J/ApJ/851/31 2FGL J0846.0+2820 opt. counterpart follow-up (Swihart+, 2017)
2FGL J0846.0+2820: a new neutron star binary with a giant secondary and variable
γ-ray emission.
Swihart S.J., Strader J., Johnson T.J., Cheung C.C., Sand D., Chomiuk L.,
Wasserman A., Larsen S., Brodie J.P., Simonian G.V., Tremou E.,
Shishkovsky L., Reichart D.E., Haislip J.
<Astrophys. J., 851, 31 (2017)>
=2017ApJ...851...31S 2017ApJ...851...31S
ADC_Keywords: Gamma rays; Binaries, spectroscopic; Photometry, UBVRI;
Radial velocities; Spectra, optical
Keywords: binaries: spectroscopic ; gamma rays: general ; pulsars: general
Abstract:
We present optical photometric and spectroscopic observations of the
likely stellar counterpart to the unassociated Fermi Large Area
Telescope (LAT) γ-ray source 2FGL J0846.0+2820, selected for
study based on positional coincidences of optical variables with
unassociated LAT sources. Using optical spectroscopy from the SOAR
telescope, we have identified a late-G giant in an eccentric (e=0.06)
8.133-day orbit with an invisible primary. Modeling the spectroscopy
and photometry together led us to infer a heavy neutron star primary
of ∼2M☉ and a partially stripped giant secondary of
∼0.8M☉. Hα emission is observed in some of the spectra,
perhaps consistent with the presence of a faint accretion disk. We
find that the γ-ray flux of 2FGL J0846.0+2820 dropped
substantially in mid-2009, accompanied by an increased variation in
the optical brightness, and since then, it has not been detected by
Fermi. The long period and giant secondary are reminiscent of the
γ-ray bright binary 1FGL J1417.7-4407, which hosts a millisecond
pulsar (MSP) apparently in the final stages of the pulsar recycling
process. The discovery of 2FGL J0846.0+2820 suggests the
identification of a new subclass of MSP binaries that are the likely
progenitors of typical field MSPs.
Description:
The γ-ray source was first detected as 2FGL J0846.0+2820 (Nolan+
2012, J/ApJS/199/31), listed in the second full catalog of Fermi-LAT
sources, based on the first two years of LAT data obtained from 2008
August to 2010 August using the earlier P7V6 instrument response
functions (IRFs). See section 2.1.
The γ-ray source 2FGL J0846.0+2820 was selected for follow-up
study based on a search of positional coincidences of periodic optical
variables found in the Catalina Sky Surveys Data Release-1 (CSS -DR1;
Drake+ 2014, J/ApJS/213/9) catalog with Fermi-LAT sources.
To analyze the CSS photometry of this variable, we retrieved 471 CSS
photometric measurements of the variable optical counterpart taken
between 2005 April 10 and 2013 September 25. See section 2.2.1.
We obtained time series photometry of the optical source in B, V, and
R bands with the 16-inch PROMPT-5 telescope at Cerro Tololo
International Observatory between 2015 February 4 and 2015 June 7.
Each observing night consisted of multiple 60s exposures of the target
field, which included the candidate optical counterpart to
2FGLJ0846.0+2820 as well as five nearby comparison stars.
Our final sample includes 1643 photometric measurements in B, 1038 in
V, and 500 in R, with mean magnitudes B=16.73, V=15.95, and R=15.40.
See section 2.2.2.
We began spectroscopic monitoring of the source with the Goodman
Spectrograph on the Southern Astrophysical Research (SOAR) 4.1m
telescope on 2014 December 8, continuing through 2016 December 31.
We obtained 19 independent SOAR epochs of velocities.
We also obtained some low-resolution spectra with OSMOS on the Hiltner
2.4m telescope at the MDM Observatory at Kitt Peak. These spectra were
obtained in two to three exposures of 20 minute each on eight epochs
from 2016 October 20 to 2017 January 8.
See section 3.1.
We obtained a high-resolution spectrum with HIRES on the Keck I
telescope on 2016 September 25 (R∼36000, wavelength range
∼3900-8100Å). See section 3.2.
Forthcoming X-ray observations will help confirm the connection
between the Fermi γ-ray source 2FGL J0846.0+2820 and the optical
binary CSS J084621.9+280839.
Objects:
--------------------------------------------------------------------------
RA (ICRS) DE Designation(s)
--------------------------------------------------------------------------
08 46 03 +28 20.9 2FGL J0846.0+2820 = 2FGL J0846.0+2820
08 46 21.88 +28 08 40.8 CSS J084621.9+280839 = CRTS J084621.9+280839
--------------------------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 25 3477 PROMPT photometry of 2FGL J0846.0+2820
table2.dat 27 20 Barycentric radial velocities of J0846
--------------------------------------------------------------------------------
See also:
B/vsx : AAVSO International Variable Star Index VSX (Watson+, 2006-2014)
II/183 : UBVRI Photometric Standards (Landolt 1992)
I/284 : The USNO-B1.0 Catalog (Monet+ 2003)
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
V/139 : The SDSS Photometric Catalog, Release 9 (Adelman-McCarthy+, 2012)
J/A+AS/146/407 : Symbiotic stars catalogue (Belczynski+, 2000)
J/ApJS/188/405 : Fermi-LAT first source catalog (1FGL) (Abdo+, 2010)
J/ApJS/199/31 : Fermi LAT second source catalog (2FGL) (Nolan+, 2012)
J/ApJS/208/17 : 2nd Fermi LAT cat. of gamma-ray pulsars (2PC) (Abdo+, 2013)
J/ApJS/213/9 : Catalina Surveys periodic variable stars (Drake+, 2014)
J/ApJS/218/23 : Fermi LAT third source catalog (3FGL) (Acero+, 2015)
J/ApJ/831/89 : Eclipsing LMXB at the center of 3FGL J0427.9-6704 (Strader+,
2016)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 F10.5 d JD [7057.6/7181.5] Julian Date, JD-2450000
12 A1 --- Band Observational band (B, V or R)
14- 19 F6.3 mag mag [6.3/17.4] Apparent magnitude in Band
21- 25 F5.3 mag e_mag [0.01/3] The 1σ error in mag
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 15 F15.7 d BJD Barycentric Julian Date
17 A1 --- f_BJD [a] a: from the Keck/HIRES spectrum
19- 23 F5.1 km/s RV [-12.4/97] Barycentric radial velocity
25- 27 F3.1 km/s e_RV [0.9/3.4] RV uncertainty
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 06-Jul-2018