The Large Area Telescope on the Fermi gamma-ray Space Telescope provides
unprecedented sensitivity for all-sky monitoring of gamma-ray activity. It has
detected a few Galactic sources, including 2 gamma-ray binaries and a
microquasar. In addition, it is an adequate telescope to detect other transient
sources. The observatory scans the entire sky every three hours and allows a
general search for flaring activity on daily timescales. This search is
conducted automatically as part of the ground processing of the data and allows
a fast response to transient events, typically less than a day. Most of the
outbursts detected are spatially associated with known blazars, but in several
cases during the first years of observations, gamma-ray flares occurring near
the Galactic plane did not reveal any initially compelling counterparts. This
prompted follow-up observations in X-ray, optical, and radio to attempt to
identify the origin of the emission and probe the possible existence of a class
of transient gamma-ray sources in the Galaxy. Here we report on these LAT
events and the results of the multiwavelength counterpart searches.Comment: Invited review, 7 pages, 3 figure

Chaty, Sylvain

English

arXiv

SYLVAIN CHATY

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<i>Fermi</i>-LAT RESULTS ON GALACTIC PLANE γ-RAY TRANSIENT SOURCES

Invited review, 7 pages, 3 figuresInternational audienceThe Large Area Telescope on the Fermi gamma-ray Space Telescope provides unprecedented sensitivity for all-sky monitoring of gamma-ray activity. It has detected a few Galactic sources, including 2 gamma-ray binaries and a microquasar. In addition, it is an adequate telescope to detect other transient sources. The observatory scans the entire sky every three hours and allows a general search for flaring activity on daily timescales. This search is conducted automatically as part of the ground processing of the data and allows a fast response to transient events, typically less than a day. Most of the outbursts detected are spatially associated with known blazars, but in several cases during the first years of observations, gamma-ray flares occurring near the Galactic plane did not reveal any initially compelling counterparts. This prompted follow-up observations in X-ray, optical, and radio to attempt to identify the origin of the emission and probe the possible existence of a class of transient gamma-ray sources in the Galaxy. Here we report on these LAT events and the results of the multiwavelength counterpart searches

