Mountain has for the Pauma Band of the
Luiseno Indians.

Palomar images of 3I/ATLAS.: https://i0.wp.com/mattbel.io/wp-content/uploads/2025/07/i3_panel-1.png
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Password Certification: Bryce Bolin (bolin.astro@gmail.com)
https://www.astronomerstelegram.org/?read=17276
==============================================================================
ATEL #17277							     ATEL #17277

Title:	EP250702a/ GRB 250702B: NuSTAR X-ray Observations
Author:	Brendan O&#39;Connor (CMU), Dheeraj Pasham (Eureka Scientific/George
		Washington), Igor Andreoni (UNC), Jeremy Hare (Catholic/GSFC):
Queries:	boconno2@andrew.cmu.edu
Posted:	7 Jul 2025; 17:47 UT
Subjects:X-ray, Gamma-Ray Burst, Transient, Tidal Disruption Event

We observed the X-ray transient EP250702a (GCN 40906), which is also associated
with the multiple gamma-ray triggers designated as GRB 250702B/D/E (GCNs
40883, 40886, 40890, 40891, 40931, 40903, 40914, 40923), with the NuSTAR
X-ray Telescope through a Target of Opportunity observation through GO
program 11282 (PI: Pasham). The observation occurred between 2025-07-03
20:45:21 and 2025-07-04 09:15:00 UTC for an on-source exposure of ~21.5
ks, simultaneous with Swift/XRT observations (ObsID: 19906002).

The source is clearly detected at an average rate of ~0.4 cts/s. The 3-79
keV X-ray lightcurve shows short term variability throughout the full exposure
in both FPMA and FPMB, but no flaring is observed. The source varies in
count rate by a factor of 2-3 on timescales as short as a few ks. Compared
to NuSTAR observations of GRB 221009A at a similar count rate (GCNs 32695,
32788), the source shows more significant variability, which we suggest
disfavors an ultralong GRB origin, especially in conjunction with the July
1st onset reported by the Einstein Probe (GCN 40906).

The X-ray spectra are featureless and do not show any Fe lines or reflection
features. This is consistent with the lack of features in the Swift/XRT
spectra. The 3-79 keV spectrum (FPMA+FPMB) is well fit by an absorbed powerlaw
with a photon index of Gamma~1.81+/-0.03, which is consistent with the
initial Swift/XRT spectral index within errors (GCN 40919). The hydrogen
column density was fixed to 1e22 cm^-2 as per the best-fit Swift/XRT spectra
(see https://www.swift.ac.uk/LSXPS/transients/9377). The unabsorbed 3-79
keV flux (time-averaged) is (2.35+/-0.05)e-11 erg/cm^2/s during our observation.

We also performed a fit with an absorbed cutoff powerlaw and find a cutoff
energy >60 keV. In this case, the best-fit photon index is Gamma~1.67+/-0.08.
This has better agreement with the EP/FXT result, although the FXT observation
was obtained at an earlier time (GCN 40917). The C-stat between both fits
to the NuSTAR data was nearly identical and the cutoff powerlaw is not
a statistically significant improvement. We suggest that the lack of Fe
lines, and lack of cutoff in the powerlaw spectra (out to at least 60 keV),
is atypical for an X-ray binary.

We performed a Lomb-Scargle analysis on the barycenter corrected X-ray
data, and did not identify any periodic signals over the frequency range
1e-3 to 100 Hz, consistent with the EP/FXT report (GCN 40917). The periodogram
becomes red noise dominated below 3e-3 Hz.

Further analysis is underway and additional NuSTAR and Swift observations
are planned.

We thank the NuSTAR SOC, and in particular Karl Forster and Brian Grefenstette,
for promptly implementing these observations.

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Password Certification: Brendan O&#39;Connor (oconnorb@gwmail.gwu.edu)
https://www.astronomerstelegram.org/?read=17277
==============================================================================
ATEL #17278							     ATEL #17278

Title:	Erratum to ATEL#17275: Follow up observations of interstellar
		comet 3I/ATLAS
Author:	M. Minev (IA and NAO, BAS, Bulgaria), A. Kostov (IA and NAO,
		BAS, Bulgaria), A. Mutafov (IA and NAO, BAS, Bulgaria)
Queries:	akostov@astro.bas.bg
Posted:	7 Jul 2025; 19:38 UT
Subjects:A Comment

The results presented in ATel #17275 were obtained with two different telescopes.
Due to an mistake, we mixed the pixel sizes of the two telescopes, when
calculating the projected size of the image. This lead to wrong, overestimated
results presented in ATel #17275.

The correct distances are as follows:

along X axis ~ 3400 km

along Y axis ~ 3000 km

photometric aperture radius ~ 8000 km

We apologize for any inconvenience this may have caused.

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Password Certification: Andon Kostov (akostov@astro.bas.bg)
https://www.astronomerstelegram.org/?read=17278
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