The INTEGRAL Galactic Plane Scanning
aa r X i v : . [ a s t r o - ph . H E ] A p r The INTEGRAL Galactic Plane Scanning
Mariateresa Fiocchi
Istituto di Astrofisica e Planetologia Spaziali. INAF. ItalyE-mail: [email protected]
Lorenzo Natalucci ∗ Istituto di Astrofisica e Planetologia Spaziali. INAF. ItalyE-mail: [email protected] on behalf of the GPS Team
After the first nine years of INTEGRAL operational life, the discovery of new sources andsource types, a large fraction of which are highly transient or highly absorbed, is certainly oneof the most compelling results and legacies of INTEGRAL. Frequent monitoring of the GalacticPlane in AO8 and AO9 campaigns allowed us to detect transient sources, both known and new,confirming that the gamma-ray sky is dominated by the extreme variability of different classesof objects. Regular scans of the Galactic Plane by INTEGRAL provide the most sensitivehard X-ray wide survey to date of our Galaxy, with flux limits of the order of 0.3 mCrab foran exposure time of ∼ ∼ hours to months) and identified by triggered followup observations, mainly bySwift/XRT and optical/infrared telescopes. These discoveries are very important to characterizethe X-ray binary population in our Galaxy, that is necessary input for evolution studies. Thetransient source monitoring is crucial to sample a variety of physical conditions corresponding toa large span in luminosity. The long exposure times available allow us to define the spectral andtiming properties of the known sources and the global properties of each object class. "An INTEGRAL view of the high-energy sky (the first 10 years)" 9th INTEGRAL Workshop and celebrationof the 10th anniversary of the launch,October 15-19, 2012Bibliotheque Nationale de France, Paris, France ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/ alactic Plane Scanning
Lorenzo Natalucci
1. The INTEGRAL Galactic Plane Scanning
For the first 5 years of INTEGRAL operational life, the scientific Core Programme includedregular scans of the Galactic Plane and Bulge. These observations led to the discovery of manynew high energy sources. Many of these are the highly absorbed HMXB, that were previouslyunknown due to the lack of sensitive hard X-ray observations. Furthermore, many new transientobjects have been found, on short timescales and with low luminosity ( < ergs − ). From AO-5onwards, these regular scans were discontinued, resulting in a significant drop in the discovery rateof new Galactic systems. From AO8 to AO10 however, a multi-year proposal for Galactic PlaneScanning has been approved and since 2011 this program is collecting data for a total exposuretime of ∼ ∼
135 Ms. The majority of these sources are best detected on timescales less than or equal to anINTEGRAL orbit ( ∼ ∼ ±
10 degree in galactic latitude and a longitude extent of about 170 deg. Due to itsunique capability to monitor the Galactic Plane for long periods at a sensitivity level unreachable byother wide-field imagers ( < ∼ . − . exposure =1Ms, ∼ exposure =10ks),INTEGRAL has proven especially capable of discovering faint and/or short-lived phenomena thatare typical of Galactic binary sources. Figure 1:
Left: JEM-X image in the 3-10 keV image of 1RXS J194211.9+255552 . Right: The XRT/Swiftimage in the 2-10 kev of the 1RXS J194211.9+255552