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Fermi-LAT RESULTS ON GALACTIC PLANEgamma-RAY TRANSIENT SOURCESSylvain ChatyTo cite this version:Sylvain Chaty. Fermi-LAT RESULTS ON GALACTIC PLANE gamma-RAY TRANSIENTSOURCES. Thibault Damour, Robert T Jantzen and Remo Ruffini. Twelfth Marcel GrossmannMeeting on General Relativity, Jul 2009, Paris, France. World Scientific, Singapore, 2010.<hal-00479473>HAL Id: hal-00479473https://hal.archives-ouvertes.fr/hal-00479473Submitted on 30 Apr 2010HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.April 30, 2010 17:36 WSPC - Proceedings Trim Size: 9.75in x 6.5in main.hyper311821Fermi-LAT RESULTS ON GALACTIC PLANE γ-RAY TRANSIENTSOURCESSYLVAIN CHATY on behalf of the Fermi-LAT collaborationLaboratoire AIM (UMR 7158 CEA/DSM-CNRS-Universite´ Paris Diderot)Irfu/Service d’Astrophysique, Centre de Saclay, Baˆt. 709FR-91191 Gif-sur-Yvette Cedex, FranceE-mail: chaty@cea.frThe Large Area Telescope on the Fermi γ-ray Space Telescope provides unprecedentedsensitivity for all-sky monitoring of γ-ray activity. It has detected a few Galactic sources,including 2 γ-ray binaries and a microquasar. In addition, it is an adequate telescope todetect other transient sources. The observatory scans the entire sky every three hoursand allows a general search for flaring activity on daily timescales. This search is con-ducted automatically as part of the ground processing of the data and allows a fastresponse to transient events, typically less than a day. Most of the outbursts detectedare spatially associated with known blazars, but in several cases during the first years ofobservations, γ-ray flares occurring near the Galactic plane did not reveal any initiallycompelling counterparts. This prompted follow-up observations in X-ray, optical, andradio to attempt to identify the origin of the emission and probe the possible existenceof a class of transient γ-ray sources in the Galaxy. Here we report on these LAT eventsand the results of the multiwavelength counterpart searches.Keywords: Fermi-LAT, γ-ray, X-ray observations, transient, Galactic.1. IntroductionThere has been one firmly established class of variable sources in the high-energyγ-ray sky. The Energetic Γ-Ray Experiment Telescope (EGRET) on the Compton Γ-Ray Observatory discovered a population of variable γ-ray blazars above 100 MeV.1However, EGRET also left the legacy of a large fraction of unidentified sources in the3EG catalog. Many of these were found at low Galactic latitudes and believed to beGalactic in nature. EGRET established γ-ray pulsars as a Galactic population andthese were thought to contribute to the unidentified sources. Several studies foundindications of variability in some of the sources along the Galactic Plane (GP),2,3a behavior not expected of the pulsars, which are steady on these timescales. Ad-ditionally, no blazar counterpart was identified in several cases. This suggested thepossible existence of a new Galactic γ-ray class. Several sources showed both strongvariability and a convincing lack of a blazar within the EGRET localization errors,like 3EGJ0241+6103, 3EGJ1824-1514 and GROJ1834-04.One of these has emerged as a new type of γ-ray source with the rediscoveryof 3EGJ0241+6103 (COS-B 2CG135+01), which was associated, although the po-sition was uncertain, with LSI+61◦303, a ”prominent radio flaring star system”,high mass X-ray binary (HMXB) system at 2 kpc constituted of a B0Ve star and aneutron star orbiting on an excentric orbit with a 26.5 days period. A daily/monthlyvariability was seen with EGRET, but no periodicity detected.4 A TeV source hasthen been detected by MAGIC and then VERITAS, at this position, with a peri-odic signal modulated at the orbital period. Fermi has detected a γ-ray source,April 30, 2010 17:36 WSPC - Proceedings Trim Size: 9.75in x 6.5in main.hyper3118220FGLJ0240.36113, at the position RA=40.076, DEC=61.233 with a 95% errorradius of 1.8′, consistent with the optical counterpart, and with a periodicity of26.6± 0.5days, the emission peaking at the periastron, and a spectrum reminiscentof the pulsars, therefore definitely identifying in the MeV-GeV domain the firstγ-ray binary source with the HMXB system.5The second interesting source is the case of 3EGJ1824-1514, detected byEGRET, but without modulation, as spatially coincident with LS 5039, an HMXBconstituted of a likely neutron star orbiting around an O6.5 star. HESS detected aperiodic signal modulated at the orbital period of 3.91 days. Fermi firmly detectedat more than 12σ a periodic source, modulated at 3.91±0.05 days, in a complicatedregion with a very intense Galactic diffuse emission.The most dynamic example from EGRET is GROJ1838-04 (3EGJ1837-0423),which produced an intense outburst in June 1995.6 The flux above 100MeV in a3.5 day period was found to be a factor of 7 brighter than in later observationsof the region. Notably, no blazar counterpart is known within the 99% EGRETerror contour. The absence of a flat spectrum radio source at the levels typical ofthe EGRET blazars made this a candidate for a different type of γ-ray emitter.The proximity of such a unique outburst to the inner Galaxy led to speculation ofa possible Galactic origin. The question of the progenitor of this activity remainsas well as the broader