Fermi LAT detection and Swift X-ray follow-up of a new gamma-ray/X-ray transient source Fermi J1544-0649 (Swift 154419.7-064915)
ATel #10482; S. Ciprini (ASI/SSDC Rome and INFN), C. C. Cheung (Naval Research Laboratory), D. Kocevski (NASA/MSFC), J. Chiang (SLAC) on behalf of the Fermi Large Area Telescope Collaboration; S. N. Shore (U. Pisa and INFN)
on 11 Jun 2017; 10:23 UT
Credential Certification: Stefano Ciprini (firstname.lastname@example.org)
Subjects: Ultra-Violet, X-ray, Gamma Ray, >GeV, AGN, Black Hole, Blazar, Cataclysmic Variable, Transient, Tidal Disruption Event
The Large Area Telescope (LAT), one of the two instruments on the Fermi
Gamma-ray Space Telescope, has observed strong gamma-ray emission from a new
source. Preliminary analysis indicates that the source was detected in gamma
rays in two consecutive weeks beginning on 2017 May 15 by the "
tool with high-significances (6 sigma in each week) in the 0.8-300 GeV band.
Taking the 2-weeks of LAT data together, and selecting photons from 300 MeV
to 300 GeV, a single power-law fit gave a best-fit average >100 MeV flux of
(8.1+/-2.3) x 10^-8 photons cm^-2 s^-1 (statistical uncertainty only) and
photon index of 1.7+/-0.1 (statistical uncertainty only). The preliminary
best-fit location of this gamma-ray source (RA=236.05 deg, Dec=-6.82 deg,
J2000) has a 95% containment radius of 0.07 deg. The source is detected by the
LAT also on a daily timescale. The significance peaked on May 21 with a
daily-averaged flux (E > 100 MeV) of (1.5+/-0.7) x10^-7 ph cm^-2 s^-1 with
corresponding photon index of 1.5 +/- 0.2, while on May 24 the daily-averaged
flux (E > 100 MeV) was (3.0+/-1.0) x10^-7 ph cm^-2 s^-1 with corresponding
photon index of 2.0+/-0.3. This source is not in any published LAT catalog and
was not detected by INTEGRAL, AGILE or EGRET.
We obtained a follow-up Swift observation of the region starting 2017 May 26.
In the 1.9 ks XRT observation, we found a new, bright X-ray source within the
LAT localization with X-ray position, R.A. = 236.08226 deg, Decl. = -6.82075 deg
(90% radius = 1.4") and an unabsorbed single power-law flux (0.3-10 keV) of
9.4 (+0.7/-0.6) x 10^-11 erg cm^-2 s^-1 with photon index 2.2 +/- 0.1 and NH
= 1.9 (+/- 0.4) x 10^21 cm^-2 similar to the Galactic value NH = 1 x
10^21 cm^-2 (Kalberla et al. 2005). This corresponds to an
about 100 times larger flux than the typical ROSAT all-sky survey (RASS) upper
limit. NH is consistent with the integrated profile.
Curiously, the X-ray source is coincident with an apparent optical transient,
ASASSN-17gs = AT2017egv, detected at V=17.3 mag on 2017-05-25 09:36 UT (i.e.,
contemporaneous to the LAT transient detections) with a potential host galaxy,
2MASX J15441967-0649156. Spectroscopic follow-up indicate hints of absorption at
the position of Hydrogen Balmer lines (TNS
page) which we estimate correspond to redshift z~0.04. However, inspection
of archival optical data for this galaxy indicate that the V=17.3 mag ASSASSN
trigger appears consistent with its nominal flux (CSS pseduo-V average ~16.8 mag
over the past decade). Radio detections have flux densities of 46.6+/-1.5
mJy at 1.4 GHz (NVSS J154419-064913) and 67 mJy at 150 MHz (TGSS alternative
data release, with a radio spectral index 0.15).
The persistent flat-spectrum radio emission could indicate a BL Lac origin
for the enhanced X-ray and hard-spectrum gamma-ray emission although the
absorption observed in the optical spectrum would be difficult to explain. The
large factor X-ray increase observed may indicate a more exotic origin (e.g.,
Because Fermi operates in an all-sky scanning mode, regular gamma-ray
monitoring of this source will continue. We encourage further multifrequency
observations of this source. For this source the Fermi LAT contact persons are
S. Ciprini (stefano.ciprini at ssdc.asi.it) and C.C. Cheung (teddy.cheung at
The Fermi LAT is a pair conversion telescope designed to cover the
energy band from 20 MeV to greater than 300 GeV. It is the product of an
international collaboration between NASA and DOE in the U.S. and many
scientific institutions across France, Italy, Japan and Sweden. We thank the
Swift team for their rapid scheduling of the observation, in particular C.
Gronwall as the Swift Observatory Duty Scientist.