The Galactic Plane observations with INTEGRAL so far have changed our view of the Galaxydoubling the number of known high-mass X-ray binaries (Bird et al. 2010), discovering twonew types of High Mass X-ray Binary (HMXB): 1) the high absorbed binary systems with N H ≥ cm − , such that these sources could not be identified earlier with soft X-ray instruments (Ku-ulkers 2005; Ibarra et al. 2007); 2) the Super Giant Fast X-ray Transient (SFXT), showing theshort (from tens of minutes to few hours) and intense flares (L ∼ − ergs − ), X-ray pulsar likespectrum, with very high dynamic range: 3-5 orders of magnitude with respect to the quiescentemission with L ∼ ergs − (for review see Sidoli 2010, Sidoli 2011). The physics of these pro-2 alactic Plane Scanning Lorenzo Natalucci . . . r m a li z ed c oun t s s − k e V − Energy (keV) data and folded model maria 21−Dec−2011 13:14 . . . r m a li z ed c oun t s s − k e V − Energy (keV) data and folded model maria 21−Dec−2011 13:15
Figure 2:
The XRT spectrum of ROSAT source 1RXS J194211.9+255552, fitted with a power law modelwith an absorption of N H ∼ . × cm − and a photon index of 0 . ± . cesses is still unknown and represent one of the main open issues in the study of compact objects inbinary system. Studies of these HMXBs classes will be fundamental to understand the populationsynthesis, to determine the chemical enrichment of the Galaxy and the evolution of massive stars inbinaries and to investigate the poorly understood accretion mechanism (Sidoli 2009 for a review)which produces the short flares observed from the SFXT.The gamma-ray satellites Fermi and AGILE and the Cherenkov telescopes (as HESS, Milagroetc.) have provided new surveys of the Galactic Plane from MeV to TeV energies. INTEGRALis and will be the unique opportunity to identifying counterparts of the unidentified very highenergy sources and observations above 20 keV are needed to understand the physical origin of theiremission. In the second Fermi/LAT catalogue about 320 MeV/GeV sources have been reportedwithin 10 degrees of the Galactic Plane and about one third are still unidentified (Nolan et al.2013). Among them, some are transient sources flaring on timescales shorter than a few days,which can be a new class of the galactic fast transient high-energy emitters. INTEGRAL representa good possibility to systematically identify the unidentified MeV/GeV/TeV sources, define thespatial distributions and luminosity functions of the each object classes.The INTEGRAL observatory strategy and its large field of view allows to monitoring a largenumber and variety of galactic sources for a long time, needed to study the physics of emissionmechanisms for white dwarfs, neutron stars and black holes. These observations allowed us tounderstand the interplay between the different components of the accretion flow onto the compactobjects such as the cold disk and the neutron star, the hot Comptonizing plasma and the possiblesynchrotron radiating jets (Cadolle Bel et al., 2009; Prat et al., 2009; Fiocchi et al. 2008).// Inparticular we report here some INTEGRAL result: 1) non-thermal components at energies >100keV from neutron star low-mass X-ray binaries (Fiocchi et al. 2006; Paizis et al. 2009; Tarana etal. 2011); 2) INTEGRAL discovered high energy emission (200-300 keV) in a strong magnetizedneutron stars, with B = − Gauss (Mereghetti et al. 2009 and and references therein) and theSoft Gamma Repeaters spectral behavior, soft at energies above 10 keV and hard below 10 keV, incontrast to spectra from Anomalous X-ray Pulsars which show an opposite spectral shape; 3) hardemission from black holes up to 1 MeV (Caballero Garcia et al., 2007; Del Santo et al. 2012).3 alactic Plane Scanning
Lorenzo Natalucci
2. The Galactic Plane Scanning throughout AO8 and AO9
We briefly report on the preliminary results from the recent AO8 and AO9 Programme, con-cerning the discovery of the new source, monitoring of the know source and the timing study onlong time scale. The results reported here are simple examples of the many different topics that canbe studied with Galactic Plane observations.