question of the existence of similar sources of this type. Asstated in Ref. 6, “other unidentified EGRET sources near the GP appear to be timevariable with V> 1.5” in Ref. 7.Finally, Fermi has recently detected a variable high energy source coincidingwith the position of the X-ray binary and microquasar Cygnus X-3, modulated atits short orbital period of 4.8 hours. Cygnus X-3 is an HMXB system located ata distance of ∼ 7 kpc, with a compact object of nature still matter in the debate,orbiting a Wolf-Rayet star.82. LAT Detection of γ-ray transientsThe Fermi Large Area Telescope (LAT) is very well suited for monitoring variabil-ity in the GeV sky. The large effective area (> 8000 cm2 on axis above 1GeV),wide field-of-view (∼ 2.4 sr), and excellent angular resolution (better than 1◦ above1GeV) greatly enhance the sensitivity to transient activity in comparison to pre-vious γ-ray instruments in this energy range. A notable departure from previousobservations is that the energy range of the LAT (20MeV to > 300GeV) extendsabove that covered by EGRET. The combination of the wide field-of-view withthe sky scanning observational mode supplies coverage of the sky every ∼ 3 hours(2 orbits). This enables the detection of fainter objects in shorter intervals thanpreviously possible. Also, the localizations for sources above threshold are muchbetter than 1◦ even on short timescales. These capabilities are critical for trigger-ing rapid multiwavelength follow-up observations of LAT transients. The LAT hasso far detected three transient events near the GP that have not been associatedApril 30, 2010 17:36 WSPC - Proceedings Trim Size: 9.75in x 6.5in main.hyper311823with blazarsa. Here, we report on the γ-ray characteristics and the multiwavelengthfollow-up observations that were triggered shortly after the detections.All the unidentified transients were first detected by the LAT automated sci-ence processing (ASP).10 The ASP flare search locates and analyzes detected pointsources in the LAT photon data as these become available for processing on theground. The search runs on 6-hour, 1-day, and 1-week intervals. The latency betweenthe time the data are acquired and when they are available for ground processingis short enough to allow alerts to be communicated and follow-up multi-wavelengthobservations triggered within a day. The LAT team continuously monitors the out-put of the automated searches for new source detections and flares from known LATsources. The automated processing reports candidate locations and flux estimates, asummary is given in weekly reports in http://fermisky.blogspot.com. The transientspresented here are high-confidence detections that appeared in multiple 6 hour anddaily ASP searches.2.1. 3EGJ0903-3531LAT observations on 6 October 2008 revealed a newly detected LAT source spa-tially associated with 3EGJ0903-353111 in the 3EG catalog1 (see Fig. 1 left panel).The preliminary localization (J2000.0: RA= 136.◦25, DEC= −35.◦45, r68 = 0.◦12;this is a correction to the originally reported ATel value) was based on one day ofobservations. Additional analysis demonstrated the source was detected (TS> 25,∼ 5σ) on 5, 6, and 7 October 2008 before falling below the daily sensitivity thresh-old for this region. The LAT flux (E> 100MeV) reported for the flare exceeded10−6 photons cm−2 s−1, which is about a factor of 5 greater than the EGRET flux,and the source brightened by a factor of 15 in 3 days. Despite being bright overseveral days, this source did not exceed the 10σ criteria for the LAT bright sourcelist,12 which includes data from 4 August 2008 through 30 October 2008.2.2. Fermi J0910-5041This source was first detected on 15 October 200813 (see Fig. 1 middle panel). Itappears in the LAT bright source list as 0FGLJ0910.2-504412 and is flagged as vari-able. The preliminary localization, (J2000.0: RA, Dec= 137.◦69,−50.◦74, r68= 0.◦07),is based on one day of data and only accounts for statistical errors. The reportedflux above 100MeV exceeded 10−6 photons cm−2 s−1. The bright source list analy-sis includes three months of observations and uses a more detailed prescription forcalculating the 95% error circle that also includes systematic effects (see Ref. 12).The preliminary coordinates are consistent with the bright source list position, pro-viding an additional confirmation of the LAT capability for consistent localizationon relatively short timescales.aThere is a fourth transient event, Fermi J0109+6134, detected on 1st of February 2010,9 whichis probably a blazar shining through the Galactic plane; therefore we will not report more aboutthis source in this paper focused on Galactic transients.April 30, 2010 17:36 WSPC - Proceedings Trim Size: 9.75in x 6.5in main.hyper3118242.3. Fermi J1057-6027The LAT detected a new transient γ-ray source in the GP with exhibited an out-burst on June 11, 2009, with a flux (E> 100MeV) of 2.4±0.7×10−6 photon.s−1cm−2(statistical)14 (see Fig. 1 right panel). The flare had the shortest duration and soft-est spectrum of the unassociated, low latitude transients making it less significant(∼ 5σ) over foreground and background emission. It coincided with a LAT sourcedetected in 9-months (Aug 2008 - Apr 2009) of all-sky monitoring data (J2000.0:RA= 164.◦308, Dec =−60.◦458, with a 95% confidence error circle radius 0.