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 (stefano.ciprini@asdc.asi.it)

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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 " FAVA" 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 N_H = 1.9 (+/- 0.4) x 10^21 cm^-2 similar to the Galactic value N_H = 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. N_H 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., TDE).

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 nrl.navy.mil).

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.