At the time of writing, the pointings of the Vela and Cygnus region starting in May 2011,allowed us to monitor many known sources: 31 HMXBs, 36 LMXBs, 5 AGN and 4 other sources(Atels 3361, 3434, 3816, 3818, 3887, 3916, 4135, 4136, 4168, 4218). The Galactic Plane Scanningobservations (from AO1 to AO9) and the follow-up soft X-ray and optical/IR allowed to identifynew sources in the Galactic plane: ∼
50 AGN hidden behind the galactic plane, ∼
50 HMXB, ∼
20 LMXB (Low Mass X-ray Binary) and ∼
20 CV (Cataclism Variables). Other ∼
120 new IGRsources have been found but their highly variable behavior makes it difficult to identify them,confirming that the gamma-ray sky is dominated by the extreme variability of different classes ofobjects (Bird et al. 2010).A recent example of detection of a new source by INTEGRAL/JEM-X happened during theGalactic Plane Scan observations performed between 2011 December 18, 13:47 UTC and De-cember 19, 8:59 UTC. This source has been promptly identified as the transient ROSAT 1RXSJ194211.9+255552, with averaged fluxes of 10 ± ± H = ( . ± . ) × cm − , a photon index of 0 . ± . ∼ . × − ergcm − s − , in 1-10 keV energy range (reduced c =1.1 for 36 degrees of freedom).Masetti et al. (Atel 4209) identified the optical counterpart using the 1.5m Cassini telescope of theAstronomical Observatory of Bologna in Loiano (Italy) and showed that the optical counterpart isa Galactic high mass X-ray binary, confirming the HMXB nature for this source as suggested byATel 3818. During the Galactic Plane Scanning INTEGRAL perform a long monitoring of GS 0834-43,a source discovered by Granat/WATCH in February 1990 with a peak flux of ∼
400 mCrab in the8-20 keV energy range (LapShov et al. 1992) and also observed at high energies by CGRO/BATSEat regular intervals until June 1993 (Wilson et al. 1997). These observations allowed to character-ize GS 0834-43 as a Be transient X-ray pulsar with a spin period of ∼ ∼
106 days), a small eccentricity between 0.10 and 0.17 and a low inclination angle (Wilson et al.1997). After 1993, no outbursts were detected by BATSE or other monitoring instruments such asSwift/BAT (Cusumano et al. 2010) or INTEGRAL/IBIS (Bird et al. 2010). During INTEGRAL4 alactic Plane Scanning
Lorenzo Natalucci
Figure 3:
Left: IBIS image in the 18-60 keV image during revolution 1184 of GS 0834-43. Right: TheIBIS/ISGRI 18 − Galactic Plane Scan observations between 2012-06-26 06:44:07 and 2012-06-26 15:27:19 UTC,IBIS/ISGRI detected this source at 47 sigma confidence level in the 18-60 keV energy range witha exposure time of 15.8 ks. It was also detected by the JEM-X instrument at a significance of 19sigma for an exposure time of 1.7 ks. The flux is 109 ± ± . ± . . × − ergcm − s − ( c =12/9 dof). The INTE-GRAL detectin of this new, strong outburst could be a signature of a renewed long term activityperiod that is being followed up by other orbiting high energy telescopes, also covering the hardX-rays. The Nuclear Spectroscopic Telescope Array (Nustar) has also observed this source in July2012 with two pointings and is expected to provide better sensitive spectral/timing measurements. The long exposure time reached in the Galactic Plane is an unique opportunity to performtiming studies, in particular to measure the orbital period of the X-ray binaries and to determine thespectral characteristics of the Galactic source. The long term 18 − ∼
10 days, whichwe interpret as the orbital period (for details see Fiocchi et al. 2012). The folded light curve usingthe best orbital period determination of ∼
10 days is shown in Figure 3 (right), where is detected asingle broad emission maximum above a quiescent level with the shape similar to those observedfrom other SFXTs such as IGR J17544-2619 (Clark et al. 2009) and IGR J16465-4507 (Clark etal. 2010).
3. Conclusions
Thanks to the combination of its large field of view and good sensitivity for short exposures5 alactic Plane Scanning
Lorenzo Natalucci (20 mCrab is reached at 50 keV with 2 standard GPS pointings of 2ksec each), timing resolutionbetter than 1ms and good broad band spectroscopy, INTEGRAL is and will be the only possibilityto study very relevant topics as the new sources, the periodicity of Galactic sources, the spectral andtemporal characteristic of the sources and of the different classes, their spatial distributions withinour Galaxy and luminosity functions. Because of the arc-minute location accuracy at energiesabove 20 keV, INTEGRAL is and will be unique to find the hard-X counterparts of new unidentifiedMeV/GeV/TeV sources, discovered with AGILE and Fermi or with the ground based Cerenkovtelescopes. Finally its instruments, sensitive in the 3 keV to 10 MeV range, provide a uniquelink between the soft X-ray band covered by X-ray telescopes such as XMM-Newton or Chandraand the energy band of the high-energy gamma-ray space missions. The future Galactic Planeobservations will provide a more uniform and longer monitoring of the known sources, new sourcedetections and will also detect outburst or in peculiar spectral states, triggering dedicated ToO.
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