◦07 (sta-tistical)). The new LAT detection increased by a factor of ∼ 10 with respect to thesource flux level detected during the earlier period. There is no previously reportedEGRET γ-ray detection at this location.3. Multiwavelength ObservationsFollowing each LAT transient detection, we triggered target of opportunity Swift15observations designed to search for plausible counterparts. A pair of snapshot (4-7 ks) exposures were obtained approximately two to three days after the onset ofγ-ray activity for the targets, with a third observation obtained 2 weeks (3EGJ0903-3531) and 1 month later (J0910-5041) in order to search for variability on longertimescales.Within the LAT r68 of 3EGJ0903-3531, there are four 2.4− 4.0σ X-ray sourcesfrom the summed XRT exposure (Fig. 2 left panel). The observed (unabsorbed) 0.3-10 keV fluxes are in the range 2.7(5.3)− 6.3(12.5)× 10−14 erg.s−1cm−2, assumingΓ = 2 and Galactic absorption = 2.6× 1021 cm−2. There are no radio counterpartsto the X-ray sources at the depth of the 1.4GHz NVSS16 image, although two faint(2.5-3.9 mJy at 1.4 GHz) radio sources are found within r68 (NVSS J090428.87-353007.5, NVSS J090458.76-353145.4).17In the case of J0910-5041, a single X-ray source (Swift J091057.47-504808.5)was detected within the LAT r68 (Fig. 3 right panel) . The observed (unabsorbed)0.3-10 keV flux of 3.2(13.3) × 10−13 erg.s−1cm−2, assuming Γ = 2 and Galacticabsorption = 1.3× 1022 cm−2. Ref 18 found weak evidence for X-ray variability inthis source and that it has a radio counterpart detected in the SUMMS image19with a flat-spectrum.20 An additional radio source is found within the LAT r68(SUMMSJ091042-504103)17 (Fig. 3 left panel).Finally, in the case of Fermi J1057-6027, a target of opportunity observation bySwift/XRT on June 13, 2009 did not detect any source within the ∼ 24′ field-of-view of XRT,21,22 and no radio counterparts have been found either (see Fig. 2 rightpanel).4. ConclusionThe task of identifying counterparts for unidentified transients remains challeng-ing. Although greatly reduced from EGRET, the LAT error circles remain largeApril 30, 2010 17:36 WSPC - Proceedings Trim Size: 9.75in x 6.5in main.hyper311825compared to the resolution of telescopes in other wavebands. Additional argumentsbased on temporal and spectral characteristics are required to support firm associ-ations. Ultimately, identifications of the Galactic transients require observations ofrelated variability between the γ-ray source and a candidate Galactic counterpartat lower frequency. In the absence of a detection of significant activity of the poten-tial radio and X-ray counterparts for the LAT transients, they remain unidentified.These sources continue to be monitored regularly for γ-ray activity as a part of theFermi sky survey observations.5. AcknowledgementsThe Fermi-LAT Collaboration acknowledges support from a number of agencies andinstitutes for both the development and operation of the LAT as well as the scientificdata analysis. These include NASA and the DOE in the United States, the CEA/Irfuand IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA inJapan, and the K.A. Wallenberg Foundation, the Swedish Research Council andthe National Space Board in Sweden. Additional support from INAF in Italy forscience analysis during the operations phase is also gratefully acknowledged. Thiswork was supported by the Centre National d’Etudes Spatiales (CNES), based onobservations obtained through MINE: the Multi-wavelength INTEGRAL & FermiNEtwork.Fig. 1. Left: LAT counts map (E> 100MeV) in celestial coordinates of 3EGJ0903-3531 from5 October through 7 October 2008. The image is smoothed with a 0.◦5 width Gaussian. Thegreen circle shows the 3EG 95% error circle. The white circle represents the LAT 95% error circlebased on the preliminary analysis of the flare period. Middle: LAT counts map (E> 100MeV)in celestial coordinates of J0910-5041 from 15 October through 16 October 2008. The image issmoothed with a 0.◦5 width Gaussian. The green circle represents the LAT 95% error circle basedon the preliminary analysis of the flare period. Right: LAT 95% confidence region for the flare(cyan) and the nine-month source position (green) for Fermi J1057-6027.References1. R. C. Hartman, D. L. Bertsch, S. D. Bloom and et al., ApJS 123, 79(July 1999).April 30, 2010 17:36 WSPC - Proceedings Trim Size: 9.75in x 6.5in main.hyper311826Fig. 2. Left: Archival radio and Swift/XRT images of the field containing 3EGJ0903-3531.Swift/XRT observations of the new LAT position (cyan) revealed a likely X-ray counterpartthat coincides with a flat-spectrum radio object. Right: Swift/XRT image of the field contain-ing Fermi J1057-6027. The white circle marks the 95% error circle for the LAT detection.Fig. 3. Archival radio (left panel) and Swift/XRT (right panel) images of the field containingFermi J0910-5041. The two white circles mark the 68% and 95% error circles for the LAT detection.2. D. F. Torres, G. E. Romero, J. A. Combi, P. Benaglia, H. Andernach and B. Punsly,A&A 370, 468(May 2001).3. P. L. Nolan, W. F. Tompkins, I. A. Grenier and P. 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Fermi-LAT results on Galactic Plane gamma-ray Transient Sources

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